Contemporary Skull Base Surgery: A Comprehensive Guide to Functional Preservation 3030993205, 9783030993207

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Contemporary Skull Base Surgery A Comprehensive Guide to Functional Preservation A. Samy Youssef Editor

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Contemporary Skull Base Surgery

A. Samy Youssef Editor

Contemporary Skull Base Surgery A Comprehensive Guide to Functional Preservation

Editor A. Samy Youssef Department of Neurological Surgery University of Colorado School of Medicine Aurora, CO, USA Department of Otolaryngology University of Colorado School of Medicine Aurora, CO, USA

ISBN 978-3-030-99320-7    ISBN 978-3-030-99321-4 (eBook) https://doi.org/10.1007/978-3-030-99321-4 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Foreword

I am proud to introduce this book Contemporary Skull Base Surgery, A Comprehensive Guide to Functional Preservation to our community of neurosurgeons. This text completes the journey of A. Samy Youssef from being my former Skull Base Fellow fellow and then colleague to becoming my teacher who is defining his vision of what skull base surgery is and how it should be applied in a more mature multi-disciplinary, multi-modality world. Samy has earned credibility in this space through countless hours in the cadaveric laboratory, numerous dissection courses around the world, and his impressive clinical experience. If you are planning to recommend or perform a typical skull base tumor operation, go to the literature to find evidence-based best practices applicable to your case. They cannot be found. That is the burden of practicing surgery in a specialty that is characterized by “orphan diseases,” too few cases for a “controlled study,” too complex and critical for patient randomization, and too controversial to expect surgeon equipoise. This world of skull base surgery will likely remain. Senior skull base surgeons manage the void by using their own best judgment acquired by a thousand personal mistakes, poor judgments, and poor outcomes. Taken together, we call this experience. Is this the path for all future skull base surgeons and their patients? The answer is no. As has been said, learn from the mistakes of others, you won’t live long enough to make them all yourself. Certainly, your patients hope to avoid being thrown upon the heap of your own cautionary anecdotes in your journey to becoming “an experienced skull base surgeon.” Contemporary Skull Base Surgery: A Comprehensive Guide to Functional Preservation will help you on this journey to become a faster learner and a safer surgeon for your patients. Dr. Youssef skillfully addresses these complex issues by gathering an impressive collection of experts who bring their personal experience to voice with each chapter. Under his guidance, this book brings reasonable consideration to the tension between endoscopic and open surgical approaches to anterior skull base tumors, trying to advance the debate from proselytization to reasonable discussion and application. Keep Contemporary Skull Base Surgery, A Comprehensive Guide to Functional Preservation near your desk, at home, or at work, wherever you go to sit and stare at the images of a specific case, with a skull in your hand, to “imagine the operation.” Find the chapter that addresses your case specifically. Follow along with the experts in the field who share their very specific v

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wisdom founded in their own experiences, and translations of their own successes and failures. With these guides helping you to avoid their mistakes, envision a surgical plan that will lead to success. And yes, unfortunately, on occasion, you will still make your own mistakes. That too is the nature of skull base surgery. As eloquently stated by Rene Leriche, “Every surgeon carries within himself a small cemetery, where from time to time he goes to pray – a place of bitterness and regret, where he must look for an explanation for his failures.” Use this book as a guide to keep your cemetery of regret as small as possible. Contemporary Skull Base Surgery: A Comprehensive Guide to Functional Preservation reflects Dr. Youssef’s vision of what is possible in our field and his passion to make a difference. Harry R. van Loveren Department of Neurosurgery and Brain Repair University of South Florida Health Morsani College of Medicine Tampa, FL, USA

Preface

For skull base neurosurgeons and neurosurgeons in training, Contemporary Skull Base Surgery: A Comprehensive Guide to Functional Preservation is a modern resource with a unifying message. My concept for this book evolved over a 3-year period to not only encompass recent technological advances in the operating room and radiology suite but to unify the achievements in our field from the past with the present. With support from Springer, the idea was launched to create a comprehensive guide/companion to facilitate the surgeon’s decision-making process and to support young surgeons in building a successful career in skull base surgery. Contemporary Skull Base Surgery’s contributors and Springer editors supported the idea of taking a progressive approach in the chapter format by creating a roadmap for devising optimum treatment strategies for complex brain pathologies. The authors generously supplemented their chapters with anatomical figures and surgical videos. Contemporary Skull Base Surgery addressed a divisive fundamental issue. As the scope of knowledge expanded within the field of neurosurgery, methodologies became more complex. However, a chiasm of debate among neurosurgeons widened: should surgery be performed by purely endoscopic approaches or only traditional open approaches? Microsurgery was no longer the only treatment choice for complex brain pathologies. The evolution of endoscopic endonasal skull base approaches and the ongoing advances in radiotherapy enhanced the surgeon’s ability to “do no harm.” Yet, without integration of these two viewpoints, the selection of an optimum treatment strategy remained a confounding step. Further impeding this learning process, current textbooks are typically either endoscopic or traditional. The few references that combine both do not guide the reader to which treatment strategy to choose. Therefore, devising an optimum treatment strategy for complex brain pathologies using the strengths of both schools of thought became a challenging enterprise. Contemporary Skull Base Surgery unifies the strengths of both of these schools of thought, preserving the wisdom of skull base traditions and promoting recent advances in the operating room and radiology. This comprehensive guide/companion aims to facilitate the surgeon’s decision-making process in optimizing treatments for our patients by functional and anatomical preservation of key neurovascular structures. Contributing authors of this

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text have been in the forefront of designing skull base laboratories; creating skull base trainings for residents, fellows, and junior-level surgeons; and leading the design and refinement of instrumentation, technique, and clinical protocols in our field.

How to Use This Book Contemporary Skull Base Surgery consists of eleven main sections. The numerous chapters are written by skull base leaders who guide the reader with their expertise in surgical technique and technology. Important to this text are the supplementary surgical and endoscopic videos. Readers are encouraged to make use of these highly effective training tools, which can enhance their understanding of the text. The first section, General Principles, consists of 13 foundational chapters essential for every skull base surgeon. Contributors acquaint the reader with the essentials, such as skull base anatomy, development of a multidisciplinary skull base team, operating room equipment, surgical instruments, and modern imaging technologies. These authors masterfully convey the key elements that play a major role in optimizing functional outcomes and patient quality of life. The two chapters on compartmental anatomy set the stage for understanding the technical and surgical nuances of each location. The ten subsequent sections are organized by anatomical compartment or region of the skull base for uniformity and ease of use. Each section describes the available choices for treatment to each compartment contributed by world-class neurosurgeons and otolaryngologists who provide top-level expertise in how to tackle each pathology. The surgical approach chapters that follow then lead to a specific anatomical section. Operative techniques are described in a clear and stepwise manner with accompanying intraoperative photos, illustrations, and surgical videos. Chapters addressing individual pathologies depict pathological subtypes with representative radiographic images of clinical case examples. Each pathology includes a treatment algorithm based on tumor morphology, preoperative clinical status, and the goal of maximum functional preservation that includes a brief description of surgical approaches. This becomes the reader’s roadmap, a guide to reach a decision treatment of each patient’s skull base pathology. (See “How to Use This Book: Example Case”) Contemporary Skull Base Surgery aims to be a comprehensive, versatile handbook as well as detailed reference and surgical atlas for skull base surgery. My deep appreciation to the collaborative of contributing authors from the fields of neurosurgery and otolaryngology who exemplify the unifying vision and progressive concept of this text, which replaces partisanship with deeper discussion. Contemporary Skull Base Surgery uniquely integrates the two main surgical schools of endoscopy and open surgery into one reference that is enhanced by treatment algorithms. Over years, trainees candidly shared their sugges-

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tions, criticisms, and ideas to better grasp the knowledge component of skull base surgery. Their input as the next generation of surgeons helped to fuel the progressive concept of this text. Finally, I wish to acknowledge Dr. Takanori Fukushima whose gracious encouragement and insight fueled my determination to envision and produce Contemporary Skull Base Surgery. Aurora, CO, USA

A. Samy Youssef, MD, PhD

How to Use This Book: Example Case

Contemporary Skull Base Surgery focuses to identify the target structures and to plan an approach that minimizes iatrogenic damage. This strategy will lead the surgeon down the road of either open, endoscopic, or a combination of both approaches. Example: Evaluating a patient with an anterior skull base meningioma. • First, go to the section Anterior Cranial Fossa and review the chapter “Meningioma” to find the optimum treatment strategy based on tumor size, location, and preoperative functional and anatomical status of surrounding neurovascular structures. • Second, follow the pictorial algorithm for selecting the steps to optimal treatment or surgical choice. • Third, read the details about the surgical approach selected (endoscopic, open, combined, or radiosurgery) in the general section Anterior Cranial Fossa and reconstruction of the skull base in the section General Principles. Summary: Use this decision-making process when seeing a patient in clinic. Follow with a more detailed study before performing the surgical procedure. Contemporary Skull Base Surgery can be a useful adjunct for multidisciplinary case discussions and clinical rounds geared toward finalizing a treatment plan.

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Contents

Part I General Principles 1 Evolution  of the Multidisciplinary Skull Base Team��������������������   3 Rafael Martinez-Perez and A. Samy Youssef 2 Surgical  Anatomy of the Cranial Nerves ��������������������������������������   9 Jaafar Basma, Kara Parikh, and Jeffrey M. Sorenson 3 Skull  Base Compartmental Anatomy: Microsurgical and Endoscopic����������������������������������������������������������������������������������������  35 Jaafar Basma, Kara Parikh, and Jeffrey M. Sorenson 4 The Operating Room ����������������������������������������������������������������������  61 Rafael Martinez-Perez and A. Samy Youssef 5 Surgical Positioning ������������������������������������������������������������������������  71 Robert S. Heller, Siviero Agazzi, and Harry R. Van Loveren 6 Cranial  Nerve Functional Preservation: Tricks of the Trade������  81 Rafael Martinez-Perez and A. Samy Youssef 7 Neurophysiologic Monitoring ��������������������������������������������������������  89 Rafael Martinez-Perez, Angela Downes, and A. Samy Youssef 8 Microdissection Tools���������������������������������������������������������������������� 101 A. Samy Youssef 9 Neuroimaging  Precision Tools and Augmented Reality���������������� 105 Torstein R. Meling and Maria-Isabel Vargas 10 Skull Base Reconstruction�������������������������������������������������������������� 131 Garni Barkhoudarian, Michael B. Avery, and Daniel F. Kelly 11 Role  of Radiotherapy in Modern Skull Base Surgery������������������ 147 Tiit Mathiesen 12 Cerebral  Revascularization for Skull Base Lesions���������������������� 157 Nickalus Khan, Turki Elarjani, and Jacques J. Morcos 13 Cranial  Nerve Repair and Rehabilitation�������������������������������������� 169 Scott Hirsch and Adam Terella

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Part II Anterior Cranial Fossa 14 Anterior  Fossa Pathology: Open Surgical Approaches���������������� 197 Ian F. Dunn, Xiaochun Zhao, Panayiotis E. Pelargos, and Ali H. Palejwala 15 Endoscopic Endonasal Approaches������������������������������������������������ 215 Michael B. Avery, Garni Barkhoudarian, Chester Griffiths, and Daniel F. Kelly 16 Anterior  Fossa: Eyebrow Keyhole Approach�������������������������������� 229 Sascha Marx and Henry W. S. Schroeder 17 Meningioma�������������������������������������������������������������������������������������� 237 Timothy H. Ung, Rafael Martinez-Perez, and A. Samy Youssef 18 Craniopharyngioma������������������������������������������������������������������������ 251 Michael Karsy and James J. Evans 19 Pituitary Adenoma �������������������������������������������������������������������������� 271 Ben A. Strickland and Gabriel Zada 20 Sinonasal Cancer������������������������������������������������������������������������������ 289 Conner J. Massey, Daniel M. Beswick, and Anne E. Getz Part III Orbit 21 Orbital Tumors�������������������������������������������������������������������������������� 303 Torstein R. Meling Part IV Middle Cranial Fossa: Cavernous Sinus 22 The  Cavernous Sinus: Surgical Approaches—Endoscopic and Open�������������������������������������������������������������������������������������������������� 331 Mohamed Labib and A. Samy Youssef 23 Cavernous Sinus Meningioma�������������������������������������������������������� 347 William T. Couldwell and Amol Raheja 24 Pituitary Adenoma �������������������������������������������������������������������������� 365 Ben G. McGahan, Giuliano Silveira-Bertazzo, Thaïs Cristina Rejane-Heim, Douglas A. Hardesty, Ricardo L. Carrau, and Daniel M. Prevedello 25 Schwannoma������������������������������������������������������������������������������������ 377 Shahed Elhamdani, Vijay A. Patel, and Paul A. Gardner 26 Chordomas  and Chondrosarcomas Involving the Cavernous Sinus������������������������������������������������������������������������ 391 Arianna Fava, Paolo di Russo, Thibault Passeri, Lorenzo Giammattei, Rosaria Abbritti, Fumihiro Matano, and Sébastien Froelich

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Part V Middle Cranial Fossa: Meckel’s Cave 27 Open  Surgical Approaches to Meckel’s Cave�������������������������������� 407 Akal Sethi and A. Samy Youssef 28 Endoscopic  Endonasal Approach to Meckel’s Cave �������������������� 415 Carl H. Snyderman and Paul A. Gardner 29 Trigeminal Schwannoma���������������������������������������������������������������� 421 Wei Huff, Benjamin K. Hendricks, and Aaron A. Cohen-Gadol Part VI Intracanalicular Vestibular Schwannoma 30 Middle  Fossa Approach for Hearing Preservation����������������������� 437 Nathan D. Cass and Samuel P. Gubbels Part VII Retrosellar Region 31 Expanded  Middle Fossa Approach: The Extradural Anterior Petrosectomy������������������������������������������������������������������������������������ 453 Lucas Troude, Guillaume Baucher, and Pierre-Hugues Roche 32 The Pretemporal Transcavernous Approach�������������������������������� 465 Vamsi P. Reddy, Arnau Benet, Mohamed Labib, and A. Samy Youssef 33 Endoscopic Endonasal Transcavernous Approach ���������������������� 475 Stephen T. Magill, Daniel M. Prevedello, and Ricardo L. Carrau Part VIII Posterior Fossa 34 Open  Surgical Approaches to the Posterior Fossa������������������������ 487 Angela M. Richardson, Burak Ozaydin, and Mustafa K. Baskaya 35 Endoscopic Endonasal Transpterygoid Approaches to the Posterior Fossa ������������������������������������������������ 501 Christina Jackson and Paul A. Gardner 36 Keyhole  Approaches to the Posterior Fossa���������������������������������� 513 Zach Folzenlogen, Alexander Yang, and A. Samy Youssef 37 Petroclival Meningiomas ���������������������������������������������������������������� 523 Steve S. Cho, Mohamed Labib, and A. Samy Youssef 38 Vestibular Schwannomas���������������������������������������������������������������� 551 Kunal Vakharia, Brian Neff, Matthew Carlson, Colin Driscoll, and Michael J. Link

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39 Cerebellopontine Angle Epidermoid Tumors�������������������������������� 569 Ehab El Refaee and Henry W. S. Schroeder 40 Chordomas  and Chondrosarcomas of the Posterior Fossa���������� 581 Thibault Passeri, Lorenzo Giammattei, Paolo di Russo, Stefan Lieber, Arianna Fava, Rosaria Abbritti, Anne Laure Bernat, and Sébastien Froelich 41 Pineal Tumors���������������������������������������������������������������������������������� 603 Rafael Martinez-Perez, Angela Downes, and A. Samy Youssef 42 Brainstem Cavernous Malformations�������������������������������������������� 621 Visish M. Srinivasan, Joshua S. Catapano, Vamsi P. Reddy, and Michael T. Lawton 43 Cranial  Nerve Hyperfunction Syndromes With and Without Vascular Compression and Tumor ������������������������������������������������ 635 Robert S. Heller, Siviero Agazzi, and Harry R. Van Loveren Part IX Jugular Foramen Region 44 Open  Surgical Approaches to the Jugular Foramen�������������������� 649 Angela M. Richardson, Burak Ozaydin, and Mustafa K. Baskaya 45 Endoscopic  Endonasal Approaches to the Jugular Foramen������ 659 Daniel Kreatsoulas, Takuma Hara, Ricardo L. Carrau, Douglas A. Hardesty, and Daniel M. Prevedello 46 Jugular Foramen Meningiomas������������������������������������������������������ 677 Kunal Vakharia and Jamie J. Van Gompel 47 Jugular Foramen Schwannomas���������������������������������������������������� 689 Kunal Vakharia, Luciano Cesar, Maria Peris-Celda, and Michael J. Link 48 Paraganglioma���������������������������������������������������������������������������������� 701 Stephen P. Cass and Olivia A. Kalmanson 49 Chondrosarcoma������������������������������������������������������������������������������ 717 Rafael Martinez-Perez and A. Samy Youssef Part X Infratemporal Fossa 50 Open  Approaches “Preauricular Transcranial Infratemporal Fossa Approaches for Radical Resection of Tumors in or Around Infratemporal Fossa”���������������������������������������������������������������������� 731 Yoichi Nonaka and Takanori Fukushima 51 Endoscopic Endonasal Approaches to the Infratemporal Fossa�������������������������������������������������������������� 747 Carl H. Snyderman and Paul A. Gardner

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52 Surgery of Paraganglioma�������������������������������������������������������������� 755 Yoichi Nonaka and Takanori Fukushima 53 Juvenile Nasopharyngeal Angiofibroma���������������������������������������� 771 Sarah A. Gitomer and Vijay R. Ramakrishnan 54 Schwannoma������������������������������������������������������������������������������������ 781 Rafael Martinez-Perez, Daniel M. Prevedello, and A. Samy Youssef Part XI Petrous Bone 55 Cholesterol  Granulomas and Endolymphatic Sac Tumors���������� 795 Rafael Martinez-Perez, Samuel P. Gubbels, and A. Samy Youssef Index���������������������������������������������������������������������������������������������������������� 807

Contributors

Rosaria  Abbritti Department of Neurosurgery, Lariboisière Hospital, Assistance Publique – Hôpitaux de Paris, University of Paris, Paris, France Siviero Agazzi  Department of Neurosurgery and Brain Repair, University of South Florida, Tampa, FL, USA Michael  B.  Avery Pacific Brain Tumor Center, Pacific Neuroscience Institute, Santa Monica, CA, USA Saint John’s Cancer Institute, Providence Saint John’s Health Center, Santa Monica, CA, USA Garni  Barkhoudarian  Pacific Brain Tumor Center, Pacific Neuroscience Institute, Santa Monica, CA, USA Saint John’s Cancer Institute, Providence Saint John’s Health Center, Santa Monica, CA, USA Mustafa  K.  Baskaya Department of Neurosurgery, University of Wisconsin – Madison, Madison, WI, USA Jaafar Basma  Department of Neurosurgery, University of Tennessee Health Science Center, Memphis, TN, USA Guillaume  Baucher Department of Neurosurgery, North University Hospital, APHM-AMU, Marseille, France Arnau  Benet Department of Neurological Surgery, Barrow Neurological Institute, Phoenix, AZ, USA Anne  Laure  Bernat Department of Neurosurgery, Lariboisière Hospital, Assistance Publique – Hôpitaux de Paris, University of Paris, Paris, France Daniel  M.  Beswick Department of Otolaryngology  – Head and Neck Surgery, University of California at Los Angeles, Los Angeles, CA, USA Matthew  Carlson Department of Otorhinolaryngology, Mayo Clinic, Rochester, MN, USA Ricardo L. Carrau  Department of Otolaryngology-Head and Neck Surgery, Center for Cranial Base Surgery, The Ohio State University Medical Center, Wexner Medical Center, Columbus, OH, USA

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Department of Otolaryngology-Head and Neck Surgery, Center for Cranial Base Surgery, The Ohio State University, Wexner Medical Center, Columbus, OH, USA Nathan D. Cass  Department of Otolaryngology—Head and Neck Surgery, University of Kentucky, Lexington, KY, USA Stephen  P.  Cass Department of Otolaryngology, University of Colorado School of Medicine, Aurora, CO, USA Joshua  S.  Catapano Department of Neurosurgery, Barrow Neurological Institute, St. Joseph’s Hospital and Medical Center, Phoenix, AZ, USA Luciano Cesar  Department of Neurosurgery, Mayo Clinic, Rochester, MN, USA Steve  S.  Cho Department of Neurological Surgery, Barrow Neurological Institute, Phoenix, AZ, USA Aaron  A.  Cohen-Gadol Department of Neurological Surgery, Indiana University School of Medicine, Indianapolis, IN, USA William T. Couldwell  Department of Neurosurgery, Clinical Neurosciences Center, University of Utah, Salt Lake City, UT, USA Paolo  di Russo Department of Neurosurgery, Lariboisière Hospital, Assistance Publique – Hôpitaux de Paris, University of Paris, Paris, France Angela  Downes Department of Neurological Surgery, University of Colorado, Aurora, CO, USA Colin Driscoll  Department of Otorhinolaryngology, Mayo Clinic, Rochester, MN, USA Ian F. Dunn  Department of Neurosurgery, University of Oklahoma Health Sciences Center, Oklahoma City, OK, USA Turki Elarjani  University of Miami, Department of Neurological Surgery, Miami, FL, USA Shahed  Elhamdani Department of Neurosurgery, Allegheny Health Network, Pittsburgh, PA, USA Ehab  El Refaee Department of Neurosurgery, Cairo University, Cairo, Egypt Department of Neurosurgery, University Medicine Greifswald, Greifswald, Germany James J. Evans  Department of Neurosurgery, Thomas Jefferson University Hospital, Philadelphia, PA, USA Arianna  Fava  Laboratory of Experimental and Skull Base Neurosurgery, Department of Neurosurgery, Lariboisière Hospital, University of Paris, Paris, France

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Zach  Folzenlogen Department of Neurological Surgery, University of Colorado, Aurora, CO, USA Sébastien  Froelich Department of Neurosurgery, Lariboisière Hospital, Assistance Publique – Hôpitaux de Paris, University of Paris, Paris, France Takanori Fukushima  Division of Neurosurgery, Duke University Medical Center, Durham, NC, USA Paul A. Gardner  Departments of Neurological Surgery and Otolaryngology, University of Pittsburgh School of Medicine, Pittsburgh, PA, USA Anne  E.  Getz  Department of Otolaryngology  – Head and Neck Surgery, University of Colorado School of Medicine, Aurora, CO, USA Lorenzo  Giammattei  Department of Neurosurgery, Lariboisière Hospital, Assistance Publique – Hôpitaux de Paris, University of Paris, Paris, France Sarah  A.  Gitomer Department of Otolaryngology—Head and Neck Surgery, University of Colorado School of Medicine, Aurora, CO, USA Chester  Griffiths Saint John’s Cancer Institute, Providence Saint John’s Health Center, Santa Monica, CA, USA Department of Head and Neck Surgery & Family Practice, Geffen School of Medicine UCLA, Los Angeles, CA, USA Pacific Neuroscience Institute, Santa Monica, CA, USA Samuel P. Gubbels  Department of Otolaryngology, University of Colorado School of Medicine, Aurora, CO, USA Takuma Hara  Departments of Neurological Surgery and Otolaryngology-­ Head and Neck Surgery, The Ohio State University Wexner Medical Center, Columbus, OH, USA Douglas  A.  Hardesty  Departments of Neurological Surgery and Otolaryngology-Head and Neck Surgery, The Ohio State University Medical Center, Wexner Medical Center, Columbus, OH, USA Robert S. Heller  Department of Neurosurgery and Brain Repair, University of South Florida, Tampa, FL, USA Benjamin K. Hendricks  Department of Neurosurgery, Barrow Neurological Institute, Phoenix, AZ, USA Scott  Hirsch Department of Otolaryngology  – Head and Neck Surgery, University of Colorado, Anschutz Medical Campus, Aurora, CO, USA Wei Huff  Department of Neurological Surgery, Indiana University School of Medicine, Indianapolis, IN, USA Christina Jackson  Hospital of the University of Pennsylvania, Philadelphia, PA, USA Olivia  A.  Kalmanson Department of Otolaryngology, University of Colorado School of Medicine, Aurora, CO, USA

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Michael Karsy  Department of Neurosurgery, Thomas Jefferson University Hospital, Philadelphia, PA, USA Daniel F. Kelly  Pacific Brain Tumor Center, Pacific Neuroscience Institute, Santa Monica, CA, USA Saint John’s Cancer Institute, Providence Saint John’s Health Center, Santa Monica, CA, USA Nickalus Khan  University of Miami, Department of Neurological Surgery, Miami, FL, USA Daniel  Kreatsoulas  Department of Neurological Surgery, The Ohio State University Wexner Medical Center, Columbus, OH, USA Mohamed Labib  Department of Neurological Surgery, Barrow Neurological Institute, Phoenix, AZ, USA Department of Neurosurgery, University of Colorado School of Medicine, Aurora, CO, USA Department of Otolaryngology, University of Colorado School of Medicine, Aurora, CO, USA Michael  T.  Lawton Department of Neurosurgery, Barrow Neurological Institute, St. Joseph’s Hospital and Medical Center, Phoenix, AZ, USA Stefan  Lieber Department of Neurosurgery, Lariboisière Hospital, Assistance Publique – Hôpitaux de Paris, University of Paris, Paris, France Michael  J.  Link Department of Neurosurgery, Mayo Clinic, Rochester, MN, USA Stephen T. Magill  Department of Neurological Surgery, Center for Cranial Base Surgery, The Ohio State University, Wexner Medical Center, Columbus, OH, USA Rafael Martinez-Perez  Department of Neurological Surgery, University of Colorado School of Medicine, Aurora, CO, USA Sascha Marx  Department of Neurosurgery, University Medicine Greifswald, Greifswald, Germany Department of Cancer Immunology and Virology, Dana-Farber Cancer Institute, Boston, MA, USA Harvard Medical School, Boston, MA, USA Conner J. Massey  Department of Otolaryngology – Head and Neck Surgery, University of Colorado School of Medicine, Aurora, CO, USA Fumihiro  Matano Department of Neurosurgery, Lariboisière Hospital, Assistance Publique – Hôpitaux de Paris, University of Paris, Paris, France Tiit  Mathiesen Department of Neurosurgery, University Hospital of Copenhagen, Copenhagen, Denmark Department of Clinical Neuroscience, Karolinska Institutet, Stockholm, Sweden

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Department of Clinical Medicine, University of Copenhagen, Copenhagen, Denmark Ben  G.  McGahan Department of Neurological Surgery, The Ohio State University Medical Center, Columbus, OH, USA Torstein  R.  Meling Department of Neurosurgery, Geneva University Hospitals, Geneva, Switzerland Faculty of Medicine, University of Geneva, Geneva, Switzerland Department of Neurosurgery, Carlo Besta Neurological Institute, Milano, Italy Jacques  J.  Morcos University of Miami, Department of Neurological Surgery, Miami, FL, USA Department of Neurological Surgery, Cranial Neurosurgery, Jackson Memorial Hospital, University of Miami, Miami, FL, USA Brian  Neff Department of Otorhinolaryngology, Mayo Clinic, Rochester, MN, USA Yoichi  Nonaka Department of Neurosurgery, Tokai University School of Medicine, Isehara, Kanagawa, Japan Burak  Ozaydin Department of Neurosurgery, University of Wisconsin  – Madison, Madison, WI, USA Ali  H.  Palejwala Department of Neurosurgery, University of Oklahoma Health Sciences Center, Oklahoma City, OK, USA Kara Parikh  Department of Neurosurgery, University of Tennessee Health Science Center, Memphis, TN, USA Thibault  Passeri Department of Neurosurgery, Lariboisière Hospital, Assistance Publique – Hôpitaux de Paris, University of Paris, Paris, France Vijay  A.  Patel Department of Otolaryngology, University of Pittsburgh School of Medicine, Pittsburgh, PA, USA Panayiotis  E.  Pelargos Department of Neurosurgery, University of Oklahoma Health Sciences Center, Oklahoma City, OK, USA Maria Peris-Celda  Department of Neurosurgery, Mayo Clinic, Rochester, MN, USA Daniel  M.  Prevedello Department of Neurological Surgery, Center for Cranial Base Surgery, The Ohio State University Medical Center, Wexner Medical Center, Columbus, OH, USA Amol  Raheja  Department of Neurosurgery, All India Institute of Medical Sciences, New Delhi, India Vijay  R.  Ramakrishnan Indiana University School of Medicine, Department of Otolaryngology-Head and Neck Surgery, Indianapolis, IN, USA

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Vamsi P. Reddy  Department of Neurological Surgery, Medical College of Georgia, Augusta, GA, USA Thaïs  Cristina  Rejane-Heim Department of Neurological Surgery, The Ohio State University Medical Center, Columbus, OH, USA Department of Pediatric Endocrinology and Neurosurgery, Jeser Amarante Faria Children’s Hospital, and Neurological and Neurosurgical Clinic of Joinville, Joinville, SC, Brazil Department of Pediatric Endocrinology, Nationwide Children’s Hospital, Columbus, OH, USA Department of Pediatric Endocrinology, Federal University of Santa Catarina, Florianopolis, SC, Brazil Angela  M.  Richardson Department of Neurosurgery, University of Wisconsin – Madison, Madison, WI, USA Pierre-Hugues  Roche Department of Neurosurgery, North University Hospital, APHM-AMU, Marseille, France Henry W. S. Schroeder  Department of Neurosurgery, University Medicine Greifswald, Greifswald, Germany Akal  Sethi Department of Neurological Surgery, University of Colorado, Aurora, CO, USA Giuliano Silveira-Bertazzo  Department of Neurological Surgery, The Ohio State University Medical Center, Columbus, OH, USA Department of Pediatric Endocrinology and Neurosurgery, Jeser Amarante Faria Children’s Hospital, and Neurological and Neurosurgical Clinic of Joinville, Joinville, SC, Brazil Carl  H.  Snyderman Departments of Otolaryngology and Neurological Surgery, University of Pittsburgh School of Medicine, Pittsburgh, PA, USA Jeffrey M. Sorenson  Department of Neurosurgery, University of Tennessee Health Science Center, Memphis, TN, USA Semmes Murphey Clinic, Memphis, TN, USA Visish  M.  Srinivasan  Department of Neurosurgery, Barrow Neurological Institute, St. Joseph’s Hospital and Medical Center, Phoenix, AZ, USA Ben  A.  Strickland Department of Neurosurgery, University of Southern California, Los Angeles, CA, USA Adam  Terella  Department of Otolaryngology  – Head and Neck Surgery, University of Colorado, Anschutz Medical Campus, Aurora, CO, USA Lucas  Troude Department of Neurosurgery, North University Hospital, APHM-AMU, Marseille, France

Contributors

Contributors

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Timothy  H.  Ung Department of Neurological Surgery, University of Colorado School of Medicine, Aurora, CO, USA Kunal  Vakharia Department of Neurosurgery, Mayo Clinic, Rochester, MN, USA Jamie J. Van Gompel  Department of Neurosurgery, Mayo Clinic, Rochester, MN, USA Harry  R.  Van Loveren Department of Neurosurgery and Brain Repair, University of South Florida Health Morsani College of Medicine, Tampa, FL, USA Maria-Isabel Vargas  Faculty of Medicine, University of Geneva, Geneva, Switzerland Department of Neuroradiology, Geneva University Hospitals, Geneva, Switzerland Alexander  Yang Department of Neurological Surgery, University of Colorado, Aurora, CO, USA A.  Samy  Youssef Department of Neurological Surgery, University of Colorado School of Medicine, Aurora, CO, USA Department of Otolaryngology, University of Colorado School of Medicine, Aurora, CO, USA Gabriel  Zada Department of Neurosurgery, University of Southern California, Los Angeles, CA, USA Xiaochun  Zhao Department of Neurosurgery, University of Oklahoma Health Sciences Center, Oklahoma City, OK, USA

Part I General Principles

1

Evolution of the Multidisciplinary Skull Base Team Rafael Martinez-Perez and A. Samy Youssef

History The evolution of collaboration between neurosurgeons and otolaryngologists dates back to the second half of the twentieth century. The development of the middle fossa approach by William House marked the birth of the first skull base team as he teamed up with a neurosurgeon, John Doyle [1]. The House/Doyle team inspired generations to come, to follow their steps and formalize otolaryngology/neurosurgery skull base teams. Schramm [2] and Sekhar [3] set another example of successful collaboration between neurosurgery and head and neck surgery. Together, they pioneered combined approaches to the infratemporal fossa, cavernous sinus, petrous bone/carotid, and upper neck. Near the turn of the century, the

R. Martinez-Perez Department of Neurological Surgery, University of Colorado School of Medicine, Aurora, CO, USA e-mail: [email protected] A. S. Youssef (*) Department of Neurological Surgery, University of Colorado School of Medicine, Aurora, CO, USA Department of Otolaryngology, University of Colorado School of Medicine, Aurora, CO, USA e-mail: [email protected]

development and rapid advances in endonasal skull base endoscopy led to the development of another team model: rhinology/neurosurgery. After a limited debut in France [4], Jho and Carrau brought purely endoscopic pituitary surgery to the United States [5, 6]. Since then, purely endoscopic skull base surgery has evolved to become an indispensable approach not only to pituitary or central skull base lesions, but also to lateral skull base complex lesions. With the continuous advances in skull base surgery, there has been a paradigm shift from radiological outcomes to functional outcomes assessment, as a measure of success of the different treatment modalities [7]. Gross total resection at the expense of functional outcomes was no longer the optimum treatment goal. As imaging technology significantly improved, delivery of safer radiation became feasible with more precision in a three-dimensional and conformal fashion. As modern radiotherapy has proved its efficacy in treatment of skull base tumors, radiation oncologists became a crucial component of the skull base team.

Establishing the Multidisciplinary Skull Base Team A starting skull base surgeon must establish an interdisciplinary program, where the expertise of multiple specialists will add to the decision-­

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 A. S. Youssef (ed.), Contemporary Skull Base Surgery, https://doi.org/10.1007/978-3-030-99321-4_1

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making process directed toward the best care of challenging pathologies. Organizations thriving to excellence in patient care should direct enough resources toward the development of interdisciplinary programs. While this model is common in academic and tertiary care centers, gathering such team of experts may not be feasible in community centers. Residency programs are c­ urrently devoted to engaging in shared learning and dialogue practices. In this regard, multidisciplinary teams should serve as educational resources not only by providing the most up to date state-of-art management, but also by endorsing a culture of dialogue and scientific discussion. Graduating skull base surgeons should adhere to this model in their job search/negotiations. Despite the establishment of skull base surgery, there have not been standard treatment protocols for skull base lesions. Evidence-based practice is almost missing, with evidence only limited to institutional series and expert opinions. Periodic service rounds provide an opportunity for exchange of expertise in the search for optimum management strategies. Open surgery is no longer the only surgical option and discussion among the different surgical disciplines will conclude the optimum surgical approach. However, when nonsurgical options are to be considered, the input of nonsurgical disciplines is paramount to the discussion. With the technological revolution in neurodiagnostics, the neuroradiologist became an invaluable member of the skull base team. The skull base nurse practitioner is the liaison between different clinical disciplines and the patients. The clinical schedulers act as service coordinators to help deliver patient care with minimal burden of multiple outpatient clinic appointments. Leadership is key to orchestrate the different roles of the individual team members. Lack of leadership has been associated with lower levels of team effectiveness, poor satisfaction among members, and burnout [8]. The directorship role for skull base programs can be shared between neurosurgery and otolaryngology. In our institution, the members of the team have joint appointments in the different departments, which strengthens collaboration and further unifies the

R. Martinez-Perez and A. S. Youssef

team. Hospital administrations should encourage multidisciplinary clinical services and physicians’ participation in teamwork seminars that have been shown to foster interprofessional teamworking [9]. Finally, the success of the multidisciplinary teamwork is measured not only by patients’ satisfaction or clinical outcomes, but also by the solid commitment of team members from different specialties.

In the Operating Room The collaboration between neurosurgeons and otolaryngologists evolved in the operating room particularly in lateral skull base procedures, and rapidly expanded to include other areas. Alternating surgeons’ roles during long procedures reduces fatigue and ensures efficiency in all different phases of the procedure. Delicate drilling of the temporal bone demands the experience and thorough knowledge of the anatomy of middle and inner ear, such as that provided by the neurotologist. The recent advent and rapid development of skull base endoscopic techniques opened another frontier for collaboration between neurosurgeons and sinus/head and neck surgeons. The surgical corridor through nasal and paranasal structures is successfully created by the rhinologist, and lesion resection is performed in a synchronized four-­ handed fashion by the neurosurgeon and the rhinologist who navigates the endoscope and optimizes surgical visualization [10]. Shortly after the endoscopic success in central skull base, endoscopic approaches to the lateral skull base were developed and refined, thanks to the collaboration between the two teams. Such collaboration has also been fruitful in improving skull base reconstruction and minimizing cerebrospinal fluid (CSF) leak rates. Jugular foramen pathologies require the collaboration of multiple teams. The otologist performs the transtemporal approaches, the head and neck surgeon performs meticulous neck dissection, and the neurosurgeon joins efforts to ligate the sigmoid sinus when needed, and resect the

1  Evolution of the Multidisciplinary Skull Base Team

lesion. Occasionally, vascular surgeons may be involved in paraganglioma surgery such as carotid body tumors. When removal of large skull base tumors requires cerebral revascularization, vascular surgeons become involved in harvesting the donor graft (radial artery or saphenous vein). Preoperative embolization of vascular skull base lesions requires involvement of interventional neuroradiology or vascular neurosurgery. Transorbital approaches have demonstrated several applications for the treatment of lesions located in the anterior and middle cranial fossae, which led to the collaboration of ophthalmologists and skull base surgeons. When skull base procedures require flap reconstruction, plastic surgeons become involved. There is a learning curve for group performance as well, and unlike other areas of neurosurgery, recent evidence suggests that the learning curve in skull base surgery is not only steep, but also never truly plateaus [11, 12]. Team stability and longstanding collaboration enhance the overall group performance. Subspecialization warrants longer training and exposure to a large number of cases in order to develop the necessary experience and skills to deliver optimal care. In addition, the ability to function within a team that puts patient care ahead of providers’ hierarchy or work politics, adds higher standards for the already steep learning curve.

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only achieve cost reduction, but also ensures the delivery of multidisciplinary care with relatively minimal burden on patients. A previous study demonstrated that, on average, this strategy helps to save 325 miles and 8 hours of travel for each patient [13]. Moreover, evaluation of patients in multidisciplinary clinics has been associated with an increase not only in physicians’ commitment and communication, but also in patients’ adherence to treatment [14, 15]. The current model of collaborative management is not hierarchically limited to physician providers. Although traditionally, there has been a tendency for the medical profession to assume leadership within multidisciplinary teams, this modern approach promotes the nurse navigator’s role in coordinating and communicating the decision-­making chain [16]. This is highly valued both by patients and all other team providers [9]. Other ancillary healthcare providers including rehab medicine, physiotherapists, occupational therapists, speech pathologists, and social workers are crucial to comprehensive postoperative patient care and return to a normal life [14].

 egular Multidisciplinary Skull Base R Meetings

The benefits of periodic team meetings have been reported, as they can potentially aid in breaking down professional barriers, improve interprofesMultidisciplinary Clinics sional communication, and avoid individual members’ bias [9]. A previous study showed that The involvement of multiple specialists in estab- recommendations made by a group of experts are lishing a plan of care requires that the patient more likely to be implemented [17]. Tumor attends different outpatient clinics. Multiple out- boards have also demonstrated to be effective in patient appointments may be weeks or even improving survival rates in head and neck tumors months apart, which may lead to an increase in [18]. Similarly, neglect of multidisciplinary manwaiting time, cost, loss to follow-up, and delay in agement can negatively impact patient outcomes delivering treatment. Complex skull base cases through errors in staging, diagnosis, or loss to are often referred to tertiary care centers with follow-up [18, 19]. Over the years, radiosurgery central geographic locations, which adds extra has emerged as an effective treatment for benign travel burden on patients. Recently, several cen- pathologies such as complex meningiomas and ters, including ours, have implemented a “one-­ vestibular schwannomas besides its traditional stop” service, in which all the consultations, adjuvant role for malignancy. Therefore, radiaincluding imaging and laboratory studies, are tion oncologists became an integral part of skull conducted the same day. Such strategy does not base practice and round table discussions.

R. Martinez-Perez and A. S. Youssef

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Moreover, with the recent advances in tumor karyotyping and the possible impact on long-­ term prognosis and treatment outcomes, the neuropathologist’s input became more and more important to making ultimate treatment choices by the group. Chemo-adjuvant therapy is often needed in skull base tumors, thus making necessary the inclusion of medical oncology. ­ Given the high incidence of primary benign tumors in and around the pituitary gland, comprehensive treatment of sellar/parasellar lesions requires coordinated assessment, treatment, and follow-up by an endocrinologist.

ensures the delivery of the highest level of care to all patients with complex skull base lesions.

Future Perspectives

References

The everchanging technology will continue to improve precision in complex brain surgery. Neuronavigation, modular approaches, intraoperative imaging systems, and robotic surgery are becoming the standard of care [20]. Such technology adds complex methodology to the daily practice and will require the integration of information technologists in the multidisciplinary team [21]. Therefore, biomedical engineers, and specialists in neuronavigation and neurophysiologists will likely become a future addition to the multidisciplinary care. Recent advances in translational medicine and new molecular targeted therapies will soon revolutionize the treatment of several pathological entities including benign skull base tumors [22– 24]. Basic science researchers should be a great addition to future multidisciplinary team discussions.

1. House WF.  Surgical exposure of the internal auditory canal and its contents through the middle cranial fossa. Laryngoscope. 1961;71(11):1363–85. 2. Schramm V. Infratemporal fossa surgery. In: Schramm VL, Sekhar LN, editors. Tumors of the Cranial Base. New York: Futura Publishing; 1987. p. 421–37. 3. Sekhar L.  Operative management of tumors involving the cavernous sinus. In: Schramm VL, Sekhar LN, editors. Tumors of the Cranial Base. New York: Futura Publishing; 1987. p. 393–419. 4. Jankowski R, Auque J, Simon C, Marchai JC, Hepner H, Wayoff M.  Endoscopic pituitary tumor surgery. Laryngoscope. 1992;102(2):198–202. 5. Jho HD, Carrau RL, Ko Y, Daly MA.  Endoscopic pituitary surgery: an early experience. Surg Neurol. 1997;47(3):213–22. discussion 222-223 6. Carrau RL, Jho HD, Ko Y. Transnasal-transsphenoidal endoscopic surgery of the pituitary gland. Laryngoscope. 1996;106(7):914–8. 7. Martínez-Pérez R, Silveira-Bertazzo G, Rangel GG, Albiña P, Hardesty D, Carrau RL, et al. The historical perspective in approaches to the spheno-petro-clival meningiomas. Neurosurg Rev. 2019;44:51. 8. Borrill C, West M, Shapiro D, Rees A. Team working and effectiveness in health care. Br J Healthc Manag. 2000;6(8):364–71. 9. Xyrichis A, Lowton K.  What fosters or prevents interprofessional teamworking in primary and community care? A literature review. Int J Nurs Stud. 2008;45(1):140–53. 10. Cappabianca P, Alfieri A, de Divitiis E.  Endoscopic endonasal transsphenoidal approach to the sella: towards functional endoscopic pituitary surgery (FEPS). Minim Invasive Neurosurg. 1998;41(2):66–73. 11. Moffat DA, Hardy DG, Grey PL, Baguley DM. The operative learning curve and its effect on facial nerve outcome in vestibular schwannoma surgery. Am J Otol. 1996;17(4):643–7.

Conclusion A successful skull base practice requires a multidisciplinary team of experts who share the same vision and commitment to provide a comprehensive care to challenging pathologies. Optimum team performance requires good coordination at all levels in outpatient consultation, preoperative planning, multidisciplinary conferences, operating room, and postoperative care. Such setup

Disclosure  Funding: This study did not receive any funding relative to its elaboration. Conflict of interest: ASY is a consultant for Stryker Corp and has received royalty from Mizuho America. Ethical approval and informed consent (to participate and for publication): Informed consent and ethical approval were not deemed necessary by the local ethics in view of the design of the study. This study did not receive financial support. Availability of data and material (data transparency): This manuscript has not been previously published in whole or in part or submitted elsewhere for review.

1  Evolution of the Multidisciplinary Skull Base Team 12. Buchman CA, Chen DA, Flannagan P, Wilberger JE, Maroon JC.  The learning curve for acoustic tumor surgery. Laryngoscope. 1996;106(11):1406–11. 13. Sadiq SA, Usmani HA, Saeed SR. Effectiveness of a multidisciplinary facial function clinic. Eye (Lond). 2011;25(10):1360–4. 14. McLaughlin N, Carrau RL, Kelly DF, Prevedello DM, Kassam AB. Teamwork in skull base surgery: an ­avenue for improvement in patient care. Surg Neurol Int. 2013;4:36. 15. Stephens MR, Lewis WG, Brewster AE, Lord I, Blackshaw GRJC, Hodzovic I, et al. Multidisciplinary team management is associated with improved outcomes after surgery for esophageal cancer. Dis Esophagus. 2006;19(3):164–71. 16. Coombs M.  Power and conflict in intensive care clinical decision making. Intensive Crit Care Nurs. 2003;19(3):125–35. 17. Lutterbach J, Pagenstecher A, Spreer J, Hetzel A, van Velthoven V, Nikkhah G, et al. The brain tumor board: lessons to be learned from an interdisciplinary conference. Onkologie. 2005;28(1):22–6. 18. Friedland PL, Bozic B, Dewar J, Kuan R, Meyer C, Phillips M. Impact of multidisciplinary team manage-

7 ment in head and neck cancer patients. Br J Cancer. 2011;104(8):1246–8. 19. Wheless SA, McKinney KA, Zanation AM.  A prospective study of the clinical impact of a multidisciplinary head and neck tumor board. Otolaryngol Head Neck Surg. 2010;143(5):650–4. 20. Castelnuovo P, Dallan I, Battaglia P, Bignami M.  Endoscopic endonasal skull base surgery: past, present and future. Eur Arch Otorhinolaryngol. 2010;267(5):649–63. 21. Olofsson J.  Multidisciplinary team a prerequisite in endoscopic endonasal skull base surgery. Eur Arch Otorhinolaryngol. 2010;267(5):647. 22. Al-Rashed M, Foshay K, Abedalthagafi M.  Recent advances in meningioma immunogenetics. Front Oncol. 2020;8(9):1472. 23. Jensterle M, Jazbinsek S, Bosnjak R, Popovic M, Zaletel LZ, Vesnaver TV, et al. Advances in the management of craniopharyngioma in children and adults. Radiol Oncol. 2019;25;53(4):388–96. 24. Yaniv D, Soudry E, Strenov Y, Cohen MA, Mizrachi A. Skull base chordomas review of current treatment paradigms. World J Otorhinolaryngol Head Neck Surg. 2020;6(2):125–31.

2

Surgical Anatomy of the Cranial Nerves Jaafar Basma, Kara Parikh, and Jeffrey M. Sorenson

Introduction A nuanced understanding of the complex anatomy of cranial nerves is crucial for the planning and execution of skull base approaches. Surgical corridors to deep targets are often hindered by one or more cranial nerves, which must be identified and possibly dissected or mobilized to address the pathology. Therefore, the surgeon must understand the limitations imposed on each approach by the cranial nerves in order to select the optimal approach for a particular lesion. Although cranial nerves can often be visualized with high-resolution imaging, it is still very helpful to know common patterns of displacement from lesions arising in various locations. Moreover, the tendency of certain lesions to adhere to, envelope, or invade cranial nerves may force the surgeon to decide between complete resection and preservation of cranial nerve function. Since cranial nerve function is the leading determinate of clinical outcome in many cases, functional preservation is increasingly a key conJ. Basma (*) · K. Parikh Department of Neurosurgery, University of Tennessee Health Science Center, Memphis, TN, USA e-mail: [email protected]; [email protected] J. M. Sorenson Department of Neurosurgery, University of Tennessee Health Science Center, Memphis, TN, USA Semmes Murphey Clinic, Memphis, TN, USA e-mail: [email protected]

cept in modern skull base surgery. In some cases, concern for cranial nerve function may lead to a decision to not to operate at all, but to instead pursue radiosurgery or observation. In this chapter, we review the course of each cranial nerve from the brain through the neural foramina of the skull base, along with their relationships to common skull base lesions and approaches. Images from the Rhoton Collection are used to illustrate these concepts.

History Likely inspired by the Alexandrian physician Marinos, Galen (129–210 AD) named the cranial nerves using an ordinal system, following the order in which these nerves exit their skull base foramina. In his scheme, the cranial nerves were limited to seven: (1) optic, (2) oculomotor (he did not distinguish between the oculomotor, trochlear, and abducens nerves), (3) sensory facial (V), (4) motor trigeminal (motor V), (5) face and hearing (combining facial and cochleovestibular nerves), (6) pharyngeal (combining glossopharyngeal, vagus, and accessory nerves), and (7) tongue (hypoglossal nerve). It is believed that Alessandro Benedetti (1445–1525 AD) was the first to recognize the olfactory nerve as such, and Niccolo Massa labeled it as the “first” nerve in 1536 AD. Alessandro Achillini (1463–1512 AD) identified the trochlear nerve, but it was not well

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depicted until the dissections performed by Vesalius and Fallopio. Several classification systems followed, including that of Thomas Willis in 1664, which included nine nerves (combining VII and VIII and IX–XI). Willis was the first to isolate the accessory nerve; however, he included it in his eighth group associated with the vagus, hence its given name “accessory.” Samuel Sömmerring of Germany was the first to classify the cranial nerves into 12 pairs in 1778, based on their cranio-caudal neural origin, and his classification was adopted by the Nomina Anatomica in 1895 [1–3]. Early neurosurgeons often faced great difficulty visualizing and preserving cranial nerves due to the lack of adequate illumination and magnification, and also because neurosurgical lesions were typically diagnosed at a more advanced stage at that time. In 1957, Theodore Kurze, inspired by the work of the neuro-otologist William House, became the first neurosurgeon to use the operating microscope. He removed a facial nerve schwannoma in a child and subsequently performed a seventh to twelfth nerve anastomosis [4]. Nonetheless, widespread adoption was slow as many surgeons remained skeptical. As a neurosurgical resident during this era, Al Rhoton Jr. later recalled that he never saw a patient maintain facial nerve function after acoustic neuroma surgery during his training [5]. This inspired him to embark on a decades long effort to help neurosurgeons create better outcomes through a more nuanced understanding of the detailed anatomy that could finally be seen clearly through the operating microscope. Through the pioneering efforts of Yaşargil [6], Rhoton, and others in the 1960s, neurosurgeons began to learn the microsurgical techniques and anatomy that would enable them to have more success preserving cranial nerves. Dr. Rhoton grouped the posterior fossa cranial nerves into three neurovascular complexes associated with the three main cerebellar arteries [7]. Branches of these arteries have intimate but variable relationships with their associated cranial nerves and it is helpful to understand the variations that may be seen intraoperatively (Fig.  2.1a, b). The upper complex is at the level of the midbrain and upper pons, and is comprised of the superior cerebellar artery (SCA) together with the oculomotor,

trochlear, and trigeminal nerves. The middle complex consists of the anterior inferior cerebellar artery (AICA), which supplies the portion of the cerebellum facing the petrous temporal bone, with the abducens, facial, and vestibulocochlear nerves, which all arise from the junction of the pons and medulla. Finally, the posterior inferior cerebellar artery (PICA) defines the lower complex at the level of the medulla, along with the glossopharyngeal, vagus, spinal accessory, and hypoglossal nerves.

Cranial Nerve Segments It is useful to learn the anatomy of cranial nerves by dividing them into distinct segments that can be identified intraoperatively and with high-­ resolution imaging. These include the brain, cisternal, venous, foraminal, and extraforaminal soft tissue segments. Modern magnetic resonance imaging (MRI) sequences such as constructive interference in steady state (CISS) provide excellent contrast between cerebrospinal fluid ([CSF] bright) and the cisternal segments of cranial nerves (dark) [8]. The cisternal segment often passes into a dural cave, such as the internal auditory canal (IAC), oculomotor cistern, or Meckel’s cave. Nerves passing through venous structures such as the cavernous sinus can be more easily identified on imaging by adding intravenous gadolinium contrast, which brightens the veins, thus making the dark cranial nerves easier to trace. The initial extraforaminal segment of cranial nerves is often visible because surrounding fat, such as in the orbit, is bright on a CISS sequence.

Olfactory Nerve (I) The olfactory nerves are often confused with the olfactory tract. These nerves are actually very short, passing from the olfactory bulb through the tiny foramina of the cribriform plate of the ethmoid bone, and then into the olfactory epithelium (Fig.  2.2b). As the olfactory bulb is part of the central nervous system, the “cisternal segment” is technically not a cranial nerve, but instead a tract (Fig. 2.2a–c). Nonetheless, for the purposes

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a

b

Fig. 2.1  Neurovascular relationships of the posterior fossa. (a) Anterior view of the brainstem, cerebellar arteries, and cranial nerves: Dr. Rhoton organized the posterior fossa cranial nerves into three groups corresponding to the cerebellar arteries. The superior cerebellar artery is associated with the oculomotor (III), trochlear (IV), and trigeminal (V) nerves (upper group); the anterior inferior cerebellar artery with the abducens (VI), facial (VII), and vestibulocochlear (VIII) nerves (middle group); and the posterior inferior cerebellar artery with the glossopharyngeal (IX), vagus (X), spinal accessory (XI), and hypoglossal (XII) nerves (lower group). (b) Closer view of the

anterior cerebellopontine angle: A loop of the superior cerebellar artery is projecting inferiorly toward the root entry zone of the trigeminal nerve (V). A loop of the anterior inferior cerebellar artery follows the facial (VII) and vestibulocochlear (VIII) nerves to the internal auditory canal. The posterior inferior cerebellar artery (PICA) is seen coursing posterior to the hypoglossal nerve rootlets (XII), and anterior to the vagus (X) and spinal accessory (XI) nerves. The relationship between PICA and the lower cranial nerves is variable. (Courtesy of the Rhoton Collection)

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a

b

c

d

e

f

Fig. 2.2  Olfactory and optic nerves. (a) Inferior view of the brain: The olfactory tract enters the olfactory trigone anterior to the anterior perforated substance, sending fibers into the medial and lateral striae. The fibers of the optic tracts synapse in the lateral geniculate nucleus (LGN), which sends optic radiations into the roof of the temporal horn of the lateral ventricle. (b) Transnasal endoscopic view: The cribriform plate and frontal bone over the ethmoid sinus have been removed to expose the olfactory bulb and tract. The olfactory bulb sends short olfactory nerves to the olfactory epithelium. (c) Anterior subfrontal view: The olfactory bulb rests in the olfactory groove of the ethmoid bone, above the cribriform plate. The olfactory tract crosses above the optic nerve and carotid bifurcation. The optic chiasm lies below the A1 segment of the anterior cerebral artery and the anterior communicating artery (ACOMM), and above the internal carotid artery. The translucent lamina terminalis leads to the anterior third

ventricle. (d) Endoscopic view: The optic chiasm is directly superior to the pituitary gland and supplied by branches of the superior hypophyseal artery. The interhemispheric fissure and ACOMM artery are seen above the nerve. This perspective is advantageous for lesions below the chiasm, such as craniopharyngiomas. (e) Medial view: The relationship between the sellar region and third ventricle is seen. The posterior portion of the optic chiasm projects into the third ventricle, while the anterior portion is below the ACOMM artery. (f) Anterior view, coronal section: The orbital apex receives the optic nerve and ophthalmic artery from the optic canal medially, and the cavernous sinus nerve veins from the medial portion of the superior orbital fissure (SOF). The ophthalmic artery and nasociliary nerve travel lateral to the optic nerve at the apex before crossing it superiorly. The lacrimal, frontal, and trochlear nerves are seen outside the annulus of Zinn and its muscular cone. (Courtesy of the Rhoton Collection)

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covers an area of 2–5  cm2. The axons of the olfactory cells enter the cranial cavity through the cribriform plate and they synapse with mitral cells in the olfactory bulb, which then Neural Segment  The paleocortex is considered project through the olfactory tract [11, 12]. The the “oldest” part of the human cortex, and in olfactory epithelium is lost with transcribriform primitive mammals it constitutes the major part endoscopic approaches, though unilateral olfacof the brain. In humans, it is limited to the olfac- tory preserving cases have been demonstrated. tory cortex (cortex pre-piriformis and peri-­ Anosmia has also been reported after transnaamygdaloid), olfactory tract, and olfactory bulb. sal, transsphenoidal approaches for pituitary Unlike other cranial nerves, olfaction involves tumors. only the cerebrum [9, 10]. of surgery, the olfactory tract can be considered to be a cranial nerve since all of the same intraoperative considerations apply.

Cisternal Segment  After originating at the olfactory bulb, the olfactory tract travels superior to the optic nerve, and just beneath the olfactory sulcus, which divides the basal frontal lobe into the gyrus rectus medially, and the orbitofrontal gyri laterally (Fig.  2.2a). Some olfactory fibers synapse with dispersed cells along the olfactory tract, called the anterior olfactory nucleus. At the olfactory trigone, the olfactory tract splits into a lateral olfactory stria and a medial olfactory stria (occasionally, there may also be an accessory olfactory stria). The lateral and medial striae engulf the anterior perforated substance (APS), which receives perforator arteries from the anterior circle of Willis. While most of the olfactory fibers follow the lateral olfactory stria to the primary olfactory cortex, some fibers decussate via the medial olfactory stria into the anterior commissure to the contralateral olfactory bulb. Other olfactory fibers in the medial olfactory stria connect to the subcallosal and septal areas [9–12]. The cisternal segment is usually exposed through anterior or anterolateral approaches to the anterior skull base or suprasellar area. It is typically injured through retraction or excessive manipulation during tumor resection. Retraction may injure the tract directly or avulse the tiny nerves passing through the cribriform plate. In some cases, loss of olfaction may occur during a craniotomy with no direct manipulation of the olfactory nerve at all. Foraminal/Extracranial  The olfactory epithelium is found in the upper posterior part of the nasal cavity (around the superior concha) and

Optic Nerve (II) As with the olfactory tracts, the “optic nerves” are actually tracts of the central nervous system myelinated with oligodendrocytes. They carry axons from the retina, also part of the central nervous system, where visual processing begins within a neural network starting with the photoreceptors at the deepest layers, and ultimately producing outputs from superficial retinal ganglion cells whose axons collect in the optic disc to form the optic nerve. Neural Segment  The optic tract originates from the lateral geniculate body of the thalamus, located in the superior anterolateral aspect of the ambient cistern, just superior and posterior to the uncus (Fig. 2.2a). It gives rise to optic radiations which fan out in the roof of the temporal horn and ultimately course along the lateral wall of the ventricle en route to the primary visual cortex. The lateral geniculate body is immediately posterior to the inferior choroidal point, where the anterior choroidal artery enters the lateral ventricle. This marks the beginnings of the choroidal fissure, which is comprised of a membrane known as the tela choroidea that connects the thalamus to the fornix and gives rise to the choroid plexus. This can be incised to separate the lateral geniculate body of the thalamus from the fimbria of the fornix to access this nucleus in the ambient cistern via a transtemporal approach working through the temporal horn (subtemporal approaches can only expose the inferior ambient cistern) [9].

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Cisternal Segments  The cisternal anatomy is more complex than other cranial nerves as the paired optic nerves partially decussate in the optic chiasm before continuing on as the optic tracts. Fibers from the nasal portion of the retina, which corresponds to the temporal visual field, cross through the chiasm, which explains the bitemporal hemianopsia seen with optic nerve compression from suprasellar lesions. The optic tract flanks the cerebral peduncle and travels superior to the uncus before continuing toward the optic chiasm medial to the anterior perforated substance and its multiple perforating vessels (Fig.  2.2a). It is typically supplied by small branches of the anterior choroidal artery laterally, and the posterior communicating artery (PCOMM) that passes medial and inferior to the tract while coursing toward the basilar apex. The optic chiasm is both cisternal and intraventricular as its posterior portion occupies the anterior-­ inferior aspect of the third ventricle, just below the lamina terminalis, and anterior to the infundibular recess and pituitary stalk arising from the floor of the third ventricle (Fig. 2.2e). The anterior portion of the chiasm lies below the anterior communicating artery (Fig. 2.2c–e). It is superior to the pituitary gland and diaphragm sellae, and medial and superior to the internal carotid artery. Normally the chiasm is located directly above the sella, whereas a “post-fixed chiasm” deviates posteriorly above the dorsum sellae and a “pre-­ fixed chiasm” is found more anteriorly toward the tuberculum sellae. The optic nerve and optic chiasm are surrounded by the chiasmatic cistern. The superior portion of this cistern (sometimes referred to as the suprachiasmatic cistern or lamina terminalis cistern) is limited by the lamina terminalis posteriorly and contains the A1 segment of the anterior cerebral artery as it approaches the anterior communicating artery, while the inferior portion abuts the interpeduncular cistern posteriorly and the Sylvian and carotid cisterns laterally. Several branches of the superior hypophyseal artery typically supply the optic nerves and chiasm inferiorly, while the superior surface of the chiasm is supplied by the anterior cerebral arteries (Fig. 2.2d). Blood supply from the superior hypophyseal arteries is more redun-

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dant, which partially explains the lower incidence of visual deficits with endonasal approaches. The inter-optic and optic-carotid windows are surgical corridors that can be dissected to expose the pituitary stalk, posterior communicating artery, and the interpeduncular cistern (after opening the Liliequist membrane), though the sensitivity of the optic nerve to retraction is limiting (Fig. 2.5c). The optic nerves and chiasm are historically exposed through anterior or anterolateral skull base approaches, though increasingly with endoscopic approaches that can minimize manipulation. Some lesions can be approached from the contralateral side to minimize optic nerve manipulation, such as a tuberculum sella meningioma invading the medial optic canal, or certain medially projecting aneurysms [11, 13]. Dural/Foraminal Segment  The falciform ligament is the extension of the clinoid dura and roof of cavernous sinus, which forms a crescent-­ shaped drape over the optic nerve as it enters the optic canal (Figs. 2.3a and 2.5b). Its mediolateral length averages around 8  mm, and the anteroposterior length over the optic nerve is 2.1 mm [14]. This ligament may strangulate the nerve when lesions such as tuberculum sella meningiomas deflect the nerve upward, so it is helpful to incise it early in the operation to allow for greater relaxation of the nerve. The optic canal is contained within the sphenoid bone, bounded superiorly by the lesser wing that attaches to the body via an anterior root (roof of the optic canal, at the junction of the planum sphenoidale), and a posterior root (floor of the optic canal, or optic strut, which separates it from the superior orbital fissure) (Figs.  2.2f and 2.3a). These two roots, along with the rest of the lesser sphenoid wing, represent the three bony attachments of the anterior clinoid process (ACP). The ACP separates the optic nerve medially from the clinoidal segment of the internal carotid artery (ICA), and its dura forms the proximal and distal rings [12, 15] (Fig. 2.3b–d). The dura of the optic nerve sheath contains arachnoid tissue and cerebrospinal fluid, which envelop the optic nerve in the optic canal and the orbit. The nerve is accompanied by the ophthalmic artery inferiorly as it enters the

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c

d

Fig. 2.3 The sphenoid bone and cranial nerves. (a) Anterior view of the orbit: The superior orbital fissure (SOF) lies between the greater and lesser sphenoid wings, while the inferior orbital fissure (IOF) is surrounded by the greater wing, zygoma, and the maxilla. The optic canal is bounded by the sphenoid body medially, the optic strut below, and the lesser wing superiorly. (b) Posterior view: The optic canal is seen medial to the anterior clinoid process. The superior orbital fissure (SOF) is a gap between the lesser and greater sphenoid wings. The foramen rotundum lies below the SOF and transmits the maxillary nerve into the pterygopalatine fossa. (c) Posterior superior view: The optic nerve is roofed by the falciform ligament dura before entering the optic canal. The oculomotor nerve passes into the roof of the cavernous sinus through the oculomotor triangle, which is bounded by the anterior and posterior petroclinoidal ligaments and the

interclinoidal ligament. The trochlear nerve enters the cavernous sinus posteriorly just below the tentorial edge. The trigeminal nerve passes below the superior petrosal sinus to enter Meckel’s cave, which lies below the medial dura of the middle fossa. The abducens nerve pierces the petroclival venous confluence as it approaches the cavernous sinus from below. (d) Posterior superior view, dura removed: The anterior clinoid processes have been removed to expose the roof of the cavernous sinus (left), and additional lesser wing of sphenoid bone removed to expose the orbital apex (right). The nerves of the cavernous sinus travel lateral to the carotid artery and funnel into the orbital apex through the superior orbital fissure, where the trochlear nerve is seen crossing over the oculomotor nerve. The abducens nerve passes under Gruber’s ligament in the petroclival venous confluence. (Courtesy of the Rhoton Collection) [12]

optic canal. Bony decompression of the optic nerve is performed intracranially by removing the ACP as well as the optic strut and lesser wing. Alternatively, it can be decompressed endoscopically by drilling the superior lateral wall of the sphenoid sinus. Whereas lesions between the optic nerves such as small tuberculum sella meningiomas are easily accessed endo-

scopically, the foraminal segment of the optic nerve is an important barrier to endoscopic surgery for lesions that extend laterally. Endoscopic exposure of the medial orbit is straightforward through an anterior ethmoidectomy with removal of the lamina papyracea, and bony removal can then be continued posteriorly into the medial optic canal.

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Extracranial Segment  Anterior to the optic strut, the superior orbital fissure and optic canal merge at the orbital apex, where the tendons of orbital muscles (superior, inferior, medial, and lateral rectus) form the annulus of Zinn. The optic nerve enters the orbit through the medial part of the annular tendon. The ophthalmic artery initially courses lateral to the nerve along with the superior ophthalmic vein and nasociliary nerve, then turns superior to the nerve and below the superior rectus muscle (Figs. 2.2f and 2.5f). The intraorbital optic nerve can be approached laterally or superiorly by retracting the orbital muscles [16].

Oculomotor Nerve (III) Neural Segment  The somatomotor fibers of the oculomotor nerve project from the nucleus of the oculomotor nerve, which is located in the mesencephalon anterior to the aqueduct of Sylvius and posteromedial to the red nucleus. The parasympathetic fibers originate at the accessory nucleus of Edinger-Westphal, which is just medial to the oculomotor nucleus, and these innervate the ciliary muscle and the pupillary sphincter. The fibers of the oculomotor nerve pass through the tegmen of the midbrain, including the red nucleus and substantia nigra, before emerging anteriorly in the interpeduncular cistern. Cisternal Segment  The oculomotor nerve arises at the sulcus oculomotorius, medial to the cerebral peduncle and lateral to the posterior perforated substance (and thus the basilar apex) (Figs. 2.1a, 2.4a, and 2.5a). It courses in the lateral wall of the interpeduncular cistern, flanking the basilar apex between the first segment of the posterior cerebral artery (P1) and the superior cerebellar artery (Fig.  2.5c). The cisternal segment of the third nerve receives many arachnoid attachments especially from the Liliequist membrane, which can be cut to expose the interpeduncular cistern [17]. As it approaches the cavernous sinus, it travels medial to the tentorial edge in close proximity to the uncus of the temporal lobe, and lateral to the internal carotid artery, with

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which it forms an anatomical space called the carotid-oculomotor triangle (Fig.  2.5c). This space is more commonly employed to reach the interpeduncular fossa than the optic-carotid window [13]. Compression of the cisternal segment of the oculomotor nerve is most commonly caused by the uncus during herniation, and aneurysms of the posterior communicating artery (PCOMM), which is located medially as the nerve approaches its dural segment. The cisternal segment is typically seen during pterional or subtemporal approaches. Dural/Foraminal Segment  The third nerve enters the roof of the cavernous sinus immediately posterior to the anterior clinoid process through a triangular-shaped cuff, formed by the medial attachments of the tentorium: the anterior and posterior petroclinoid ligaments, and the interclinoid ligament (Fig. 2.3c). These form the oculomotor triangle, which creates a small cistern around the oculomotor nerve before it pierces the roof of the posterior cavernous sinus (Fig. 2.3d). As the nerve passes anteriorly beneath the anterior clinoid process, the third nerve becomes embedded in the dura of the lateral wall of the cavernous sinus (Fig. 2.5b). There, it establishes dural attachments with the carotid artery, the carotid-oculomotor membrane, which is continuous with the roof of the cavernous sinus and the proximal dural ring of the carotid artery beneath the anterior clinoid process (Figs. 2.3d and 2.5d). This segment of the nerve, as well as other nerves of the cavernous sinus (ophthalmic division V1, abducens, and trochlear), can be injured by neoplastic, inflammatory, infectious, or vascular lesions within the cavernous sinus; or compressive conditions such as pituitary apoplexy. Exposure of this segment is obtained by peeling the dura propria of the temporal lobe off the lateral wall of the cavernous sinus, combined with an anterior clinoidectomy to expose the roof of the cavernous sinus as well as the superior orbital fissure. This allows the surgeon to identify the oculomotor nerve within the oculomotor triangle and follow it into the cavernous sinus [15, 18]. The oculomotor nerve within the cavernous sinus can be accessed from a medial trajectory endo-

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a

b

Fig. 2.4  Brainstem and cranial nerves. (a) Anterior view: The oculomotor nerve emerges from the interpeduncular fossa along the medial aspect of the cerebral peduncle. The trigeminal nerve exits from the mid-pons, anterior to the middle cerebellar peduncle, into the cerebellopontine angle. The middle cranial nerves (VI, VII, and VIII) are seen exiting the pontomedullary sulcus. The facial and vestibular nerves exit superior and anterior to the foramen of Luschka. Cranial nerves IX, X, and XI exit from the

postolivary sulcus, with IX exiting at the level of the foramen of Luschka. (b) Posterior view: The trochlear nerve exits the midbrain below the inferior colliculus (IC) after its fibers decussate behind the cerebral aqueduct. The nerve travels in the cerebello-mesencephalic fissure and courses laterally and anteriorly into the ambient cistern, then below the tentorium as it approaches the cavernous sinus. It should be identified and preserved when opening the tentorial incisura. (Courtesy of the Rhoton Collection)

scopically by opening the lateral wall of the sphenoid sinus (Fig. 2.5e). The abducens nerve can be seen lateral to the ICA as well as the other cranial nerves in the lateral wall of the cavernous sinus.

As with the falciform ligament and the optic nerve, incision of the dura of the oculomotor cistern allows greater relaxation of the oculomotor nerve during exposure of the interpeduncular cis-

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a

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f

Fig. 2.5  Cavernous sinus and Meckel’s cave. (a) Right anterolateral view: The tentorium has been removed and the superior cerebellar artery is seen bifurcating into its rostral and caudal branches, which may impinge upon the trigeminal root entry zone. The fourth nerve exits the ambient cistern and courses laterally to the posterior cavernous sinus at its junction with the tentorial edge. The oculomotor nerve travels between the posterior cerebral (PCA) and superior cerebellar (SCA) arteries. (b) Right lateral view: The temporal dura has been peeled from the cavernous sinus and Meckel’s cave, which is a cerebrospinal fluid cistern containing the trigeminal nerve and ganglion. As seen in this picture, most cranial nerve foramina also transmit veins. (c) Right anterolateral view: The anterior and posterior clinoid processes and the dorsum sellae have been removed, and the posterior cavernous sinus has been dissected to expose the basilar artery and its superior branches. The corridor between the carotid artery and oculomotor nerve is usually favored over the optic-carotid window to access the basilar apex and interpeduncular cistern. (d) Right lateral view: The middle fossa floor has been removed and the pterygoid process has been drilled to expose the infratemporal and

pterygopalatine fossae. The branches of the mandibular nerve (V3) are seen, along with the lesser petrosal nerve and otic ganglion in the infratemporal fossa. The Vidian nerve and maxillary nerve (V2) enter the pterygopalatine fossa through their foramina in the base of the pterygoid process, and send some of their branches into the inferior orbital fissure. Other branches are seen turning inferiorly as the descending palatine nerves. (e) Right medial view: The sphenoid sinus has been opened and the base of the pterygoid process has been drilled to expose the cavernous sinus, foramen rotundum, and Vidian canal. The carotid sympathetic fibers travel through the petrous temporal bone and one of its branches, the deep petrosal nerve, merges with the greater superficial petrosal nerve to form the Vidian nerve. (f) Left lateral view: The orbit has been exposed and the annulus of Zinn has been opened to reveal the nerves of the cavernous sinus entering the orbital apex, along with the optic nerve, ophthalmic artery, and ophthalmic veins. The lacrimal, trochlear, and frontal nerve travel outside the annulus of Zinn. The parasympathetic, sensory, and sympathetic roots of the ciliary ganglion are seen. (Courtesy of the Rhoton Collection)

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tern or clipping of certain PCOMM aneurysms. Once the oculomotor and trochlear nerves are skeletonized posteriorly, the posterior clinoid and ipsilateral dorsum sella can be drilled for increased anterior midline exposure (Fig. 2.5c) [19]. Extracranial Segment  The oculomotor nerve enters the orbit through the medial portion of the superior orbital fissure (oculomotor foramen), where it divides into superior and inferior divisions, both of which remain within the annular tendon of Zinn (Figs.  2.2f and 2.5f). While the superior division innervates the superior rectus and levator muscles, the inferior division courses more medially to reach the inferior rectus, inferior oblique, and medial rectus muscles. The inferior division also sends a parasympathetic, or motor root, to the ciliary ganglion [16].

Trochlear Nerve (IV) Neural Segment  The nucleus of the trochlear nerve is located anterior to the aqueduct of Sylvius, below the oculomotor nucleus. Its fibers then course posteriorly around the aqueduct and decussate, before exiting below the contralateral inferior colliculus. Cisternal Segment  The fourth cranial nerve emerges from the dorsal aspect of the brainstem into the quadrigeminal cistern below the inferior colliculus (Fig. 2.4b). It initially courses in the cerebello-mesencephalic fissure from medial to lateral, eventually crossing lateral and superior to the rostral and caudal trunks of the superior cerebellar artery (Fig. 2.5a). In the ambient cistern, it is found between the superior cerebellar artery inferiorly and the tentorium superiorly. When the incisura is cut during subtemporal approaches to open the ambient cistern, the trochlear nerve is usually protected by an arachnoid membrane [17]. The cisternal segment is also frequently encountered just below the tentorium during retrosigmoid approaches for cerebellopontine (CP) angle tumors and microvascular decompression.

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Dural/Foraminal Segment  The trochlear nerve enters the free edge of the tentorium posterior and inferior to the oculomotor triangle (Fig. 2.3c, d) [15, 20]. The nerve travels within the anterior extension of the tentorium, or the petroclinoid fold, for a short distance, before becoming incorporated within the lateral wall of the cavernous sinus below the oculomotor nerve (Fig. 2.5b–d). Thus, the trochlear nerve is most likely to be inadvertently cut when opening the most anterior portion of the tentorial edge during exposure of certain posterior communicating aneurysms or during transcavernous approaches. Extracranial Segment  To reach the superior oblique muscle, which is located in the medial and superior aspects of the orbit, the trochlear nerve shifts superiorly and medially, crossing over the oculomotor nerve below the anterior clinoid as it approaches the superior orbital fissure (Figs. 2.3d and 2.5d, f). The nerve is at risk here during drilling of the clinoid. It enters the orbit outside the annulus of Zinn, then courses further superiorly above the frontal nerve and the levator muscle, to finally reach the superior oblique muscle (Figs. 2.2f and 2.5f) [16, 20].

Trigeminal Nerve (V) The trigeminal nerve originates in the posterior fossa and passes into the middle fossa before its three divisions exit into distinct compartments for wide distribution of its branches: the orbit, pterygopalatine fossa, and the infratemporal fossa. Therefore, branches of this nerve may be encountered in a wide variety of anterior and lateral skull base approaches [21]. Neural Segment  The trigeminal nerve exits the brainstem at the junction of the pons and middle cerebellar peduncle (Fig. 2.4a). It can be seen as one large sensory root and a smaller motor root (innervating the masticators, mylohyoid, and anterior belly of the digastric, tensor veli palatini, and tensor tympani muscles). In the brainstem, its sensory nuclei are located along the dorsolat-

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eral aspect of the brainstem, and include a pontine nucleus (nucleus principalis), a medullary nucleus and tract (spinal tract), and a small mesencephalic nucleus. Its motor nucleus is located in the pons, anterior to the sensory nucleus [22].

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petrosal sinus and tentorium. Conversely, these structures can be cut from a retrosigmoid approach to follow the nerve from the posterior fossa into Meckel’s cave. This exposure can be increased by drilling the suprameatal tubercle, but not beyond the horizontal portion of the Cisternal Segment  From the level of the mid-­ petrous carotid artery [7]. pons, the trigeminal nerve travels superiorly and laterally in the cerebellopontine cistern toward Beyond the semi-lunar ganglion, the divisions the petrous apex and has a close relationship with of the trigeminal nerve fan out in the medial porthe superior cerebellar artery in the upper neuro- tion of the middle fossa before exiting through vascular complex of the posterior fossa separate foramina of the sphenoid bone. The oph(Figs. 2.1a, b and 2.5a) [7]. While the trochlear thalmic branch of the trigeminal nerve (V1) and oculomotor nerves cross above the superior enters the lateral wall of the cavernous sinus cerebellar artery (SCA), the trigeminal nerve below the trochlear nerve and just lateral to the passes just below it, and is often surrounded by abducens nerve before reaching the medial aspect the superior petrosal vein (Dandy’s vein) and its of the superior orbital fissure (Fig. 2.5d, e) [15]. branches. This vein is sometimes sacrificed dur- The maxillary nerve (V2) exits the middle fossa ing exposure of the nerve, but there is small risk through the foramen rotundum just inferior to the of venous infarction in case of large tumors with superior orbital fissure and lateral to the sphenoid venous compromise. The SCA bifurcates into sinus to enter the pterygopalatine fossa rostral and caudal trunks, either of which may (Figs. 2.3b, d and 2.5d, e). The mandibular nerve contact the trigeminal nerve at its root entry zone (V3) travels toward the foramen ovale in the to cause trigeminal neuralgia [22]. greater wing of the sphenoid bone, just lateral Cerebellopontine angle tumors often displace or and posterior to the lateral pterygoid process, to compress the trigeminal nerve and may cause reach the infratemporal fossa (Fig.  2.5d) [21]. facial pain, numbness, or weakness. Lesions While the ophthalmic branch of the trigeminal involving the cisternal segment of the trigeminal nerve (V1) can be dissected in the lateral wall of nerve are most often exposed through retrosig- the cavernous sinus and superior orbital fissure moid, petrosal, or middle fossa approaches. through a pterional approach, the maxillary (V2) and mandibular (V3) branches and Meckel’s cave Dural/Foraminal Segment  The trigeminal are found in the middle fossa floor as they course nerve crosses over the trigeminal depression of toward their foramina. These branches can be the petrous apex, below the superior petrosal easily reached through a subtemporal approach, sinus, to exit the posterior fossa and enter which can be extended with an anterior petrosecMeckel’s cave (Figs. 2.3d and 2.5b). The latter is tomy (“Kawase approach”), providing a window a CSF-filled cavern in the floor of the middle to the posterior fossa between the trigeminal and fossa, enclosing the trigeminal nerve and its facial nerves [23]. Alternatively, an endoscopic semi-lunar (Gasserian) ganglion (Fig.  2.5d). Its transpterygoid approach provides a medial peroverlying dura is continuous with the lateral wall spective of the branches of the trigeminal nerve of the cavernous sinus medially and the tento- and Meckel’s cave, as well as the cavernous sinus rium laterally (Fig. 2.3c) [22]. The petrous carotid and the Vidian nerve in the floor of the sphenoid artery and the greater petrosal nerve course deep sinus, which can be followed posteriorly to the to the trigeminal nerve in the petrous apex to petrous apex (Fig. 2.9b) [24]. reach the foramen lacerum before entering the cavernous sinus (Figs. 2.3d, 2.5d, and 2.9b). An Extracranial Segment  At the distal end of the incision into Meckel’s cave in the middle fossa cavernous sinus, V1 divides into the lacrimal, can be easily extended along the trigeminal nerve frontal, and nasociliary nerves as it approaches into the posterior fossa by dividing the superior the orbital apex (Figs. 2.2f and 2.5f). The lacri-

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mal nerve travels laterally outside the annulus of Zinn above the lateral rectus muscle, where it receives the parasympathetic fibers destined to the lacrimal gland from the zygomatic nerve of the pterygopalatine ganglion. The frontal nerve also travels outside and lateral to the annulus and divides into the supratrochlear and supraorbital nerves. The nasociliary nerve, however, travels medially inside the annulus, crossing over the optic nerve and the oculomotor branches, and terminating in the inferior trochlear, and anterior and posterior ethmoidal nerves. It also sends a sensory root to the ciliary ganglion and long ciliary branches to the globe [15, 16]. In the pterygopalatine fossa, and the infraorbital and zygomatic nerves, branches of V2 pass through the inferior orbital fissure to reach the orbit (Figs.  2.3a and 2.5d). The pterygopalatine (sphenopalatine) ganglion receives communicating branches from V2, and most importantly, the Vidian nerve (union of greater superficial and deep petrosal nerves), as it exits its canal in the lateral floor of the sphenoid sinus [24]. This segment is seen during maxillotomy or endoscopic transpterygoid approaches. In the infratemporal fossa—between the medial and lateral pterygoid muscles—the mandibular nerve (V3) divides into an anterior trunk (giving rise to the deep temporal and masseteric nerves and nerve of lateral pterygoid), and a lateral posterior trunk (buccal, lingual, inferior alveolar, and auriculotemporal nerves). After it exits the skull through the petrotympanic fissure, the chorda tympani joins the lingual nerve which then conveys its taste fibers of the anterior tongue. The otic ganglion receives the parasympathetic fibers of the lesser superficial petrosal nerve (LSPN), which exits through the canaliculus innominatus (between foramen spinosum and ovale) (Fig. 2.5d). Its fibers are then carried through the auriculotemporal nerve to the parotid gland.

Abducens Nerve (VI) Neural Segment  The nucleus of the abducens nerve is located within the pontine floor of the fourth ventricle. The facial nerve loops around

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the abducens nerve, forming its first genu, and these create a protrusion of the floor of the fourth ventricle called the facial colliculus. The abducens fibers project anteriorly from the nucleus and the nerve emerges from the anterior portion of the pontomedullary sulcus, above the pyramid (Fig. 2.4a) [25]. Cisternal Segment  The abducens nerve ascends in a superior and lateral trajectory toward the inferior portion of the cavernous sinus, crossing the anterior inferior cerebellar artery and basilar trunk in the prepontine cistern (Fig. 2.1a, b) [17]. The cisternal segment of the abducens nerve is the most vertically oriented among the cranial nerves, hence its sensitivity to stretch injuries with increased intracranial pressure (Fig.  2.9a). The cisternal segment is commonly seen in retrosigmoid, petrosal, or anterior endoscopic approaches through the clivus. Dural/Foraminal Segment  The abducens nerve penetrates the dura at the petroclival venous confluence, where the inferior petrosal, basilar, and cavernous sinuses communicate (Figs.  2.3d and 2.5e) [26]. This defines the most inferior portion of the posterior wall of the cavernous sinus, which is also demarcated by the posterior clinoid and the entry point of the trochlear nerve (inferomedial paraclival triangle). The abducens nerve then typically passes below the petrosphenoid (Gruber’s) ligament, which extends from the petrous apex to dorsum sellae, but it may also be found above this ligament. This region is also known as Dorello’s canal (Fig. 2.5d). From there it courses within the cavernous sinus lateral to the cavernous carotid artery, medial to the ophthalmic branch of the trigeminal nerve, then passing through the superior orbital fissure [15] (Figs. 2.5d, e and 2.9b). The inferolateral trunk is a branch of the intracavernous ICA vascularizing the cranial nerves within the cavernous sinus which can be found coursing just between V1 and cranial nerve VI (CN-VI) [15]. This segment of the abducens nerve may be injured when performing an anterior petrosectomy if drilling extends too medially toward the clivus, or during endoscopic approaches through the clivus.

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Extracranial Segment  While the abducens nerve is the most medial of the cranial nerves in the cavernous sinus, it innervates the most lateral of the orbital muscles. Just as the trochlear nerve crosses the oculomotor nerve to reach its medial target, the abducens emerges from behind the ophthalmic nerve (V1) and its nasociliary branch as it approaches the superior orbital fissure to course laterally. It enters the orbit through the annulus of Zinn and innervates the medial aspect of the lateral rectus muscle [16].

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related fibers of the nervus intermedius carry the pre-ganglionic parasympathetic and sensory components of the facial nerve arising from the superior salivatory nucleus that provide parasympathetic fibers to the lacrimal, nasopharyngeal, submandibular, and sublingual salivary glands; and gustatory fibers of the nucleus solitarius that supply the anterior two-­thirds of the tongue [27].

Neural Segment  The facial motor nucleus is located in the dorsolateral tegmentum of the pons. Its axons course medially to loop around the abducens nucleus within the facial colliculus before turning laterally again to exit the brainstem at the pontomedullary junction between the abducens and cochlear nerves, and just above the exit of the glossopharyngeal nerve (Fig.  2.4a). The closely

Cisternal Segment  Upon exiting the brainstem, the facial nerve is located in the cerebellopontine cistern anterior and superior to the flocculus and foramen of Luschka [7]. The root entry zone is exposed by retracting the flocculus (Fig.  2.6a). The facial nerve joins the vestibulocochlear nerve en route to the internal auditory canal (IAC). As part of the middle neurovascular complex of the posterior fossa, it can be found in close proximity to the anterior inferior cerebellar artery (AICA), which can compress the nerve root entry zone (causing hemifacial spasm), form a loop that extends into the IAC (causing geniculate neuralgia), or pass between the facial and cochleovestibular nerves. Occasionally, the posterior inferior cerebellar artery (PICA) loops superiorly, close to the VII-VIII complex, before turning back caudally toward the lower cranial nerves (Figs. 2.1a, b and 2.6b). The nervus intermedius travels par-

Fig. 2.6  Middle cranial nerves. (a) Perspective from a right retrosigmoid infra-floccular exposure: The glossopharyngeal nerve (IX) can be followed to the foramen of Luschka by retracting the flocculus. The facial nerve root entry zone is superior and anterior to the entry zone of IX.  The labyrinthine artery follows the VII/VIII complex into the internal auditory canal (IAC), and the subarcuate artery enters the subarcuate canal of the petrous temporal bone. (b) Right posterior view: The internal auditory canal has been opened posteriorly to expose the facial and superior vestibular nerves above the transverse crest entering the fundus of the IAC anteriorly and posteriorly, respectively. The nervus intermedius travels with the facial nerve into the facial canal. The cochlear nerve enters the cochlear area below the facial nerve. A loop of AICA is seen displacing the nervus intermedius, which may cause geniculate neuralgia. (c) Right superior view: The floor of the middle fossa has been drilled to expose the internal and external auditory canals, tympanic cavity, cochlea, labyrinth, petrous carotid artery, and mastoid antrum. The facial nerve is seen passing into the IAC from the cerebellopontine cistern, then entering the facial canal at the fundus. The tympanic segment passes below the lateral semicircular canal (SSC) before tuning inferiorly as the mastoid segment (not

visible). (d) Right superior view: A closer view of the fundus of the IAC exposing the vertical crest (Bill’s bar), between the facial and superior vestibular nerves. After its labyrinthine segment, located just posterior to the cochlea, the facial nerve enters the geniculate ganglion along with the fibers of the nervus intermedius. (e) Right posterior view: The mastoid has been drilled and the tympanic cavity has been opened. The tympanic segment of the facial nerve is seen turning inferiorly to become the mastoid segment below the lateral semicircular canal (SSC) at the second genu, which the short process of the incus points to. The chorda tympani nerve courses superiorly into the tympanic cavity, between the incus and malleus, before turning inferiorly again to exit into the infratemporal fossa. (f) Left lateral view, postauricular transtemporal approach: The sternocleidomastoid muscle has been reflected, the digastric muscle has been cut, and the mastoid has been drilled. The facial nerve is seen exiting the stylomastoid foramen, coursing above the transverse process of the atlas and below the internal auditory canal en route to the parotid gland. Below the transverse process the spinal accessory nerve (XI) turns posteriorly and crosses the jugular vein (retracted) to enter the belly of the sternocleidomastoid muscle. (Courtesy of the Rhoton Collection)

Facial Nerve (VII) The course of the facial nerve is difficult to learn due to its multiple segments within the temporal bone and its special relationships to middle and inner ear structures.

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allel to the facial and vestibulocochlear nerves as it enters the IAC [7]. The cisternal segment of the facial nerve is most commonly seen through retrosigmoid, petrosal, and middle fossa approaches. It can be injured while dissecting it from tumors, by inadvertent traction while manipulating a tumor, or while operating on a deep-seated lesion that requires working past the nerve. Dural/Foraminal Segment  The meatal segment of the facial nerve enters the IAC with the vestibular and cochlear nerves in a specific arrangement that divides the IAC into four quadrants (Fig. 2.6b, d). The transverse crest of the fundus separates the nerves into superior and inferior groups. The facial canal is found in the anterior superior quadrant, anterior to the superior vestibular nerve. These two structures are separated at the fundus of the IAC by the vertical crest (“Bill’s bar”). The nervus intermedius is found in the anterior superior quadrant between the facial nerve and vertical crest. Below the transverse crest, the cochlear nerve is found in the anterior inferior quadrant, whereas the inferior vestibular nerve occupies the posterior inferior quadrant. Bony Segment  Upon entering the facial (Fallopian) canal, the facial nerve travels between the cochlea and labyrinth (labyrinthine segment) before entering the geniculate ganglion. This segment is vulnerable to injury when drilling the fundus of the IAC in the middle fossa due to the thin layer of bone over the nerve in this area. At the geniculate ganglion, it abruptly turns posteriorly (first genu), while the greater superficial petrosal nerve (GSPN) continues anteriorly in the middle fossa floor, traveling above the horizontal part of the petrous carotid artery and carrying pre-ganglionic parasympathetic fibers for ­lacrimation (Fig. 2.6c). The facial nerve courses medial to the incus in the tympanic cavity (tympanic segment). At the pyramidal eminence, under which lies the stapedius muscle, the facial nerve takes another turn in a downward direction below the lateral semicircular canal (second genu), continuing into its mastoid segment (Fig. 2.6e). The short process of the incus points toward this genu. The mastoid segment gives off

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a branch to the stapedius muscle several millimeters above the stylomastoid foramen. An injury proximal to this branch point may cause hyperacusis. Next, the chorda tympani branches from the mastoid segment and ascends over and around the tympanic membrane, between the incus and malleus, before emerging into the infratemporal fossa from the petrotympanic fissure (Fig. 2.7b). The bone between the mastoid segment of the facial nerve and the chorda tympani (facial recess) can be drilled to access the tympanic cavity. Injury to the facial nerve within the bony segment can occur during drilling, so a thin layer of cortical bone is often preserved over the nerve. Weakness will usually result from mobilization of the nerve during transcochlear approaches, which requires cutting the GSPN, and by anterior transposition during postauricular transtemporal approaches [28]. Extracranial Segment  The facial nerve exits the skull at the stylomastoid foramen, which lies between the styloid process and the mastoid tip, deep to the tympanomastoid suture, which is just posterior to the external auditory canal (Figs. 2.6f and 2.8d). The stylomastoid artery, a branch of the posterior auricular artery, enters this foramen and should be preserved as it supplies the facial nerve (Fig. 2.7b). Shortly after its exit, the facial nerve curves anteriorly below the external auditory canal, just superior but deep to the edge of the posterior belly of the digastric muscle (Fig. 2.7a). It crosses the styloid process superficially as it approaches the parotid gland. The cartilaginous “tragal pointer” is often utilized to locate the nerve, which is approximately 1–2 cm deep and inferior to this landmark. The transverse process of the atlas is another palpable landmark in this area that is consistently below the course of the facial nerve (Fig. 2.6). Immediately upon exiting the foramen, the nerve gives rise to the branchial motor fibers that innervate the auricular, stylohyoid, posterior belly of the digastric, and the occipitalis muscles branch. After these early muscular innervations, the remaining fibers then travel through the parotid gland where the nerve splits into five branches that supply motor function to the facial muscles (listed rostral to

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Fig. 2.7  Lateral skull base. (a) Left lateral view: The parotid gland has been removed to reveal the branches of the facial nerve. The nerve exits the stylomastoid foramen and turns anterior to cross the styloid process before branching. It is found superior and deep to the posterior belly of the digastric muscle. The spinal accessory nerve (XI) is seen turning posteriorly below the transverse process of the atlas (hidden beneath the digastric muscle) and superficial to the internal jugular vein to enter the medial sternocleidomastoid muscle (reflected). (b) Left lateral

view: A mastoidectomy has been performed, the styloid process has been removed, and the jugular foramen and carotid canal have been opened laterally. The chorda tympani is seen curving over the tympanic membrane within the tympanic cavity, medial to the malleus, before turning inferiorly again to exit into the infratemporal fossa through the petrotympanic fissure. The stylomastoid artery is entering the stylomastoid foramen as the facial nerve exits. (Courtesy of the Rhoton Collection)

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Fig. 2.8  Lower cranial nerves. (a) Right posterolateral view: A retrosigmoid craniotomy, upper cervical laminectomies, condylectomy, and paracondylar bone removal have been performed for extensive far lateral exposure. The vertebral artery has been transposed from the transverse foramen of the atlas. It pierces the dura medial to the atlanto-occipital joint, and courses deep to the lower cranial nerves toward the vertebrobasilar junction. The cervical contribution to the spinal accessory nerve is seen coursing toward the jugular foramen to join the cranial accessory and vagus nerve rootlets. The glossopharyngeal nerve enters a separate meatus, anterior and superior to the vagal meatus. Extensive drilling of the occipital condyle has opened the hypoglossal canal. (b) Right posterolateral view: The jugular bulb has been removed to reveal the lower cranial nerves in the jugular foramen. After exiting its foramen, the hypoglossal nerve joins the nerves exiting the jugular foramen medial to the jugular vein. (c) Left posterior view: The posterior jugular fossa and hypoglossal canal have been opened to expose the lower cranial nerves, located medial to the jugular bulb and vein. The glossopharyngeal and vagus nerves are initially separated by a venous channel from the inferior petrosal vein

draining into the jugular bulb. The vagus and spinal accessory nerves are conjoined in the foramen. Two hypoglossal foramina are seen medially, converging to a single canal laterally. (d) Right inferior view: The nerves of the jugular foramen are seen medial to the jugular vein just below the jugular foramen, along with the hypoglossal nerve. The facial nerve is exiting lateral to the jugular foramen between the mastoid and styloid process. The mandibular nerve (V3) exits the foramen ovale into the infratemporal fossa. The sympathetic deep petrosal and parasympathetic greater superficial petrosal nerves follow the petrous carotid, then merge to become the Vidian nerve at the foramen lacerum. (e) Right anterolateral view: The lower cranial nerves begin medial to the internal jugular vein at the skull base and then diverge. Soon after exiting the jugular foramen, the glossopharyngeal nerve courses anteriorly to cross the internal carotid artery, while the hypoglossal nerve is seen turning anteriorly at the level of the hyoid bone. The spinal accessory nerve courses posteriorly at the level of the transverse process of the atlas toward the sternocleidomastoid muscle. The vagus nerve gives rise to several branches while remaining in the carotid sheath. (Courtesy of the Rhoton Collection)

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caudal): temporal, zygomatic, buccal, mandibular, and cervical [27] (Fig. 2.7a). The chorda tympani branch of the facial nerve passes through the tympanic cavity, following the curve of the tympanic membrane between the malleus and incus, then exits the petrotympanic fissure into the infratemporal fossa, where it joins the lingual branch of V3 (Fig.  2.7b). Its presynaptic parasympathetic fibers supply the submandibular ganglion, which innervates the submandibular and sublingual glands. The chorda tympani also carries sensory afferents of taste and sensation from the anterior two-thirds of the tongue [27].

Cochleovestibular Nerve (VIII) Neural Segment  The vestibulocochlear nerve emerges from the brainstem lateral to the facial nerve, carrying fibers to the two lateral cochlear nuclei and four more medially located vestibular nuclei (Fig. 2.6a). These nuclei are found at the lateral pontomedullary junction. The macula of the utricle detects linear acceleration in the horizontal plane and is innervated by the superior vestibular ganglion, which relays to the lateral vestibular nucleus. The macula of the saccule detects linear acceleration in the vertical plane and is innervated by the inferior vestibular ganglion, which in turn relays to the inferior vestibular nucleus [28]. Auditory electrodes are typically implanted into the cochlear nuclei through the foramen of Luschka into the lateral recess of the fourth ventricle. Cisternal Segment  The nerve passes through the cerebellopontine angle cistern before entering the IAC, along with the facial nerve and nervus intermedius. In this segment, it is found in close relation to the AICA and two of its branches, the subarcuate and labyrinthine arteries, with the latter following the nerve into the IAC. Occasionally, the labyrinthine artery arises from the basilar artery [7]. Injury to the cochlear nerve often results from manipulation while dissecting it from a tumor, but can also occur with nothing more than retraction of the cerebellum during a retrosigmoid approach (such as for a trigeminal

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microvascular decompression)—presumably related to stretching the nerve or avulsing its fibers as they enter the fundus of the IAC. Dural/Foraminal/Bony Segment  Upon entering the IAC, the vestibulocochlear bundle separates into the cochlear, superior, and inferior vestibular nerves (Fig.  2.6b). As they approach the fundus of the IAC, these nerves pass through the four quadrants of the IAC with the cochlear nerve located anteriorly below the facial nerve as its fibers pass into the cochlea, which is located just anterior to the fundus (Fig.  2.6c, d). The cochlear nerve carries axons from the cells of the spiral ganglia found in the center of the cochlea to the cochlear nuclei. Posteriorly, the superior and inferior vestibular nerves send fibers to the vestibular organs, which are posterior to the IAC. The superior vestibular nerve transmits sensory fibers from the hair cells of the anterior and lateral semicircular canals and the utricle, while the inferior vestibular nerve carries sensory fibers from the saccule.

Glossopharyngeal Nerve (IX) Neural Segment  The glossopharyngeal nerve arises from the medulla as a linear cluster of rootlets in the postolivary sulcus, which is situated between the olive and the posterolateral surface of the medulla (Fig. 2.4a). It arises inferior to the pontomedullary junction and facial nerve root entry zone, and anterior to the foramen of Luschka [7]. This area is easily exposed by retracting the flocculus through a retrosigmoid approach (Fig. 2.6a), but can also be seen during a telovelar approach that opens the lateral recess of the fourth ventricle and the foramen of Luschka [29]. The motor fibers of the glossopharyngeal nerves that innervate the pharyngeal muscles originate in the nucleus ambiguus. Its parasympathetic fibers innervating the parotid gland arise in the inferior salivary nucleus, and its sensory fibers supplying the middle ear and gustatory bodies of the posterior third of the tongue arise in the solitary nucleus. These are located in the dorsolateral and rostral aspects of the medulla.

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Cisternal Segment  The glossopharyngeal nerve courses posterior to the vertebral artery within the cerebello-medullary cistern toward the jugular foramen, along with the vagus and spinal accessory nerves (Fig.  2.8a). The tonsillo-­ medullary segment of the PICA is usually found in close proximity but with highly variable relationships to these nerves—typically passing between nerves or rootlets of a nerve, and sometimes forming a complex loop that distorts the nerves [7, 29]. The cisternal segment glossopharyngeal, vagus, and accessory nerves are most commonly seen through retrosigmoid and far lateral approaches [7]. These nerves are often injured when dissecting the rootlets from tumors or through retraction to access an anterior lesion. Dural/Foraminal Segment  The glossopharyngeal nerve, along with the vagus and accessory nerves, enters the jugular foramen through the dura on the medial aspect of the intrajugular process of the temporal bone before taking a sharp turn inferiorly (Fig.  2.8b, c). The glossopharyngeal nerve enters the more superior and anterior glossopharyngeal meatus, which is separated from the vagal meatus by a dural septum. Its entry into the foramen is just inferior to the cochlear aqueduct [30]. As a complex and variable region, the jugular foramen is most easily understood by dividing it into three parts: the venous petrosal (anterior) and sigmoid (posterior) parts that are initially separated by the intrajugular septum before converging to form the jugular bulb; and the medial nervous intrajugular part in between [30]. Often, the glossopharyngeal and vagus nerves are initially separated inside the foramen by a venous channel communicating the inferior petrosal sinus with the jugular bulb (Fig. 2.8c, d). The glossopharyngeal nerve then joins the vagus and spinal accessory nerves medial to the jugular bulb and vein [30]. A branch of the glossopharyngeal nerve, the tympanic nerve (or Jacobson’s nerve), courses within the tympanic canal between the carotid canal and the jugular foramen to reach the tympanic cavity, where it forms the tympanic plexus [28]. It then innervates the middle ear before sending the lesser superficial petrosal nerve, which exits the skull through the canalicu-

lus innominatus, synapses within the otic ganglion (of V3), and innervates the parotid gland through the auriculotemporal nerve (Figs.  2.5d and 2.8c). The foraminal segments of the lower cranial nerves can be seen in postauricular transtemporal approach to the jugular foramen after removing the jugular vein and bulb, though it is safer to preserve the medial wall of the vein to avoid dissecting it from the nerves [28]. Extracranial Segment  The glossopharyngeal, vagus, and accessory nerves all exit the skull medial to the jugular vein and posterior to the ICA (Fig. 2.8d, e). The hypoglossal nerve, which exits from a separate foramen, briefly joins the other lower cranial nerves just outside the jugular foramen. As it descends, the glossopharyngeal quickly turns anteriorly, crossing laterally to the ICA below the styloid process, and then passing between the stylopharyngeus and stylohyoid muscles as it approaches the pharynx [30]. It innervates several pharyngeal muscles along its path toward the palatine tonsil, the mucous glands of the mouth, and the base of the tongue.

Vagus Nerve (X) Vagus is Latin for “wanderer,” which is an apt descriptor of its extensive meandering course throughout the body. Neural Segment  There are four nuclei from which the vagus nerve arises, including two motor and two sensory nuclei. The motor nuclei are the nucleus ambiguus, giving rise to branchial motor fibers, and the dorsal vagal nucleus, giving rise to the parasympathetic motor fibers. The two sensory nuclei include the solitary tract nucleus responsible for visceral sensory function, and the spinal trigeminal nucleus, which carries general sensory information. Unlike the glossopharyngeal nerve, the vagus nerve typically has multiple distinct rootlets. These exit the medulla inferior to the glossopharyngeal nerve and superior to the accessory nerve rootlets in the postolivary sulcus, between the olive and the inferior cerebellar peduncle [7] (Fig. 2.4a).

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Cisternal Segment  The vagus nerve rootlets travel within the lateral cerebello-medullary cistern below the glossopharyngeal nerve to reach the jugular foramen. The PICA may be found either anterior or posterior to this nerve in the cistern [7] (Fig. 2.8a). Dural/Bony Segment  The vagus nerve enters the vagal meatus of the dura on the medial aspect of the intrajugular process of the temporal bone, which is separated from the more anterior and superior glossopharyngeal meatus by the previously mentioned dural septum. The vagus and accessory nerves both immediately turn inferiorly, as opposed to the frequently displaced course of the glossopharyngeal nerve due to the petrosal drainage into the jugular bulb (Fig. 2.8b, c). The fibers of the vagus nerve continue on to the superior ganglion and the distally located inferior ganglion within the jugular fossa of the skull base. The auricular branch (Arnold’s nerve) arises at the level of the superior ganglion and courses along the anterior wall of the jugular fossa, entering the mastoid canaliculus in the lateral wall of the jugular fossa [30]. Extracranial Segment  After exiting the skull in the jugular foramen, the vagus nerve continues inferiorly within the carotid sheath, posterior and lateral to the internal and common carotid arteries, and medial to the internal jugular vein [30] (Fig. 2.8e). The vagus nerve has an extensive extracranial course. In the neck it supplies motor fibers to several muscles as well as the larynx and pharynx, to which it also provides sensation. It innervates nearly all of the organs of the chest and abdomen with parasympathetic fibers.

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as rootlets from the accessory nucleus and C1–6 of the upper cervical cord. Cisternal Segment  The spinal portion of the accessory nerve rootlets run superiorly and posterior to the dentate ligament to enter the foramen magnum and then the cerebello-medullary cistern, to join the cranial rootlets (Fig. 2.8a, b). The spinal accessory rootlets either join with the cranial accessory rootlets to enter the vagal meatus together, or occasionally they enter the lower border of the vagal meatus separated from the cranial accessory rootlets by a dural septum [30]. Bony/Foraminal Segment  The united spinal and cranial portions of the accessory fibers then blend into the vagus nerve’s inferior margin at the jugular foramen and run laterally then sharply inferior just posterior to the vagus nerve in the medial aspect of the jugular foramen (Fig. 2.8c). The accessory nerve can often be seen closely tethered to the vagus or to the hypoglossal nerve within the jugular foramen [30]. Extracranial Segment  After exiting the jugular foramen, the accessory nerve turns posteriorly and crosses the internal jugular vein to innervate the sternocleidomastoid and the upper trapezius muscles (Figs. 2.6f and 2.7a). It is usually found lateral to the internal jugular vein, near the lateral inferior border of the transverse process of the atlas, but can also be found medial to the vein [30]. Some of the fibers of the cranial portion of the accessory nerve are distributed by the vagus nerve, contributing to innervation of the laryngeal and pharyngeal muscles. Excessive ­mobilization of the sternocleidomastoid muscle during lateral high cervical exposures can cause a stretch injury or avulsion of this nerve [30].

Accessory Nerve (XI) Neural Segment  The spinal accessory nerve is formed by rootlets originating from the upper cervical spine and medulla. Its cranial portion arises from the nucleus ambiguous and exits the medulla as multiple rootlets in the postolivary sulcus, located inferior to the rootlets of the vagus nerve (Fig. 2.4a). Its spinal portion arises

Hypoglossal Nerve (XII) Neural Segment  The hypoglossal nerve arises from the hypoglossal nucleus, which is found in the floor of the fourth ventricle deep to the hypoglossal triangle. It exits the brainstem as nerve rootlets between the pyramid and the olive in the

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anterolateral sulcus, inferior to the spinal accessory nerve rootlets [31] (Fig. 2.4a). Cisternal Segment  The hypoglossal rootlets pass through the subarachnoid space posterior to the vertebral artery in the nerve’s course toward the hypoglossal canal (Fig. 2.8a, b). In patients in whom the vertebral artery has a tortuous course, the hypoglossal rootlets can be stretched posteriorly over the dorsal aspect of the artery, which sometimes displaces them so far that it can be difficult to distinguish them from the other lower cranial nerve rootlets. Occasionally, the vertebral artery may even pass between the hypoglossal nerve rootlets. The PICA has a highly variable course but commonly arises at the level of the hypoglossal rootlets and may deflect them [31]. Dural/Foraminal Segment  The rootlets group into two clusters prior to the hypoglossal nerve’s entry into the hypoglossal canal (Fig. 2.8c). The hypoglossal canal is located between the occipital condyle and jugular tubercle of the occipital a

Fig. 2.9  Anterior endoscopic perspective. (a) Anterior endoscopic view: The clivus, pterygoid process and petrous apex have been drilled, and the Eustachian tubes have been mobilized. The jugular tubercle has been drilled to expose the nerves in the jugular foramen, and the petrous temporal bone has been drilled to open the internal auditory canal. Endoscopic exposure of the cavernous sinus allows access to medial lesions without dissecting the nerves. Lesions in the clivus and anterior to the brainstem can be reached without working between cranial

bone. The hypoglossal canal may begin with two foramina intracranially that unite to form one nerve and foramen laterally before the nerve exits medial to the jugular foramen. During far lateral approaches, drilling the occipital condyle increases the exposure of the cervico-medullary junction, and especially when carried to the jugular tubercle, allows easier access to the ventral medulla. It is recommended to avoid drilling more than half of the condyle to avoid cranio-­ cervical instability. Drilling the cancellous bone of the condyle while preserving the cortical shell around the hypoglossal nerve may help preserve its function [32]. Extracranial Segment  The hypoglossal nerve briefly joins the other lower cranial nerves below the jugular foramen medially (Figs. 2.8b, c, d and 2.9a). It then courses inferiorly in the upper neck posterior to the inferior ganglion of the vagus nerve and travels a short distance between the internal carotid artery and internal jugular vein, beneath the posterior belly of the digastric musb

nerves. Note the vertical orientation of the abducens nerve (VI). (b) Right anterior endoscopic view: The pterygoid base has been drilled to expose Meckel’s cave below the cavernous sinus, lateral to the carotid artery. The abducens nerve is seen coursing medial to the ophthalmic nerve (V1). The Vidian nerve has been cut at the foramen lacerum, where it is formed by the greater superficial petrosal nerve (GSPN) and deep petrosal nerve. (Courtesy of the Rhoton Collection)

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cle, before curving anteriorly to cross the internal and external carotid arteries in a trajectory similar to the glossopharyngeal nerve, but more inferior [30] (Fig. 2.8e). It then passes medial to the stylohyoid muscle en route to muscles of the tongue. It is frequently found in close proximity to the occipital artery as it arises from the external carotid [30].

Cranial Nerve Preservation The work of Al Rhoton Jr. and others to describe microneurosurgical anatomy of the cranial nerves has been motivated by the historical difficulties with preserving cranial nerve function during skull base surgery. This knowledge allows surgeons to choose a trajectory to obtain a perspective that balances their ability to address the lesion while also preserving nerve function. As it is clear that loss of cranial nerve function frequently has a significant impact on quality of life, a tailored approach can be worth the effort. Decades of microsurgical experience have distilled several lessons. 1. Complex sensory nerves such as the olfactory, optic, and auditory usually do not recover well after injury, so preservation of these nerves requires delicate dissection. In some cases, judgment may lead the surgeon to stop the dissection if the likelihood of injury is high. 2. It is helpful to reduce tension on cranial nerves resulting from the pathology or the approach. This can be done by cutting the nerve’s dural investment (e.g., falciform ligament or oculomotor cistern) [19], early internal debulking, and minimizing tumor manipulation. 3. Transposition of the facial nerve during transcochlear and preauricular transtemporal approaches usually results in postoperative weakness. A transotic approach reduces this risk significantly by skeletonizing the fallopian canal instead of transposing the nerve. 4. Blood supply to cranial nerves should be preserved if possible. Sacrifice of superior hypophyseal branches to the optic nerves and

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chiasm may result in visual loss, and loss of the stylomastoid artery may contribute to facial nerve weakness. 5. The posterolateral approaches (retrosigmoid, far lateral, and petrosal) are extremely versatile as all of the cranial nerves of the posterior fossa can be accessed and the internal auditory canal can be drilled to expose the intracanalicular nerves [7, 28]. Nonetheless, it is difficult to operate through a curtain of cranial nerves between the surgeon and an anterior lesion such as a petroclival meningioma or clival chordoma. The middle and lower cranial nerves are susceptible to inadvertent manipulation from instruments working between nerves. Anterior petrosectomy and endoscopic approaches can provide more direct access to anterior lesions of the posterior fossa with less risk to cranial nerves, though these approaches incur other limitations. 6. Endoscopic approaches also improve visualization and access to midline lesions below the optic chiasm, such as craniopharyngiomas and smaller tuberculum sella meningiomas. Unfettered exposure of these lesions means less manipulation of the optic nerves and chiasm is needed. Transcranial approaches rely upon corridors between the optic nerves, between the carotid artery and optic nerve, and between the carotid artery and oculomotor nerve. All of these corridors suffer from limited visualization, thus tempting the surgeon to retract neurovascular structures. 7. The surgeon must recognize lower cranial nerve dysfunction in the early postoperative period in order to prevent aspiration or airway compromise. Visualization of the vocal cords upon extubation and early swallowing assessments should be strongly considered if these nerves or their rootlets have been dissected. Embolization of glomus tumors has reduced the risk of inadvertent coagulation of the lower cranial nerves, and preservation of the medial venous wall eliminates the need to dissect nerves in the jugular foramen. 8. Olfactory testing can detect unilateral dysfunction pre- or postoperatively. Often, an

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olfactory groove meningioma has destroyed only one nerve, which can be identified with preoperative testing or visualized on preoperative imaging. The functional nerve can often be preserved by choosing a contralateral approach and through careful dissection.

Conclusion Knowledge of cranial nerve anatomy is critical to success in skull base surgery because injuries to these nerves often have a significant impact on quality of life, and, in some cases, life expectancy. For each lesion, the surgeon must visualize how the cranial nerves might be displaced or otherwise affected, and which nerves are at risk during the surgical approach. This understanding will lead to the selection and execution of an approach that balances risks to cranial nerves with the quality of exposure, or perhaps to a decision to recommend nonoperative treatment.

References 1. Ng ALC, Rosenfeld JV, Di Ieva A.  Cranial nerve nomenclature: historical vignette. World Neurosurg. 2019;128:299–307. 2. Davis MC, Griessenauer CJ, Bosmia AN, Tubbs RS, Shoja MM. The naming of the cranial nerves: a historical review. Clin Anat. 2014;27(1):14–9. 3. Simon F, Marečková-Štolcová E, Páč L.  On the terminology of cranial nerves. Ann Anat. 2011;193(5):447–52. 4. Kurze T.  Microtechniques in neurological surgery. Clin Neurosurg. 1964;11:128–37. 5. Matsushima T, Matsushima K, Kobayashi S, Lister JR, Morcos JJ.  The microneurosurgical anatomy legacy of Albert L.  Rhoton Jr., MD: an analysis of transition and evolution over 50 years. J Neurosurg. 2018;129(5):1331–41. 6. Yasagil GM, Krayenbuhl HA, Donaghy RMP.  Microsurgery: applied to neurosurgery. Stuttgart: Georg Thieme Verlag/Academic Press; 1969. 7. Rhoton AL Jr. The cerebellopontine angle and posterior fossa cranial nerves by the retrosigmoid approach. Neurosurgery. 2000;47:S93–S129. 8. Blitz AM, Macedo LL, Chonka ZD, Ilica AT, Choudhri AF, Gallia GL, Aygun N.  High-resolution CISS MR imaging with and without contrast for evaluation of the upper cranial nerves: segmental anatomy

and selected pathologic conditions of the cisternal through extraforaminal segments. Neuroimaging Clin N Am. 2014;24(1):17–34. 9. Rhoton AL Jr. The cerebrum. Anatomy. Neurosurgery. 2007;61(1 Suppl):37–118; discussion 118–9. 10. Ribas EC, Yağmurlu K, de Oliveira E, Ribas GC, Rhoton A. Microsurgical anatomy of the central core of the brain. J Neurosurg. 2018;129(3):752–69. 11. Inoue K, Seker A, Osawa S, Alencastro LF, Matsushima T, Rhoton AL Jr. Microsurgical and endoscopic anatomy of the supratentorial arachnoidal membranes and cisterns. Neurosurgery. 2009;65(4):644–64. 12. Rhoton AL Jr. The sellar region. Neurosurgery. 2002;51(4 Suppl):S335–74. 13. Rhoton AL Jr. The supratentorial arteries. Neurosurgery. 2002;51(4 Suppl):S53–120. 14. Altafulla JJ, Iwanaga J, Kikuta S, Prickett J, Ishak B, Uz A, Dumont AS, Tubbs RS. The falciform ligament: anatomical study with microsurgical implications. Clin Neurol Neurosurg. 2020;195:106049. 15. Rhoton AL Jr. The cavernous sinus, the cavernous venous plexus, and the carotid collar. Neurosurgery. 2002;51(4 Suppl):S375–410. 16. Rhoton AL Jr. The orbit. Neurosurgery. 2002;51(4 Suppl):S303–34. 17. Rhoton AL Jr. The posterior fossa cisterns. Neurosurgery. 2000;47(3 Suppl):S287–97. 18. Martins C, Yasuda A, Campero A, Rhoton AL Jr. Microsurgical anatomy of the oculomotor cistern. Neurosurgery. 2006;58(4 Suppl 2):ONS-220-7. 19. Basma J, Ryttlefors M, Latini F, Pravdenkova S, Krisht A.  Mobilization of the transcavernous oculomotor nerve during basilar aneurysm surgery: biomechanical bases for better outcome. Neurosurgery. 2014;10(Suppl 1):106–14. 20. Joo W, Rhoton AL Jr. Microsurgical anatomy of the trochlear nerve. Clin Anat. 2015;28(7):857–64. 21. Tanriover N, Sanus GZ, Ulu MO, Tanriverdi T, Akar Z, Rubino PA, Rhoton AL Jr. Middle fossa approach: microsurgical anatomy and surgical technique from the neurosurgical perspective. Surg Neurol. 2009;71(5):586–96. 22. Joo W, Yoshioka F, Funaki T, Mizokami K, Rhoton AL Jr. Microsurgical anatomy of the trigeminal nerve. Clin Anat. 2014;27(1):61–88. 23. Kawase T, Shiobara R, Toya S. Anterior transpetrosal-­ transtentorial approach for sphenopetro-clival meningiomas: surgical method and results in 10 patients. Neurosurgery. 1991;28:869–76. 24. Osawa S, Rhoton AL Jr, Seker A, Shimizu S, Fujii K, Kassam AB. Microsurgical and endoscopic anatomy of the vidian canal. Neurosurgery. 2009;64(5 Suppl 2):385–411. 25. Joo W, Yoshioka F, Funaki T, Rhoton AL Jr. Microsurgical anatomy of the abducens nerve. Clin Anat. 2012;25(8):1030–42. 26. Destrieux C, Velut S, Kakou MK, Lefrancq T, Arbeille B, Santini JJ.  A new concept in Dorello’s canal microanatomy: the petroclival venous confluence. J Neurosurg. 1997;87(1):67–72.

2  Surgical Anatomy of the Cranial Nerves 27. Yang SH, Park H, Yoo DS, Joo W, Rhoton A.  Microsurgical anatomy of the facial nerve. Clin Anat. 2021;34(1):90–102. 28. Rhoton AL Jr. The temporal bone and trans temporal approaches. Neurosurgery. 2000;47(3 Suppl):S211–65. 29. Mussi ACM, Rhoton AL Jr. Telovelar approach to the fourth ventricle: microsurgical anatomy. J Neurosurg. 2000;92:812–23.

33 30. Rhoton AL Jr. Jugular foramen. Neurosurgery. 2000;47(3 Suppl):S267–85. 31. Wen HT, Rhoton AL Jr, Katsuta T, de Oliveira E. Microsurgical anatomy of the transcondylar, supracondylar, and paracondylar extensions of the far-­ lateral approach. J Neurosurg. 1997;87(4):555–85. 32. Rhoton AL Jr. The far-lateral approach and its transcondylar, supracondylar, and paracondylar extensions. Neurosurgery. 2000;47(3 Suppl):S195–209.

3

Skull Base Compartmental Anatomy: Microsurgical and Endoscopic Jaafar Basma, Kara Parikh, and Jeffrey M. Sorenson

Introduction

partments provides the surgeon with a reliable framework for surgical planning. The base of the human skull, which transmits all The skull base is most simply divided into neural and vascular connections between the intracranial and extracranial compartments. brain and body, challenges the modern surgeon Intracranially, the anterior fossa floor is located with its anatomical and surgical complexity. below the frontal lobes, the middle fossa below the Approaches to this area must be designed to pro- temporal lobe, and the posterior fossa, containing vide good exposure while at the same time limit- the cerebellum and brainstem, below the tentoing the risks to neurovascular structures. From a rium. At the center of these lies the sella turcica. developmental perspective, the skull base dis- Hidden within the temporal bone, there are several plays transitional anatomical features with the important compartments including the internal neighboring cervical spine, calvarium, and face, and external auditory canals, tympanic cavity, labresulting in variations in its regional anatomy. yrinth, cochlea, and carotid canal. Extracranial Nonetheless, it can be conceptualized as a well-­ compartments associated with the skull base defined arrangement of interconnected compart- include the orbit, nasal cavity and associated ments that must be safely traversed in order to sinuses, pharynx, and the temporal, infratemporal, reach the pathology (Fig.  3.1). Open and endo- and pterygopalatine fossae. The sphenoid bone scopic approaches through these compartments faces all of the major skull base compartments and have been developed and extensively refined over many of the extracranial compartments (Fig. 3.2). the past half century to create a wide variety of Adjacent compartments may be navigated by widsurgical trajectories that minimize brain retrac- ening natural windows such as sinus ostia or the tion. Thus, an understanding of skull base com- sphenopalatine foramen. Alternatively, new openings through partitions such as the clivus or tentorium must be created. Often, one or more extracranial compartments are traversed to reach J. Basma (*) · K. Parikh an intracranial compartment containing the tarDepartment of Neurosurgery, University of Tennessee Health Science Center, Memphis, TN, USA geted lesion. In some cases, these roles may be e-mail: [email protected] reversed as an intracranial compartment may be J. M. Sorenson opened to expose an extracranial lesion. For examDepartment of Neurosurgery, University of Tennessee ple, a superior orbitotomy is often performed to Health Science Center, Memphis, TN, USA enhance access to an anterior fossa lesion; and Semmes Murphey Clinic, Memphis, TN, USA conversely, a frontal craniotomy can also be e-mail: [email protected]

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 A. S. Youssef (ed.), Contemporary Skull Base Surgery, https://doi.org/10.1007/978-3-030-99321-4_3

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Fig. 3.1  Skull base compartments and their surgical relationships. The major intracranial compartments (blue) are approached through various incisions (red). Anterior approaches often involve passage through sinuses (yel-

low), whereas lateral and posterior approaches present obstacles such as skull base bones, venous sinuses, or the tentorium (green)

employed to approach an orbital lesion through the floor of the anterior fossa. Large lesions often require an extended approach through additional compartments, or even multiple approaches. In this chapter, we provide an anatomical overview of skull base compartments and their relationships using images from the Rhoton Collection with multiple perspectives.

support the frontal lobes and the olfactory tracts and bulbs (Figs. 3.3a, 3.5a, and 3.8b). It is semicircular in shape anteriorly, tapering posteriorly along the sphenoid ridge of the lesser wing of the sphenoid bone to form the anterior clinoid processes that flank the medially located chiasmatic sulcus (Fig. 3.3a). These structures form the posterior boundary of the anterior fossa. The medial portion of the anterior fossa floor is comprised of the portion of the frontal bone overlying the frontal and ethmoid sinuses, the cribriform plate and crista galli of the ethmoid bone, and the planum of the sphenoid body. The cribriform plate is a perforated portion of the ethmoid bone supporting the olfactory bulb, through which very short olfactory nerve filaments exit the skull base to

Anterior Fossa Boundaries and Contents The floor of the anterior cranial fossa is formed by the frontal, ethmoid, and sphenoid bones that

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Fig. 3.2  The sphenoid bone and its relationships to skull base compartments. (a) Anterior view: The sphenoid bone interfaces with multiple skull base compartments, including the orbit, anterior fossa, temporal fossa, infratemporal fossa, pterygopalatine fossa, posterior fossa, and pharynx. Two nerves pass through the base of the pterygoid process: the maxillary and Vidian nerves, which exit into the pterygopalatine fossa. The nerves of the cavernous sinus pass through the superior orbital fissure (SOF) into the orbital apex. (b) Posterior view: The Vidian canal begins posteriorly, at the lateral base of the sphenoid body, near the groove for the paraclival carotid artery. This canal can be followed to reach the carotid artery as it exits the foramen lacerum. (c) Right lateral view: In addition to contributing to the orbit, the greater wing of the sphenoid forms the anterior floor of the middle fossa, medial wall of the temporal fossa, and roof of the infratemporal fossa. The optic strut is seen extending from the body of the sphenoid to the anterior clinoid process—it borders the optic canal laterally (not seen) and the SOF medially. The cavernous sinus extends from the superior orbital fissure anteriorly to the dorsum sella posteriorly, and the anterior portion is roofed by the anterior clinoid process. (d) Superior view: The compartments surrounding the greater wing of the sphenoid are seen. Drilling through the temporal fossa allows access to both the orbit and the middle

f

fossa. (e) Anterior view of coronal section through the sphenoid bone with surrounding structures: The orbital apex occupies the space between the body, lesser wing, and greater wing of the sphenoid. The temporal fossa, which contains the temporalis muscle, is lateral and superior to the infratemporal fossa. The pterygoid process borders the infratemporal fossa medially and the nasopharynx laterally. (f) Lateral view of sphenoid bone with associated structures of the middle, infratemporal, and pterygopalatine fossae and the orbit: The cavernous sinus is bounded medially and the sphenoid sinus anteriorly, and the sella and dorsum sellae posteriorly. The nerves of the cavernous sinus converge in the superior orbital fissure to enter the orbit at the superior orbital fissure (SOF), where the trochlear nerve (IV) crosses over the oculomotor nerve (III). The three branches of the trigeminal nerve exit the middle fossa into the orbit, pterygopalatine fossa, and the infratemporal fossa. The floor of the middle fossa and the pterygoid process have been drilled to expose the infratemporal fossa, pterygopalatine fossa, Vidian canal, and foramen rotundum. The Vidian and maxillary (V2) nerves enter the pterygopalatine fossa, and several of their branches are seen exiting through the inferior orbital fissure (IOF), while others turn inferiorly toward the palate. The mandibular (V3) and lesser petrosal nerves are seen in the infratemporal fossa

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Fig. 3.3 Superior perspective. (a) Bony features of the intracranial skull base. The floor of the anterior fossa is comprised of the crista galli and cribriform plate of the ethmoid bone covering the nasal cavity in the midline, and the frontal bone covering the ethmoid air cells and orbit. Posteriorly, the planum sphenoidale, which covers the sphenoid sinus, and the lesser wings of the sphenoid, which cover the cavernous sinus and superior orbital fissure, complete the anterior fossa floor. The middle fossa floor is made up of the greater wing of the sphenoid anteriorly and the temporal bone posteriorly. Medially, the sella is bounded laterally by the cavernous sinus and posteriorly by the dorsum sellae. The triangular-shaped anterior portion of the posterior fossa is flanked by the petrous temporal bones, which taper anteriorly toward the clivus. (b) The anterior fossa floor has been removed to reveal the nasal sinuses, orbit, and optic canal; and the infratemporal fossa is visible after removing the middle fossa floor. The cavernous sinus nerves are seen converging at the superior orbital fissure (SOF) before diverging again in the orbit. Branches of the ophthalmic artery supply the orbit, ethmoid sinuses, and associated skull base. The mandibular nerve (V3) and lesser petrosal nerve are visible in the infratemporal fossa. (c) The tentorium has been removed to expose the cerebellum and brainstem in the posterior fossa. A venous ring is formed at the edges of the tento-

rium by the transverse sinuses and superior petrosal sinuses, and by the basilar sinus. (d) The sella and anterior portion of the middle fossa are separated by the cavernous sinus, which contains the internal carotid artery and cranial nerves entering the orbit through the superior orbital fissure. The sella and dorsum sellae are encircled by a ring of venous sinuses: cavernous, intercavernous, and basilar. The anterior clinoid process, which roofs the cavernous sinus anteriorly, has been removed. The dura above the clinoid is continuous with the distal dural ring of the carotid artery and falciform ligament, whereas the dura below the clinoid that roofs the cavernous sinus forms the proximal ring of the carotid. (e) The posterior portion of the middle fossa has been drilled to reveal structures hidden within the petrous temporal bone, which present obstacles for lateral approaches to the posterior fossa. The external auditory canal, internal auditory canal, and greater superficial petrosal nerve (GSPN) form a “Y” shape with the tympanic cavity at its center. Translabyrinthine approaches through the mastoid allow access to the posterior fossa and IAC without cerebellar retraction. Transcochlear approaches take a more lateral to medial trajectory, requiring sacrifice of the external auditory canal, tympanic cavity, and cochlea; and transposition or skeletonization of the facial nerve

reach the olfactory epithelium in the superior portion of the nasal cavity. The anterior portion of the cribriform plate is divided in the midline by the crista galli, which attaches to the falx cerebri. The foramen caecum, which may transmit emissary veins from the nasal cavity to the superior sagittal sinus, lies just anterior to the crista galli at its junction with the frontal bone. Lateral to the cribriform plate and medial to the orbit, a narrow strip of frontal bone roofs the ethmoid air cells. Posterior to the cribriform plate, the planum sphenoidale roofs the sphenoid sinus. The remainder of the frontal bone and the superior surface of the lesser sphenoid wing form the lateral portion of the anterior cranial fossa, which overlies the orbit, optic canal, cavernous sinus, and internal carotid artery [1].

tal craniotomy usually traverses the frontal sinus, giving a wide view that is particularly useful if anterior skull base reconstruction is required. The disadvantages include ligation of the origin of the superior sagittal sinus, and increased risk of olfactory loss and postoperative edema due to retraction of both frontal lobes. Unilateral approaches such as the supraorbital, lateral supraorbital, and pterional can reduce these risks while providing a more lateral perspective of the lesion and related neurovascular structures; however, this can create more difficulty when addressing large bilateral lesions. Removal of the orbital rim during bifrontal or unilateral craniotomies permits more superior trajectories with less brain retraction, which may be lessened further through the use of endoscopy. Other than brain retraction and loss of olfaction, neurological risks of these approaches include injury to the frontalis branch of the facial nerve, which transitions above the zygomatic arch to be superficial to the temporalis fascia and then superficial to the pericranium anteriorly as it approaches the frontalis muscle, or to the supraorbital nerve as it exits the supraorbital foramen.

Surgical Considerations The anterior fossa is typically exposed anteriorly through bifrontal or unilateral craniotomies, anterolaterally through a pterional approach, or endoscopically through the sinuses [2]. A bifron-

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The extended endonasal transsphenoidal approach can expose the anterior fossa through the frontal sinuses, cribriform plate, planum sphenoidale, and tuberculum sella to access midline lesions without lateral extension beyond the a

supraclinoid carotid or the midportion of the superior orbit [3] (Figs. 3.3a, b, 3.4d, and 3.7b). The advantages of the approach include elimination of the need for brain retraction, early identification and takedown of the anterior and

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Fig. 3.4  Inferior perspective. (a) The infratemporal fossa is roofed by the greater wing of the sphenoid anteriorly and the temporal bone posteriorly. It is separated from the more lateral and superior temporal fossa by the infratemporal crest. The oropharynx and nasopharynx are located anterior to the lower clivus, and the latter is also between the pterygoid processes. The jugular foramen is formed by the temporal and occipital bones, between the petroclival fissure anteriorly and the occipitomastoid suture posteriorly. The roof of the jugular fossa, formed by the temporal bone, is seen from this perspective but not its medially directed turn into the posterior fossa. (b) At the anterior border of the infratemporal fossa, the pterygomaxillary fissure leads to the pterygopalatine fossa, which is a slender compartment between the pterygoid process and the posterior wall of the maxillary sinus. It communicates with the inferior orbital fissure superiorly. (c) Inferior view of the right infratemporal fossa and jugular foramen. The jugular foramen is bounded medially by the occipital condyle, posteriorly by the jugular process and rectus capitis lateralis muscle, laterally by the styloid process

d

and facial nerve, and anteriorly by the internal carotid artery. The nerves of the jugular foramen are seen exiting medial to the internal jugular vein and are joined by the hypoglossal nerve after it exits the hypoglossal canal. The inferior petrosal sinus courses along the petroclival fissure and empties into the jugular bulb through a channel between the glossopharyngeal (IX) and vagus (X) nerves. The greater superficial petrosal nerve and deep petrosal nerve accompany the internal carotid artery in the petrous carotid canal before merging to become the Vidian nerve at the foramen lacerum. The Eustachian tube is lateral to the petrous carotid. (d) Inferior view of the nasal cavity, infratemporal fossa, pterygopalatine fossa, and maxillary sinuses. The more superior exposure on the right side obtained by drilling the pterygoid process shows the nerves of the pterygopalatine fossa—the Vidian coursing through its canal in the pterygoid base, and the maxillary (V2) exiting the middle fossa through foramen rotundum. A branch of V2 continues through the inferior orbital fissure into the floor of the orbit (roof of maxillary sinus) as the infraorbital nerve

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posterior ethmoid arteries supplying the tumor base, and access to lesions beneath the chiasm and optic nerves without significant manipulation of these structures. Disadvantages include loss of olfaction, increased risk of postoperative cerebrospinal fluid (CSF) leak, poor access to the superior or lateral optic canal, and increased difficulty addressing lesions with significant neurovascular involvement. The transbasal approach is essentially a reversal of the endonasal approach— but with more extensive exposure—opening the anterior fossa floor from above to enter the orbits, nasal cavity, and all of its sinuses, then possibly into the sella, and finally through the clivus into the posterior fossa. The optic canal can be opened superiorly through the anterior fossa floor, or ­laterally by removing the anterior clinoid process (Figs. 3.3d and 3.8d).

Middle Cranial Fossa and Sella Boundaries and Contents Dr. Albert Rhoton defines the middle cranial base as the area posterior to the sphenoid ridge and chiasmatic sulcus, and anterior to the border created by the petrous ridge, dorsum sella, and posterior clinoid processes. He divides this into two distinct compartments: the sellar region medially, and the middle fossa laterally (Fig. 3.3a, d) [1, 4]. Some authors, however, apply the term middle fossa to both of these compartments. For surgical access purposes only, the sella is placed in the anterior fossa section in this book. The middle fossa supports the mesial and basal temporal lobes, and its floor has contributions from both the temporal and sphenoid bones. The greater sphenoid wing accounts for a large area of the middle fossa, whereas the lesser sphenoid wing makes a small contribution to the anterior border, above the superior orbital fissure. Petrous and squamosal portions of the temporal bone compose the remainder of the middle fossa floor [1]. The middle fossa exhibits complex anatomy, with the anterior half featuring a plexus of cranial nerves inside sleeves of dura routed to three different extracranial compartments, whereas the

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floor of the posterior portion covers the facial nerve, mastoid antrum, and the middle and inner ear with their associated canals (Figs. 3.3d, e and 3.9d). The cavernous sinus forms the medial wall of the middle fossa anteriorly, separating it from the sphenoid sinus, sella, and dorsum sella [4] (Figs. 3.2d and 3.11f). This small compartment is created by an outer layer of dura facing the temporal lobe, and an inner layer of periosteal dura. It is roofed by the anterior clinoid process anteriorly and the petroclinoid and interclinoid ligaments forming the oculomotor triangle posteriorly (Fig.  3.3a, d). As a venous nexus, it communicates with the ophthalmic veins, sphenoparietal sinus, basilar sinus, superior and inferior petrosal sinuses, intercavernous sinuses, and the pterygoid plexus (Figs. 3.2c, 3.3d, and 3.11e, f). Just inside its lateral dural wall, the cavernous sinus funnels the cranial nerves destined for the orbit— oculomotor, trochlear, abducens, and ophthalmic—into the medial part of the superior orbital fissure, which is formed by the space between the lesser and greater sphenoid wings that opens into the posterior orbit, just lateral to the optic canal (Figs.  3.2a, 3.3b, and 3.11f). The carotid artery courses below the middle fossa floor before leaving the petrous carotid canal to occupy the medial part of the cavernous sinus, forming its characteristic posterior and anterior genua, and giving off small branches to the pituitary, dura, and the cavernous sinus itself (Figs. 3.3b, d, e, 3.4c, 3.5b, d, 3.6f, and 3.7d, e). Microsurgical and endoscopic anatomy of the cavernous sinus is more detailed in Chap. 22. Adjacent to the inferior edge of the cavernous sinus another dural compartment, Meckel’s cave, is also found in the anterior part of the middle fossa. The dura of this CSF-filled cistern follows the trigeminal nerve from the posterior fossa into the middle fossa. It contains the trigeminal (semilunar) ganglion as it gives rise to three divisions: the ophthalmic, which enters the cavernous sinus and then into the orbit through the superior orbital fissure; the maxillary, which exits the middle fossa anteriorly through the foramen rotundum to enter the pterygopalatine fossa; and the mandibular, which exits the floor of the middle fossa through the foramen ovale to enter the infratem-

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Fig. 3.5  Medial perspective. (a) Nasal endoscopy can expose a wide swath of the skull base in the sagittal plane, from the frontal sinus, anterior fossa, sella, posterior fossa, and finally the foramen magnum and upper cervical spine. The middle turbinate covers the maxillary sinus, and leads to the sphenoid sinus, which provides access to the sella and upper clivus. The lower clivus is accessed through the nasopharynx. (b) The turbinates have been removed along with the ethmoid air cells, lamina papyracea, and lateral wall of the sphenoid sinus to expose the medial orbit and the cavernous sinus. Below the orbit, the maxillary sinus and pterygopalatine fossa are seen as the palatine bone has been removed and the pterygoid process has been drilled. The maxillary (V2) and Vidian nerves are seen entering the pterygopalatine fossa, and the infraorbital and greater palatine nerves are seen exiting. (c) A series of prominences and depressions is seen along the superior aspect of the petrous temporal bone. The trigemi-

nal depression forms the floor of the ostium of Meckel’s cave, roofed by the superior petrosal sinus. The trigeminal prominence is anterior to the internal auditory canal (IAC), corresponding to the bone removed during an anterior petrosectomy. The meatal depression roughly corresponds to the IAC, while the arcuate eminence corresponds to the location of the superior semicircular canal. This is not always a reliable landmark. The tegmen covers the mastoid antrum, and is a common site of spontaneous cerebrospinal leaks. The inferior petrosal and sigmoid sinuses converge at the jugular foramen. The jugular bulb rises to the level of the labyrinth in this specimen. The endolymphatic sac is frequently encountered during presigmoid approaches. (d) The petrous temporal bone has been drilled to expose the carotid artery, which turns from vertical to horizontal just inferior and anterior to the cochlea (not exposed) and internal auditory canal

poral fossa (Fig.  3.2f). Beneath Meckel’s cave lies the apex of the petrous temporal bone and the foramen lacerum, above which the carotid artery exits the carotid canal and enters the cavernous sinus (Figs. 3.2f and 3.3a). The petrous part of the floor of the middle fossa exhibits a series of prominences and depres-

sions, starting with the trigeminal depression, where the trigeminal nerve passes over the petrous apex; the trigeminal prominence, which is posterior to Meckel’s cave and often drilled during an anterior petrosectomy; the meatal depression, which roughly approximates the internal auditory canal (IAC); the arcuate emi-

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Fig. 3.6  Anterior perspective. (a) The pterygoid process of the sphenoid bone articulates anteriorly with the palatine bone, which in turn articulates with the maxilla. These two bones form the hard palate, which separates the oral and nasal cavities, and they both contribute to the floor of the orbit. The zygoma and greater wing of the sphenoid form the lateral wall of the orbit, with the inferior orbital fissure occupying the gap between the lateral wall and the floor. The superior orbital fissure lies between the lesser and greater sphenoid wings. (b) The pterygopalatine fossa and infratemporal fossa can be seen through the maxilla, which has been opened anteriorly and posteriorly. (c) The face of the sphenoid sinus and posterior wall of the ethmoid sinus are seen medial to the orbital apex. The vomer, which forms the inferior part of the nasal septum and attaches to the rostrum of the sphenoid, is the best anatomical marker for midline during transsphenoidal approaches. (d) The middle and inferior turbinates border the maxillary sinus medially in the nasal

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cavity, and are mobilized during transnasal endoscopic exposures through the maxillary sinus. (e) The posterior wall of the maxillary sinus has been removed to expose the pterygopalatine fossa. The internal maxillary artery enters laterally from the infratemporal fossa through the pterygomaxillary fissure and continues medially into the nasal cavity as the sphenopalatine artery. The sphenoid sinus has been opened and the impression of the optic canal can be seen superolaterally, posterior to the orbital apex. The clivus has been drilled to expose the basilar sinus. (f) The pterygoid plates have been removed to increase exposure of the infratemporal fossa (left), and the pterygoid base and petrous bone have been drilled to expose the middle fossa and petrous carotid artery (right). Meckel’s cave and the branches of the trigeminal nerve are seen lateral to the petrous and paraclival carotid. The posterior fossa has been exposed through a transclival approach, though the sella still obstructs the view of the basilar apex and interpeduncular fossa

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Fig. 3.7 Anterior endoscopic perspective. (a) Anterior view into the nasal cavity: The middle turbinate, which leads to the sphenoid ostium, has been removed to open the ethmoid air cells above. The inferior turbinate has also been removed and the sphenopalatine foramen can be seen, which transmits the sphenopalatine artery from the pterygopalatine fossa (PPF). (b) Expanded endonasal exposure: The frontal, ethmoid, and sphenoid sinuses have been opened to provide access to the anterior fossa superiorly and the orbit laterally. The impressions of the optic nerve (ON) and internal carotid artery (ICA) are seen with the opticocarotid recess between them, corresponding to a pneumatized optic strut. The posterior wall of the maxillary sinus has been removed (right side) to expose the pterygopalatine (PPF) and infratemporal (ITF) fossae. (c) Maxillary sinus from endonasal perspective: A window into the pterygopalatine fossa (PPF) has been created through the posterior wall of the maxillary sinus. (d) Expanded endonasal

exposure: The tuberculum sella has been removed to expose the chiasmatic cistern, and the carotid arteries and cavernous sinus have been exposed laterally in the sphenoid sinus. The optic nerve courses along the superior lateral aspect of the sphenoid sinus in the optic canal to enter the orbital apex, whereas the nerves of the cavernous sinus enter through the superior orbital fissure. The Vidian nerve has been preserved in its canal, which leads to the foramen lacerum and petrous carotid artery. (e) The base of the pterygoid process has been removed to expose Meckel’s cave in the middle fossa, below the cavernous sinus and lateral to the petrous and paraclival carotid artery. (f) Transpterygoid exposure of the petrous apex and jugular foramen. The Eustachian tubes have been retracted to expose the jugular and hypoglossal foramina. The occipital condyle has been drilled to reach the hypoglossal canal, and the jugular tubercle has been drilled above the hypoglossal canal to expose the nerves approaching the jugular foramen

nence, which approximates the superior semicircular canal; and the tegmen, which roofs the mastoid antrum (Figs.  3.5c and 3.11e) [5]. The internal auditory canal can be opened through the middle fossa floor, starting at the wider medial opening (porus acusticus) for increased safety. The narrow fundus of the internal auditory canal is roofed by a thin layer of bone and surrounded by the cochlea anteriorly, the labyrinth posteriorly, and the geniculate ganglia laterally (Figs. 3.3e and 3.5d). Also lateral to the IAC and labyrinth is the tympanic cavity, which transduces sound from the external auditory canal into vibrations of the ossicles and cochlear endolymph via the oval window. The greater superficial petrosal nerve (GSPN) is an important landmark of the middle fossa floor, which runs through the sphenopetrosal groove and approximates the position of the carotid canal (Fig. 3.3b, e). It is typically seen directly beneath the periosteal dura, but is sometimes under a thin layer of bone and can be located with a facial nerve stimulator [6].

and the lesser wing of the sphenoid. The lesser wing is typically drilled away, along with the anterior clinoid process, to increase exposure of the anterior Sylvian fissure, carotid artery, optic canal, and cavernous sinus. Additional bony structures can be removed, including the superior and lateral orbital rims, zygoma, and zygomatic arch (frontotemporal-orbitozygomatic craniotomy), to create additional trajectories to the middle fossa floor [7], pretemporal area, superior orbital fissure, and orbit with less temporal lobe retraction (Fig. 3.8a, b). Access to the posterior fossa is accomplished by dissecting the anterior tentorium and posterior dura of the cavernous sinus, along with removal of the posterior clinoid process, which exposes the interpeduncular cistern and upper basilar artery (Fig.  3.8c, d). Medially, the sphenoid sinus can be opened through the space between the ophthalmic and maxillary nerves (anteromedial triangle); the pterygoid base, found between the maxillary and mandibular nerves (anterolateral triangle), can be drilled to access the pterygopalatine fossa (Fig.  3.2f). From opposite direction, the middle fossa can be reached endoscopically through the sphenoid sinus and through the pterygopalatine fossa by drilling the pterygoid base (Fig.  3.7e). The pterygopalatine fossa is typically reached by traversing the maxillary sinus endonasally or through a sublabial trajectory (Caldwell-Luc) [8] (Figs. 3.6b and 3.7b, c).

Surgical Considerations A pterional craniotomy provides a direct route to the optic apparatus, internal carotid artery, and parasellar region through the Sylvian fissure. The main anatomical barriers are the temporal lobe

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Fig. 3.8 Anterolateral perspective. (a) Drilling the greater wing of the sphenoid in the temporal fossa exposes the middle fossa posteriorly and the orbit anteriorly. A burr hole at the frontosphenoid suture allows for both an orbitotomy and anterior fossa craniotomy. (b) A right frontotemporal-orbitozygomatic craniotomy has been created, allowing subtemporal, pretemporal, trans-­ Sylvian, and anterior fossa trajectories. Removal of the anterior clinoid can provide additional exposure of the

carotid, optic nerve, and cavernous sinus. (c) The oculomotor cistern has been opened to allow mobilization of the oculomotor nerve and increased access to the posterior clinoid process. (d) The posterior clinoid process and the dorsum sellae have been drilled, and the posterior cavernous sinus has been dissected to increase exposure of the superior midline posterior fossa. The carotid-oculomotor triangle is a commonly used corridor to the basilar apex

The posterior middle fossa is typically exposed through an extradural subtemporal approach, which can provide access to the entire internal auditory canal without violating the labyrinth, though it provides fairly limited access to the cerebellopontine angle [7]. The key to this approach is identifying the orientation of the internal auditory canal (Chap. 30). The greater superficial nerve is at risk as it may be injured when dissecting the dura from the floor of the middle fossa or when drilling. The facial nerve is particularly vulnerable at the fundus of the internal auditory canal, where it is covered by a thin layer of bone, as is the geniculate ganglion lateral

to that. The highly variable vein of Labbé is tethered to the junction of the transverse and superior petrosal sinuses; therefore, it may be injured during temporal lobe retraction, possibly leading to venous infarction (Fig. 3.9c). Laterally, the floor of the middle fossa can be drilled through a subtemporal approach for easy access to the infratemporal fossa (Fig.  3.9e). Middle fossa approaches can be extended to the posterior fossa by opening the tentorium, which can expose the superior cerebellum, superior cerebellar artery, and trochlear and oculomotor nerves (Fig. 3.9b) [7]. Moving further inferiorly, Kawase described the removal of the petrous

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Fig. 3.9  Lateral perspective. (a) Left subtemporal view: The temporal lobe has been elevated to expose the incisura and the ambient and crural cisterns. The oculomotor nerve (III) is seen exiting the interpeduncular cistern between the superior cerebellar artery (SCA) and the posterior cerebral artery (PCA), which is coursing laterally from the basilar apex. The trochlear nerve (IV) is seen in the ambient cistern before diving below the tentorium. (b) The tentorium has been incised to expose the superior surface of the cerebellum and cerebellomesencephalic fissure. The trochlear and oculomotor nerves are well visualized, but the trigeminal nerve (V) is not. (c) An anterior petrosectomy has been performed and the tentorium has been divided to communicate the middle and posterior fossa, exposing the midbrain and upper pons. Drilling is limited by the greater superficial petrosal nerve (GSPN) and carotid laterally, and the clivus medially. The vein of Labbé is seen under tension, which can lead to avulsion or thrombosis. (d) Middle fossa floor, right: The greater

superficial petrosal nerve (GSPN), which is variably covered by a thin layer of bone, estimates the location of the petrous carotid artery. Kawase’s (posteromedial) triangle is bounded laterally by the GSPN, anteriorly by the trigeminal nerve, posteriorly by the arcuate eminence, and medially by the superior petrosal sinus. Glasscock’s (posterolateral) triangle is formed by the angle between the GSPN and the mandibular nerve. Drilling through this triangle exposes the petrous carotid, lesser petrosal nerve, and infratemporal fossa. (e) Preauricular exposure of the infratemporal fossa: The temporalis muscle has been reflected anteriorly after removal of the zygomatic arch. Following a frontotemporal craniotomy, the dura is elevated, and the middle fossa floor is drilled lateral to the superior orbital fissure and foramen ovale to expose the trigeminal nerve in the infratemporal fossa. The carotid artery has been mobilized from its canal in the petrous temporal bone. The mandible has been partially resected to increase inferior exposure

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apex to expose the trigeminal nerve and pons (Fig. 3.9c). Drilling proceeds anterior to the IAC, medial to the GSPN and cochlea, and lateral to the clivus and inferior petrosal sinus [9]. The abducens nerve can be injured if drilling violates the inferior petrosal sinus. This trajectory provides more direct exposure of anterior lesions from the incisura to the level of the internal auditory canal, though lesions extending across the midline are difficult to address. The view may be extended more inferiorly with an angled endoscope. With tumors involving both compartments, such as trigeminal schwannomas, the petrous bone is often eroded so less drilling is required.

Orbit Boundaries and Contents The orbit is bounded by seven bones. The frontal bone’s orbital plate, along with the lesser sphenoid wing, forms the orbital roof, while the boundary of the orbital floor is created by the zygoma, maxilla, and palatine bones. The orbit is bounded medially by the maxilla, lacrimal, and ethmoid bones; and laterally by the zygoma and greater wing of the sphenoid bone [4]. The orbit is surrounded by and often accessed from adjacent compartments, such as the temporal fossa through the greater wing of the sphenoid and zygoma (Figs. 3.2a, d, 3.6a, and 3.8a), the anterior fossa through the frontal bone (Fig. 3.3a, b), and the ethmoid sinuses through the lamina papyracea (Figs. 3.5b and 3.7b). The inferior orbit may also be reached through the maxillary sinus, which forms the floor of the orbit (Figs. 3.4d and 3.7b). The anterior portion of the orbit houses the globe, while the posterior portion houses the muscular, nervous, and vascular structures found in the retro-orbital space (Fig. 3.3b). The lacrimal gland is tucked in the superolateral portion of the orbit. Neurovascular structures enter the orbit through the optic canal, superior orbital fissure, and inferior orbital fissure. A key anatomical landmark in the orbital apex that envelopes the

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medial superior orbital fissure and the optic canal is the annulus of Zinn (Figs. 2.2e, f and 2.5b in Chap. 2). This ring is formed by a fibrous tendon which serves as an attachment point for the origin of the superior, inferior, medial, and lateral rectus muscles [4]. As described in Chap. 2, nerves serving the globe and its muscles travel within the annulus, except for the trochlear nerve. The lacrimal nerve and nerves that eventually leave the orbit (frontal branch of V1) travel outside the annulus. The lacrimal gland is innervated by parasympathetic and sympathetic fibers of the greater and lesser petrosal nerves, respectively. These are carried by the Vidian nerve into the pterygopalatine ganglion, which then sends branches to the zygomatic nerve that traverses the lateral orbit to join the lacrimal nerve above the lateral rectus muscle. The nerves of the superior orbital fissure pass medially through the oculomotor foramen. The oculomotor foramen is the portion of the opening in the annulus through which the superior and inferior divisions of the oculomotor nerve, the nasociliary nerve, and abducens nerve pass. The abducens nerve enters the medial aspect of the lateral rectus muscle. The remaining three extraocular muscles (levator palpebrae superioris, and superior and inferior oblique) are found outside of the annulus. The superior and inferior oblique muscles arise along the superomedial and inferomedial orbital walls, respectively [4]. The superior orbital fissure is separated from the optic canal by the optic strut, which connects the base of the anterior clinoid to the sphenoid body (Figs. 3.2c and 3.3b). When pneumatized, it appears in the sphenoid sinus as the opticocarotid recess (Fig. 3.7b). The optic canal transmits the optic nerve and the ophthalmic artery into the orbit (Fig. 3.3a, b, d). The optic nerve exits the optic canal superiorly and medially into the orbital apex, enveloped by the dura of the optic nerve sheath, and the ophthalmic artery initially courses below and lateral to the optic nerve (Figs.  3.6f and 3.7d). More anteriorly, the ophthalmic artery turns medially between the optic nerve and the superior rectus muscle on its journey to the medial aspect of the orbit where it

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branches into the anterior and posterior ethmoid arteries (Fig.  3.3b). The superior and inferior ophthalmic veins provide the main venous drainage of the orbit and exit the orbit outside of annulus of Zinn through the superior orbital fissure. The inferior orbital fissure is formed by the greater sphenoid wing superiorly and posteriorly, and the maxillary and palatine bones inferiorly and anteriorly. It contains fibrous tissue and orbital muscle, but it also opens into the pterygopalatine fossa to transmit nerves such as the zygomatic and infraorbital branches of V2, and branches of the pterygopalatine ganglion [4] (Figs. 3.2f, 3.4b, d, and 3.5b).

Surgical Considerations The orbit may be accessed through adjacent compartments, such as the temporal fossa through the greater wing of the sphenoid and zygoma, the anterior fossa through the frontal bone, and the ethmoid sinuses through the lamina papyracea. The inferior orbit may also be reached through the maxillary sinus, which forms the floor of the orbit. The lateral orbit can be accessed through a lateral orbitotomy (Chap. 21). The orbital apex can be accessed through a medial transorbital approach for medial lesions or transcranially through a supraorbital approach. The orbit contains a significant amount of fat, which is often encountered during cranio-orbital approaches. The ethmoid arteries are coagulated for tumor devascularization during various expanded endoscopic skull base procedures; in such case, they must be adequately exposed in the ethmoid sinuses and coagulated before they retract into the medial orbit causing orbital hematomas. Also, they can be coagulated for the same purpose in medial transorbital transcaruncular approach (Chap. 17).

Sella Situated between the cavernous sinuses is the sella, which holds the pituitary gland under the diaphragma sellae. It indents the sphenoid sinus

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between the dorsum sellae posteriorly and the tuberculum sellae anteriorly, taking on the shape of a saddle (Figs. 3.2c, d and 3.3a). The gland is invested in dura, which may contain both inferior and superior intercavernous venous sinuses [4] (Figs. 3.3d, 3.7d, and 3.11f).

Surgical Considerations The sella is most commonly approached through a transnasal trajectory through the sphenoid sinus (Fig.  3.5a). The bony nasal septum is usually fractured posteriorly and the anterior wall of the sphenoid sinus is then removed. The corridor is sometimes expanded by mobilizing or resecting the middle turbinate, and by removing posterior ethmoid air cells. The internal carotid artery is at risk as it forms its anterior genu lateral to the sella (Fig.  3.7d). Several bony impressions and recesses can be recognized within the sphenoid sinus, such as the optic canal, carotid artery, tuberculum sella, and optic strut (Fig.  3.7b). Occasionally, the carotid artery is not covered by bone. Cranial approaches to the sella are possible, but it is often difficult to obtain good ­visualization of the sellar contents due to neurovascular barriers laterally, such as the cavernous sinus, optic nerve, and internal carotid artery. The window between the optic nerves is sufficient for addressing some lesions involving the sella, such as tuberculum sellae meningiomas and craniopharyngiomas (Fig. 3.6c). Use of the endoscope has allowed surgeons to follow sellar lesions into the suprasellar space with good visualization medial to the oculomotor nerves, but with more limited lateral access than transcranial approaches.

Infratemporal Fossa Boundaries and Contents The infratemporal fossa is an irregular anatomical compartment of the external skull base located below the greater sphenoid wing and the zygomatic arch (Figs. 3.2a–c and 3.4a–d). It contains

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the pterygoid muscles, the pterygoid venous plexus, the maxillary artery and its branches, and the mandibular nerve and its branches [10] (Figs. 3.2e, f, 3.3b, 3.4c, d, 3.6e, f, and 3.9e). The otic ganglion is located below the foramen ovale medial to the mandibular nerve, where it receives the lesser superficial petrosal nerve carrying parasympathetic fibers to the parotid gland (Fig. 3.2f). The chorda tympani, a branch of the facial nerve that carries taste fibers from the anterior tongue as well as parasympathetic fibers to the submandibular ganglion, emerges from the petrotympanic fissure into the infratemporal fossa where it joins the lingual branch of the mandibular nerve. The infratemporal fossa is bounded medially by the lateral pterygoid plate and laterally by the mandible (Figs. 3.2e and 3.4d). The lateral pterygoid muscle inserts at the condylar process, whereas the medial pterygoid muscle attaches more inferiorly to the ramus of the mandible, thus contributing to the inferior border of the infratemporal fossa. Posterior to the lateral pterygoid plate, the medial border is formed by the parapharyngeal space, Eustachian tube, and levator and tensor veli palatini muscles. The parapharyngeal space is divided into pre-styloid and post-styloid compartments, separated by the styloid and digastric muscles (stylo-digastric diaphragm). The more superficial pre-styloid compartment contains the facial nerve as it courses into the parotid gland, while the deeper post-styloid compartment contains the carotid artery, jugular vein, and early extracranial segments of the lower cranial nerves [10, 11] (Fig. 3.4c). The roof of the infratemporal fossa is largely constituted by the external surface of the greater sphenoid wing, through which the foramen spinosum and ovale open from the middle fossa (Fig. 3.6e, f). Superolaterally, the infratemporal crest demarcates the boundary between infratemporal fossa and the temporal fossa, which continues more superiorly and contains the temporalis muscle (Fig. 3.4b). The posterior limit is demarcated by the styloid process and tympanic segment of the temporal bone (Fig. 3.4c). Anteromedially, the infratemporal fossa communicates through the pterygomaxillary fissure with the pterygopalatine fossa—a slender com-

partment between the posterior wall of the maxilla anteriorly and the pterygoid process posteriorly that tapers inferiorly (Fig.  3.4b). Superiorly, the pterygopalatine fossa communicates with the orbit through the inferior orbital fissure. It is limited medially by the palatine ­perpendicular plate (Fig.  3.6a), and laterally by the pterygomaxillary fissure through which the internal maxillary artery enters from the infratemporal fossa [10] (Figs. 3.4d, 3.5b, and 3.6e). After coursing across the pterygopalatine fossa, this artery terminates as the sphenopalatine artery as it passes through the sphenopalatine foramen along with the sphenopalatine nerves to supply the nasal cavity (Fig.  3.6e). The Vidian nerve emerges from the pterygoid (Vidian) canal into the pterygopalatine fossa, where it innervates the parasympathetic pterygopalatine ganglion, which supplies the nasal cavity, nasopharynx, and oral mucosa of the maxilla as well as the lacrimal gland (Figs.  3.2f, 3.4d, 3.5b, and 3.7d). It also carries sympathetic fibers into the ganglion. Many of the autonomic fibers of the pterygopalatine ganglion are distributed by the maxillary nerve, which enters this fossa through the foramen rotundum. The maxillary nerve sends two sensory roots to the ganglion before splitting into several branches, the largest of which is the infraorbital nerve that continues anteriorly through the inferior orbital fissure (Figs. 3.4d and 3.5b). Inferiorly, the pterygopalatine fossa tapers into the greater palatine canal, which carries the descending palatine artery and the lesser and greater palatine nerves toward their foramina into the oral cavity (Fig. 3.5b).

Surgical Considerations The infratemporal fossa can be accessed through a preauricular incision by drilling the floor of the middle fossa lateral to the foramen ovale to expose the mandibular nerve [12, 13] (Fig. 3.9d, e). The mandible may be drilled for further exposure [14]. The Eustachian tube is found just posterior to the foramen ovale and lateral pterygoid plate, but lateral to the petrous internal carotid artery, which can be mobilized from its canal

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with additional drilling in Glasscock’s triangle (Figs. 3.4c and 3.9d). Further posterior extension of the approach can be accomplished by removing the styloid process and muscles [12]. Transfacial maxillotomy approaches to the infratemporal fossa have largely been replaced by extended endoscopic endonasal approaches (EEA) [15, 16] (Chap. 51). These typically utilize a large maxillary antrostomy, followed by identification and sacrifice of the sphenopalatine artery (Fig.  3.7b, c). The pterygopalatine ganglion is then found posterior to the artery, leading to the Vidian nerve medially (Fig.  3.6e, f). Drilling the lateral pterygoid plate then exposes the foramen ovale. Adding a lateral transmaxillary route through a sublabial incision (Caldwell-­ Luc) offers an expanded and more direct exposure of the infratemporal fossa lateral to the foramen ovale [8] (Fig. 3.6b).

Posterior Cranial Fossa Boundaries and Contents The posterior cranial fossa is a bowl-shaped compartment, formed by the occipital, sphenoid, and temporal bones, with a dural covering shaped like a pitched tent (tentorium). Accommodating the cerebellum and the brainstem, the posterior fossa transitions from a larger semicircular shape posteriorly formed by the squamosal part of the occipital bone and the mastoid, to a smaller rounded triangle anteriorly formed by the petrous temporal bones and clivus [17] (Fig. 3.3a, c). At the most superior border, the tentorial incisura transmits the midbrain and other neurovascular structures into the supratentorial space, while at the inferior border the foramen magnum opens into the spinal canal. The posterior fossa contains a network of venous sinuses, which function both as important landmarks and obstacles to entry. Both the tentorium and the medial face of the petrous temporal bone are encircled by venous sinuses. The transverse and superior petrosal sinuses flow within dural leaflets at the lateral attachment of the tentorium, communicating with the basilar sinus at the clivus to form a com-

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plete venous ring along the superior perimeter of the posterior fossa (Fig. 3.3c). The sigmoid sinus occupies a prominent groove in the temporal bone before emptying in the jugular bulb. The inferior petrosal sinus courses along the petroclival fissure from the basilar sinus to the jugular foramen (Fig. 3.11e). The superior petrosal sinus then completes the venous ring around the petrous temporal bone. The basilar, superior, and inferior petrosal sinuses also drain the posterior cavernous sinus (Fig. 3.11f). The neural foramina of the posterior fossa, middle and inner ear, carotid and vertebral arteries, and jugular bulb are all located laterally, which complicates approaches from this direction (Figs. 3.3e, 3.4c, and 3.11c, d). Other cranial nerves exit the posterior fossa more anteriorly to enter other intracranial compartments: the trigeminal nerve enters Meckel’s cave over a bony depression of the petrous apex; the abducens nerve crosses into the posterior aspect of the cavernous sinus through Dorello’s canal; and the oculomotor and trochlear nerves enter the superior and posterior portions of the cavernous sinus, respectively (Fig. 3.11f). Albert Rhoton Jr. has proposed the rule of three to systematize the posterior fossa contents and their relationships into neurovascular complexes associated with each cerebellar artery, which we have described in the cranial nerve chapter (Chap. 2) along with their associated cisterns [17].

Surgical Considerations Because there are so many important obstacles to avoid, approaches to the posterior fossa often require a compromise between achieving the desired angle of attack and limiting approach-­ related morbidity. The most commonly used corridor is the retrosigmoid approach, which provides extensive access along the medial face of the petrous temporal bone into the cerebellopontine angle, from the tentorium to the foramen magnum [17]. Extensions into the middle fossa are possible by drilling the suprameatal tubercle [18] (Fig.  3.10b). The retrosigmoid exposure is created by drilling squamosal occipital and pos-

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Fig. 3.10  Posterolateral perspective. (a) Left lateral view of posterolateral skull base: The sigmoid sinus divides posterolateral approaches into presigmoid (petrosal) or retrosigmoid. The mastoid has been drilled to expose the labyrinth and the mastoid segment of the facial nerve. Note the working corridor between the sigmoid sinus and the labyrinth is quite small. The asterion is the confluence of the occipitomastoid, lambdoid, and parietomastoid sutures, and it is typically located posterior to the junction of the sigmoid and transverse sinuses. The vein of Labbé is seen coursing toward the superior petrosal sinus. (b) A left retrosigmoid exposure of the posterior fossa exposing the cranial nerves of the posterior fossa: The relationship of the posterior inferior cerebral artery (PICA) and the lower cranial nerves is variable. Here, it is emerging between the vagus and spinal accessory nerves. The root entry zone of the facial and vestibulocochlear nerve is obscured by the flocculus. A large suprameatal tubercle is obscuring the distal trigeminal nerve. The superior petrosal vein typically has tributaries from the cerebellum and the pons. A “curtain of cranial nerves” hinders access to anterior lesions involving the clivus or petroclival fissure. (c) Perspective of right far lateral approach with additional dissection to show surrounding structures: A more medial trajectory to midline anterior structures is achieved by drilling the condyle. The vertebral artery has been transposed out of the transverse foramen of the atlas. In this specimen, PICA is coursing posterior to the

all of the lower cranial nerves. (d) A petrosal exposure has been augmented by a temporal craniotomy. Division of the superior petrosal sinus and tentorium allows retraction of the sigmoid sinus to enlarge the presigmoid corridor, though in this case the vein of Labbé is under tension. This combined approach allows lesions to be addressed from multiple trajectories from the posterior and middle fossae. (e) A complete left petrosectomy has been performed. The lower cranial nerves are seen entering the jugular foramen medially, but the glossopharyngeal nerve (IX) is separated from the vagus nerve (X) by a venous channel connecting the inferior petrosal sinus to the jugular bulb. (f) Left-sided posterior and middle fossa craniotomy for combined petrosal approach: The posterior and middle fossa dura is opened and the venous drainage is inspected before dividing the superior petrosal sinus and tentorium. (g) Left translabyrinthine approach with exposure of the internal auditory canal and mastoid segment of the facial nerve: This is an excellent approach for vestibular schwannomas when hearing preservation is not a consideration, as the tumor is approached from a more lateral trajectory that does not require cerebellar retraction. (h) Left combined transcochlear approach: The facial nerve has been transposed, which results in weakness. This approach allows access to the petrous carotid, which is located anterior and inferior to the cochlea, as well as the basilar artery due to the more lateral trajectory

terior mastoid bone to open the intracranial groove of the sigmoid sinus (Figs.  3.3a and 3.10a). The sigmoid-transverse junction is typically superior and anterior to the asterion [19]. The drawbacks of this approach are cerebellar retraction, sigmoid sinus injury or thrombosis, and difficult visualization of anterior lesions requiring the surgeon to work between cranial nerves. The far lateral approach extends the retrosigmoid approach inferiorly by exposing the upper cervical region and performing hemilaminectomies of the atlas and axis, providing increased access to the cervicomedullary junction [20] (Fig. 3.10c). The occipital condyle and the jugular tubercle can be drilled to increase exposure of the lower clivus and ventral medulla (extreme lateral infrajugular transcondylar–transtubercular exposure). Condylar drilling is limited anteriorly by the hypoglossal canal, which is located at the superior aspect of the occipital condyle, just inferior to the jugular foramen [20]. Presigmoid approaches allow for more direct anterior visualization. A retrolabyrinthine “minimal mastoidectomy” requires less cerebellar retraction than a retrosigmoid exposure while

still preserving hearing [6] (Fig.  3.10d). More extensive posterior petrosal approaches require the surgeon to consider obstacles within the petrous temporal bone such as the labyrinth, facial nerve, tympanic cavity, internal and external auditory canals, cochlea, and the internal carotid artery (Figs. 3.3e and 3.10a, e). The facial nerve turns inferiorly into its mastoid segment just below the lateral semicircular canal, and this segment is typically identified and preserved during a translabyrinthine approach (Fig. 3.10f, g). More anterior bony removal of the cochlea requires the surgeon to cut the GSPN and mobilize the facial nerve, causing weakness, or work around a skeletonized nerve (transotic approach) (Fig. 3.10h). The jugular bulb limits presigmoid approaches inferiorly and its height is variable, sometimes reaching the labyrinth (Figs. 3.5d and 3.10f). Superiorly, the superior petrosal sinus and tentorium are limiting, but these can be divided to allow the sigmoid and transverse sinuses to be retracted posteriorly [21]. This widens the presigmoid corridor, while also opening the exposure into the supratentorial compartment to better address lesions extending through the incisura

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(Fig.  3.10a, d). Loss of vein of Labbé drainage into the superior petrosal sinus is a serious risk during this maneuver; hence, all veins in this area should be radiographically analyzed before planning the tentorial incision. The trochlear ­ nerve, which is usually separated from the tentorium by arachnoid, should be identified and preserved as the incision reaches the incisura by preserving the arachnoid. Tentorial incisions are also useful when approaching the posterior fossa from above, providing access to the quadrigeminal cistern through an occipital craniotomy and to anterior structures through middle fossa craniotomy as previously discussed. Middle fossa anterior petrosectomy can be combined with a posterior petrosal approach to create multiple trajectories for difficult lesions, including those extending above the incisura [21]. As previously discussed, an anterolateral trajectory to the upper posterior fossa can be achieved through variants of the pterional approach [22]. This requires a more complex dissection than the subtemporal approach because the incisura has reached the anterior and poste-

rior clinoid processes where the tentorial edge splits into the anterior and posterior petroclinoid ligaments, which also define the roof of the posterior cavernous sinus. Therefore, to maximize this exposure, the surgeon must first drill the anterior clinoid process, then dissect and mobilize the oculomotor and trochlear nerves as they enter the superior and posterior cavernous sinus, and then drill the posterior clinoid process (Fig. 3.8b–d) [23, 24]. This allows access to the interpeduncular fossa and upper pons through the carotid-oculomotor space. With more limited drilling, the interpeduncular fossa can be reached through the optic-carotid space. Direct anterior endonasal approaches to the posterior fossa through the pharynx inferiorly and the sphenoid sinus superiorly require drilling the clivus [25]. As the posterior fossa dura is opened, the basilar sinus can be a source of brisk bleeding (Figs. 3.5b, 3.6e, f, 3.7d, and 3.11e, f). Inferiorly, this expanded endonasal approach can extend below the foramen magnum to the odontoid process. Straightforward drilling of the clivus allows exposure of the pons, medulla,

Fig. 3.11  Posterior perspective. (a) A wide suboccipital craniotomy has been performed, exposing the cerebellum and the transverse and sigmoid sinuses. The posterior approach also provides excellent exposure of the dorsal medulla, fourth ventricle, and distal PICA lesions, but not the cerebellopontine angle. (b) The superior posterior fossa is viewed from a posterior perspective and the superior cerebellum and quadrigeminal cistern, pineal, and posterior third ventricle are seen. These structures can be reached through a supracerebellar infratentorial approach or an occipital transtentorial approach. (c) The cerebellum has been removed and the brainstem and fourth ventricles are seen. The flocculus is found just below the middle cerebellar peduncle (MCP). It must be retracted to expose the foramen of Luschka and the root entry zone of the glossopharyngeal, vestibulocochlear, and facial nerves. PICA is seen emerging between the vagus and spinal accessory nerves to form its caudal loop between the medulla and the cerebellar tonsil (removed). (d) The brainstem has been removed to expose the anterior neurovascular structures of the posterior fossa. The vertebrobasilar junction and basilar artery are often eccentric as seen here. In extreme cases, this can cause neurovascular compression symptoms. The origin of PICA is variable, occurring anywhere along the intracranial vertebral artery and sometimes extracranial. (e) The venous sinuses and neural foramina of the posterior fossa are seen. The petrous tem-

poral bone is encircled by a ring of venous sinuses—sigmoid, superior petrosal, basilar, and inferior petrosal. The jugular foramen has major venous contributions from the petrosal sinus anteriorly and the sigmoid sinus posteriorly. The posterior fossa foramina—internal auditory, jugular, and hypoglossal—are all oriented vertically in the lateral aspect of the posterior fossa. Other nerves exiting the posterior fossa do not immediately become extracranial, but instead transition to the middle fossa into the cavernous sinus or Meckel’s cave. A dural septum separates the glossopharyngeal nerve from the vagus and spinal accessory nerve as they enter the jugular foramen. Two dural openings into the hypoglossal canal are seen. (f) The sellar region and cavernous sinus are seen from a posterior perspective. The nerves of the cavernous sinus form a funnel that empties into the superior orbital fissure (SOF). The abducens nerve (VI) is piercing the petroclival venous confluence beneath Gruber’s ligament at the posterior inferior corner of the cavernous sinus before crossing lateral to the paraclival carotid and traveling medially to the ophthalmic nerve (V1). The oculomotor nerve enters its cistern in the oculomotor triangle before joining the cavernous sinus below the anterior clinoid process. The trochlear nerve enters the cavernous sinus at the posterior superior corner, just medial to the tentorial edge. It travels below the oculomotor nerve before crossing above it at the superior orbital fissure (SOF)

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vertebrobasilar junction, basilar trunk, and abducens nerve, but exposure of the interpeduncular fossa requires traversal through the sella and mobilization of the pituitary to remove the bone of the dorsum sella (Figs. 3.6f and 3.7f). Lateral extensions of the approach through the maxillary sinus and pterygopalatine fossa permit drilling through the pterygoid process to reach the petrous apex (Fig. 3.7d, e).

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Intrajugular processes from the occipital and temporal bones protrude into the fossa and divide it into a petrosal space anteromedially and a sigmoid space posterolaterally. Furthermore, the dural leaflets over the intrajugular processes form a neural space for the lower cranial nerves 9–11, and the ganglia of the glossopharyngeal and vagus nerves. The jugular foramen thus contains three main subcompartments: a lateral sigmoid venous part (sigmoid sinus), a medial petrosal venous part (inferior petrosal sinus), Jugular Fossa and a neural intrajugular part between the first two harboring cranial nerves 9–11 (Figs. 3.5c, d Boundaries and Contents and 3.12d). Although variable, typically a venous channel passes between the ninth and tenth Rather than a two-dimensional opening in a sin- nerves to connect the petrosal part to the sigmoid gle bone, this region is a tortuous three-­ part as they unite to become the jugular bulb dimensional space, or hiatus, between the (Fig. 3.10e). Condylar emissary veins also empty temporal and occipital bones. Thus, although into the jugular bulb. Other contents of the fossa commonly known as the jugular “foramen,” it include meningeal branches of the ascending constitutes a true anatomical “fossa.” It is formed pharyngeal and occipital arteries, Jacobson’s by the petrous part of the temporal bone antero- nerve (tympanic branch of CN-9), Arnold’s laterally and the condylar part of the occipital nerve (auricular branch of CN-10), and the bone posteromedially [11] (Figs. 3.3a and 3.4a). cochlear aqueduct [11]. The jugular foramen is surrounded by the otic capsule superiorly, carotid canal anteriorly, sigmoid groove of the occipital bone posteriorly, Surgical Considerations jugular tubercle and occipital condyle medially, and mastoid air cells laterally (Figs.  3.3a, 3.4c, Depending on which border of the foramen is and 3.5c, d). The jugular tubercle lies between involved, approaches to the jugular foramen were the hypoglossal canal and the jugular foramen classified by Dr. Albert Rhoton into anterior, lat(Figs. 3.3a and 3.11e). From an extracranial per- eral, and posterior groups [11]. Also, they are comspective, the posterior border of the jugular fora- monly combined into two groups: anterolateral and men is formed by the jugular process of the posterolateral approaches. The anterior approaches occipital bone to which the rectus capitis lateralis traverse the middle and infratemporal fossa (preaumuscle attaches (Figs.  3.4c and 3.12b, c). This ricular subtemporal-infratemporal fossa approach) muscle inserts on the transverse process of the [13], and require addressing multiple barriers such atlas, separating the jugular foramen anteriorly as the mandibular condyle, temporomandibular from the vertebral artery and the “condylar fossa” joint, styloid process, and Eustachian tube; these posteriorly. Laterally, the jugular foramen region are often resected or dislocated when maximal is flanked by the mastoid tip, base of the styloid exposure is required. The tympanic and petrous process, and the facial nerve; and more superfi- portions of the temporal bones can be drilled cially by the occipital artery and digastric muscle through the floor of the middle fossa to open the (Fig.  3.12a). Anteriorly, it is bounded by the infratemporal fossa and expose the carotid artery internal carotid artery as it enters the carotid within the carotid canal as necessary (Fig.  3.9e). canal. Medially, the jugular foramen is limited by This approach is rarely used. the occipital condyle, hypoglossal canal, and recLateral approaches are more common, partictus capitis anterior muscle [11] (Fig. 3.4c). ularly the postauricular transtemporal infralaby-

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Fig. 3.12  Jugular foramen: lateral and posterolateral views. (a) Lateral view of the jugular foramen region: The parotid gland has been removed. The facial nerve is seen exiting its foramen posterior to external auditory canal and turning anteriorly above the digastric muscle to cross superficial to the styloid process before branching. The sternocleidomastoid muscle has been mobilized posteriorly. The spinal accessory nerve turns posteriorly below the transverse process of the atlas (hidden) and deep to the digastric muscle, crossing lateral to the internal jugular vein toward the medial aspect of the sternocleidomastoid muscle. It may also pass deep to the jugular vein. (b) Posterolateral view of the jugular foramen: The squamosal paracondylar bone and the occipital condyle have been removed to expose the posterior jugular bulb and internal jugular vein, which is anterior to the rectus capitis muscle.

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The nerves are seen entering the foramen medially. The occipital condyle has been drilled, and the segment of the vertebral artery that wraps around the atlantal condyle has been removed. (c) The postauricular transtemporal approach to the jugular foramen involves a mastoidectomy to expose the jugular bulb, labyrinth, and facial nerve, which is also exposed extracranially as it crosses the styloid process before entering the parotid gland. The C1 transverse process and rectus capitis lateralis form the posterior border of the extracranial jugular foramen region. (d) The jugular foramen has been exposed and the ninth and tenth nerves can be seen. The styloid process has been resected and the facial nerve has been transposed anteriorly for more exposure of the petrosal portion of the foramen and the carotid canal

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rinthine approach [11, 26, 27]. Much like the retrosigmoid approach, petrosal approaches can also be extended inferiorly with an upper cervical dissection to work above and below the jugular bulb and in the extracranial jugular foramen region [21] (Fig. 3.12c, d). This allows excellent exposure of all venous components of the jugular foramen for glomus tumors. Drilling the mastoid bone below the labyrinth exposes the superior aspect of the jugular fossa, while detaching and retracting the sternocleidomastoid muscle (SCM) and the digastric muscle uncovers the lateral wall of the jugular vein inferiorly. Further drilling of the jugular process of the occipital bone and resection of the rectus capitis lateralis can expose the posterolateral aspect of the foramen. When necessary, the styloid process and overlying muscles can also be removed to gain further access anterolaterally to the carotid artery [11] (Fig. 3.12d). The facial nerve is at risk during the mastoid drilling and extracranially as it passes anteriorly from the stylomastoid foramen, just above and deep to the posterior belly of the digastric muscle toward the parotid gland. The spinal accessory nerve courses posteriorly either deep or superficial to the jugular vein, just below the transverse process of the atlas. It is at risk when retracting the SCM or when resecting the jugular vein. The posterior group of approaches includes the retrosigmoid approach, which is helpful when the pathology involves the intracranial portion of the jugular foramen and the posterior fossa, as the lower cranial nerves would be a barrier to the lateral approach in this case (Fig. 3.10b). Drilling the petrous bone below the internal auditory meatus can widen this exposure (inframeatal extension) [28], but more commonly the far lateral approach is used (Fig.  3.10c). Drilling the occipital condyle can expose the hypoglossal canal [29], whereas drilling the jugular tubercle opens the space superior to the hypoglossal canal and just inferior and medial to the jugular foramen [30]. The paracondylar portion of the occipital bone can also be drilled to better expose the posterior border [20] (Fig. 3.12b). The vertebral artery is at risk during this exposure and can be injured when drilling the occipital condyle or

when using electrocautery along the lateral portion of the C1 lamina where it is traveling just above in the sulcus arteriosus. The posterior inferior cerebellar artery may arise extradurally from the vertebral artery and be mistaken for the posterior meningeal artery. Progress has been made in the development of endoscopic endonasal approaches (EEA) for lesions involving the ventral aspect of the jugular foramen [31–33]. In this approach, termed infrapetrous “extreme-medial” EEA, the medial pterygoid plate is drilled after exposure of the pterygopalatine fossa through a transmaxillary corridor (Chap. 45). Drilling below the Vidian nerve leads to exposure of the foramen lacerum below the internal carotid artery as it exits the petrous carotid canal (Figs.  3.4c and 3.7e, f). After the rectus capitis anterior muscle and longus capitis muscles are resected, the clivus is drilled, and the occipital condyle identified using the supracondylar groove. The occipital condyle and jugular tubercle are then drilled exposing the hypoglossal canal. Further extension of the approach can be done with resection or mobilization of the Eustachian tube (extreme-medial EEA), providing wider access to the petroclival region below the petrous bone [33] (Fig. 3.7f).

Conclusion Surgeons should understand the anatomy within each skull base compartment as well as the relationships between them, so that surgical approaches traversing multiple compartments can be easily understood and executed. For each trajectory, the surgeon must know the potential barriers and structures at risk. When possible, the fundamental skull base surgery strategy of increasing bone removal to improve exposure while minimizing neurovascular injury should be followed. In order to select the optimal trajectory for each case, both transcranial and endoscopic approaches must be considered. Modern endoscopic techniques have greatly expanded the surgical corridors available to skull base surgeons, but they require mastery of additional complex anatomy.

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References 1. Rhoton AL Jr. The anterior and middle cranial base. Neurosurgery. 2002;51(Suppl 4):273–302. 2. Morales-Valero SF, Van Gompel JJ, Loumiotis I, Lanzino G.  Craniotomy for anterior cranial fossa meningiomas: historical overview. Neurosurg Focus. 2014;36(4):E14. 3. Dehdashti AR, Ganna A, Witterick I, Gentili F. Expanded endoscopic endonasal approach for anterior cranial base and suprasellar lesions: indications and limitations. Neurosurgery. 2009;64(4):677–87. 4. Rhoton AL Jr, Natori Y. The orbit and sellar region: microsurgical anatomy and operative approaches. New  York: Thieme Medical Publishers, Inc.; 1996. p. 3–25. 5. Peris-Celda M, Perry A, Carlstrom LP, Graffeo CS, Driscoll CLW, Link MJ.  Key anatomical landmarks for middle fossa surgery: a surgical anatomy study. J Neurosurg. 2019;131(5):1561–70. 6. Pait TG, Zeal AA, Harris FS, Paullus WS, Rhoton AL Jr. Microsurgical anatomy and dissection of the temporal bone. Surg Neurol. 1977;8:363–91. 7. Diaz Day J. The middle fossa approach and extended middle fossa approach: technique and operative nuances. Oper Neurosurg. 2012;70(Suppl_2):192–201. 8. Macbeth R.  Caldwell, Luc, and their operation. Laryngoscope. 1971;81(10):1652–7. 9. Kawase T, Shiobara R, Toya S. Anterior transpetrosal-­ transtentorial approach for sphenopetro-clival meningiomas: surgical method and results in 10 patients. Neurosurgery. 1991;28:869–76. 10. Joo W, Funaki T, Yoshioka F, Rhoton AL Jr. Microsurgical anatomy of the infratemporal fossa. Clin Anat. 2013;26(4):455–69. 11. Rhoton AL Jr. Jugular foramen. Neurosurgery. 2000;47(3 Suppl):S267–85. 12. Fisch U. Infratemporal fossa approach to tumours of the temporal bone and base of the skull. J Laryngol Otol. 1978;92(11):949–67. 13. Sekhar LN, Schramm VL Jr, Jones NF. Subtemporal-­ preauricular infratemporal fossa approach to large lateral and posterior cranial base neoplasms. J Neurosurg. 1987;67(04):488–99. 14. Fortes FS, da Silva ES, Sennes LU. Mandibular subluxation for distal cervical exposure of the internal carotid artery. Laryngoscope. 2007;117(05):890–3. 15. Theodosopoulos PV, Guthikonda B, Brescia A, Keller JT, Zimmer LA.  Endoscopic approach to the infratemporal fossa: anatomic study. Neurosurgery. 2010;66(1):196–202. 16. Cavallo LM, Messina A, Gardner P, Esposito F, Kassam AB, Cappabianca P, de Divitiis E, Tschabitscher M.  Extended endoscopic endonasal approach to the pterygopalatine fossa: anatomical study and clinical considerations. Neurosurg Focus. 2005;19(1):E5. 17. Rhoton AL Jr. The cerebellopontine angle and posterior fossa cranial nerves by the retrosigmoid approach. Neurosurgery. 2000;47:93–129.

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18. Seoane E, Rhoton AL Jr. Suprameatal extension of the retrosigmoid approach: microsurgical anatomy. Neurosurgery. 1999;44(3):553–60. 19. Ribas GC, Rhoton AL Jr, Cruz OR, Peace D.  Suboccipital burr holes and craniectomies. Neurosurg Focus. 2005;19(2):E1. 20. Wen HT, Rhoton AL Jr, Katsuta T, de Oliveira E.  Microsurgical anatomy of the transcondylar, supracondylar, and paracondylar extensions of the far-­ lateral approach. J Neurosurg. 1997;87(4): 555–85. 21. AI-Mefty O, Fox JL, Smith RR.  Petrosal approach for petro-clival meningiomas. Neurosurgery. 1988;22:510–7. 22. Yasargil MG.  Microneurosurgery, vol. 2. Stuttgart: Georg Thieme Verlag; 1984. 23. Dolenc VV, Skrap M, Sustersic J, Skrbec M, Morina A. A transcavernous transsellar approach to the basilar tip aneurysms. Br J Neurosurg. 1987;1:251–9. 24. Krisht AF. Transcavernous approach to diseases of the anterior upper third of the posterior fossa. Neurosurg Focus. 2005;19:E2. 25. Stippler M, Gardner PA, Snyderman CH, Carrau RL, Prevedello DM, Kassam AB. Endoscopic endonasal approach for clival chordomas. Neurosurgery. 2009;64(2):268–77. 26. Gardner G, Cocke EW Jr, Robertson JT, Trumbull ML, Palmer RE.  Combined approach surgery for removal of glomus jugulare tumors. Laryngoscope. 1977;87(5 Pt 1):665–88. 27. Michael LM, Hamm W, Robertson JH. Surgical management of intracranial glomus tumors. In: Badie B, editor. Neurosurgical operative atlas: neuro-oncology. 2nd ed. New York: Thieme; 2006. p. 251–9. 28. Samii M, Metwali H, Samii A, Gerganov V.  Retrosigmoid intradural inframeatal approach: indications and technique. Neurosurgery. 2013;73(1 Suppl Operative):ons53–60. 29. Heros RC.  Lateral suboccipital approach for vertebral and vertebrobasilar artery lesions. J Neurosurg. 1986;64(04):559–62. 30. George B, Lot G, Tran Ba Huy P.  The juxtacondylar approach to the jugular foramen (without petrous bone drilling). Surg Neurol. 1995;44(03):279–84. 31. Vaz-Guimaraes F, Nakassa ACI, Gardner PA, Wang EW, Snyderman CH, Fernandez-Miranda JC.  Endoscopic endonasal approach to the ventral jugular foramen: anatomical basis, technical considerations, and clinical series. Oper Neurosurg (Hagerstown). 2017;13(4):482–91. 32. Zimmer LA, Hirsch BA, Kassam A, Horowitz M, Snyderman CH.  Resection of a recurrent paraganglioma via an endoscopic transnasal approach to the jugular fossa. Otol Neurotol. 2006;27(3): 398–402. 33. Simal-Julián J, Mirnda-Lloret P, Beltrán-Giner A, Plaza-Ramirez E, Botella-Asunción C.  Full endoscopic endonasal extreme far-medial approach: eustachian tube transposition. J Neurosurg Pediatr. 2013;11:584–90.

4

The Operating Room Rafael Martinez-Perez

The Operative Checklist The operative checklist has been widely implemented in Western countries as a way to avoid surgical complications [1, 2]. As described by Laws et  al. [3], operative checklists have three main goals: (1) to promote teamwork and an effective communication; (2) to ensure that basic safety checks have been performed; and (3) to ascertain that all surgical equipment is present and works properly. Prior to surgery, all members of the surgical team need to be identified by the name, as well as the role in the operating room. A recent review showed that most of the adverse outcomes in neurosurgical procedures resulted from difficulties in communication or variations in the equipment [4]. Therefore, there should be a clear communication between team members about the surgical plan, equipment, and goals of the procedure taking place. Likewise, as part of the preoperative planning, a list of the required surgical equipment needed by R. Martinez-Perez Department of Neurological Surgery, University of Colorado School of Medicine, Aurora, CO, USA e-mail: [email protected] A. S. Youssef (*) Department of Neurological Surgery, University of Colorado School of Medicine, Aurora, CO, USA Department of Otolaryngology, University of Colorado School of Medicine, Aurora, CO, USA e-mail: [email protected]

and A. Samy Youssef

the different teams should be provided to avoid delays or equipment malfunctioning. Before induction, there must be an open communication between the surgeon and anesthesia team in regard to patient’s medical condition and any additional specific precautions. For example, comorbidities associated with some skull base pathologies, such as Cushing disease or acromegaly, may have an impact on the ease of intubating the patient or accessing the sella through a transsphenoidal approach. Similarly, craniocervical junction pathologies such as chordomas may require additional precautions during positioning or intubation, to avoid spinal cord injury or brain stem compression. Ideally, awake or fiberoptic intubation may be performed. Prior to skin incision, the surgical pause should be read aloud including the type of surgery, site and side of operation, expected operative time, summary of the key steps of the operation, and possible complications pertaining to the surgical procedure. Anticipated blood requirements and potential autograft use should also be disclosed at this point.

The Skull Base Crew The Surgeon(s)  Skull base procedures are usually performed in close collaboration between neurosurgeons, otolaryngologists, head and neck

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 A. S. Youssef (ed.), Contemporary Skull Base Surgery, https://doi.org/10.1007/978-3-030-99321-4_4

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surgeons, and other surgical specialties (see Chap. 1). Coordination between surgical teams is essential to eliminate surgical errors, promote efficiency, and improve outcomes [5].

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to be reduced. Inducing Valsalva maneuver is helpful to confirm watertight dural closure. In addition, the anesthesia team is directly in charge of administering often needed intravenous contrast agents such as indocyanine green (ICG) or fluorescein as well as holding the operating table control for positional changes.

Scrub Nurse and Circulator  The surgeon coordinates the operating room setup for each procedure with a dedicated scrub team in order to optimize the use of space and surgical equipment, and reduce the amount of time needed for such setup. An experienced team of nurses and scrub technicians/nurses will become acquainted of a standard setup for each of the various procedures based on the surgeon’s style and preferences. Skull base procedures comprise so many important details that a dedicated team will become acquainted of, with time and repetition. Ideally, circulators, as well as scrub nurses, should be familiar with the equipment regularly used in skull base procedures [3]. Microscopes and endoscopes are vital for skull base procedures, and their appropriate and timely preparation is key to smooth progression of surgery. Likewise, setup of neuronavigation system and other instrument sets employed in skull base surgery require thorough knowledge and detailed training to ensure its correct functioning during the procedure.

Assisting Surgeon  Skull base surgery is founded on teamwork for various reasons. Besides the collaboration among the different surgical disciplines, having an assistant is critical. Developing surgical assisting skills is among the most important training milestones for young surgeons. Training residents and fellows to perform the surgical approach saves the senior surgeon’s energy and focus for the critical part of the procedure. Additional suction, providing cotton patties, hemostatic material, assisting with brain retraction and irrigation are among surgeon assistant’s duties. A well-rested and yet involved assistant will still be more focused to perform a final hemostasis and meticulous closure at a later phase of the procedure. A good anatomical closure will minimize postoperative complications such as cerebrospinal fluid (CSF) leak, hemorrhage, and infection.

Anesthesia Team  A dedicated neuro-anesthesia team usually comprises the attending, a resident, and/or one anesthesiologist assistant. Open communication between anesthesia and surgery teams should be maintained from the beginning until the end of surgery. Basic induction requirements such as corticosteroids, hyperosmolar agents, antiepileptic drugs, and antibiotics should be confirmed. Any anticipated intraoperative issues should be disclosed. For instance, autonomic changes can result from surgical manipulations near brain stem, trigeminal nerve/ branches, and cavernous or supraclinoid carotid artery segments [6]. In other situations, hyperventilation, use of hyperosmolar agents (e.g., mannitol, or hypertonic saline), and head elevation can be desired if intracranial pressure needs

Neuromonitoring Clinicians  Intraoperative neuromonitoring has become the standard of care in skull base procedures involving cranial nerves and brain stem [7]. Neurophysiologic monitoring techniques are cost-effective in the current paradigm of “function preservation” in skull base surgery, thus having a positive impact on minimizing morbidity and improving quality of life [8]. They have demonstrated to be safe and effective when used in both transnasal and transcranial approaches [9]. For instance, intraoperative neuromonitoring has been useful in determining intracranial blood flow, cranial nerve function, and long tract fiber integrity in the brain stem [9]. A more detailed description of the current techniques available in intraoperative neuromonitoring is provided in Chap. 7.

4  The Operating Room

Operating Room Setup Although transcranial (Fig. 4.1) and endoscopic approaches (Fig. 4.2) differ in the final arrangement of the operating room, as well as the equipment employed for each approach, there are some principles that can be followed in every setup [10]: 1. Room setup must guarantee surgeon’s optimal position in relation to the operative field (see section “Surgeon’s Position”). The surgeon needs to have a comfortable position with direct access to instruments, operative field, microscope, or endoscope with the least movements or rearrangements. 2. The anesthesiologist has to have full access to the patient’s airway, as well as to the intrave-

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Fig. 4.1 Operative room layout for a transcranial approach. (a) Setup starts prior to the patient entering the room. Anesthesia team should have direct access to the arterial and intravenous (IV) lines, as well as to the airway for a fast and effective induction and intubation. Anesthesia monitors should be preoperatively checked to avoid any delays due to malfunctioning. In the meantime, the scrub nurse is preparing surgical instruments that are going to be required during the surgical intervention. Once the patient is intubated, the bed orientation is rotated 180°, to position and fix the patient’s head, while anesthesia team checks the access to arterial and IV lines. Neuronavigation setup is performed after all lines and airway are secured. The screen of the navigation monitor is placed in the contralateral side of the craniotomy to facilitate its visualization from main surgeon’s perspective. (b)

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nous and arterial access and operating table control. In our practice, anesthesia is positioned at the foot of the table in every case in order to avoid crowding at the head. 3. Setup should anticipate the space needed for the surgeon to move around the operative field to ensure good visualization and surgical maneuverability. 4. The scrub nurse should be positioned in a way that allows unimpeded rapid exchange of instruments with the surgeon, ideally across the table from the surgeon if the assistant is on the right side of the surgeon. 5. Setup should allow comfortable access for the assistant to the endoscope or microscope. 6. Video monitors showing surgery in real-time are essential to facilitate coordination and engagement of the members of the skull base

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Left pterional (supratentorial) approach: The surgeon is seated on the left of the patient’s head. The assistant surgeon is positioned on the main surgeon’s right and the scrub nurse in front of the surgical team, on the patient’s right, to facilitate the tool exchange and direct visual contact between surgeon and scrub nurse. The microscope tower is positioned on the patient’s left and behind the main surgeon. The anesthesiologist stands at the patient feet to have direct access to the patient IV line on the left side and the airway. Neuronavigation system is placed on the patient’s right side with the screen facing the main surgeon. The display is located on the left side of the room oriented toward the scrub nurse to facilitate coordination between the scrub nurse and the surgical team, while the monitor can also be seen from the anesthesia team location. The position is reversed for a right-side craniotomy

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Fig. 4.2  Operative room layout for an endoscopic transnasal transsphenoidal approach. (a) The patient head is slightly extended and tilted to the left side, so both nostrils face the surgical team that is positioned on the patient’s right. (b) The surgeon holding the endoscope stands on the left side of the operating surgeon. The scrub nurse is situ-

ated on the patient’s left, facing the surgical team, whereas the anesthesiologist is located at the patient’s feet. The endoscope tower is placed on the opposite side of the head of the patient so it can be seen by both surgeons and the scrub nurse. The neuronavigation system with its layout is placed behind the patient head, next to the endoscope tower

team. An important video monitor is the one used by the scrub nurse, which must be placed in front of him/her, in order to be able to follow the progress of the surgery and anticipate the surgeon’s needs. A video display for anesthesia will keep the anesthesiologist updated on the surgical progress and any intraoperative complications such as uncontrolled bleeding.

• The back of the surgeon should be straight against the back rest. • The height of the chair and armrest should be adjustable to provide wide view of the surgical field from different angles. • The arms should rest relaxed with the elbow slightly flexed between 45° and 90°. • The entire forearm, from the elbow to the wrist, should be supported on the armrest, allowing only free movement of the wrist and fingers, thereby reducing any muscle fatigue. • Wrist height and hence hand position should be located right above the operative field in a slight slouched position, so that the microsurgical maneuverability is optimized. • The eyepiece of the microscope must be positioned in such a way that the surgeon does not require more than a slight cervical flexion to adapt his posture and his vision under the microscope. • A mouthpiece or a foot pedal can be used to adjust the focus and the magnification of the microscope without the need of moving the hands away from the operative field. • The foot pedals for bipolar coagulator, drill, and ultrasonic aspirator should be arranged on the chair pedestal for easy access according to the surgeon’s preferences.

Surgeon’s Position Ergonomics and maintaining a relaxed and comfortable position are very important to reduce stress, fatigue, and fine tremor during long skull base procedures (Chap. 5). The concept of ­surgical ergonomics is often neglected particularly by inexperienced surgeons. Microsurgery (Fig. 4.3)  Although many experienced neurosurgeons prefer operating while standing, most of the skull base surgeons prefer the sitting position. We prefer an armed operating chair with a pedestal in order to provide ideal surgical ergonomics. The ideal position for the operating surgeon should follow the following principles:

4  The Operating Room

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b

Fig. 4.3 (a) Surgeon’s chair: Seat and armrest height can be adjusted by foot operation. Orientation on the axial plane and inclination of the armrest can be modified to accommodate individual ergonomics. (b) Surgeon’s position: Elbow is flexed between 45° and 90°. The forearm rests on its entire extension from the elbow to the wrist

over the armrest, to facilitate delicate free movement of the hands that lie right above the operative field. The surgeon’s back rests comfortably over the backrest to reduce fatigue. Foot pedals control the operating table; bipolar or ultrasonic aspirator are placed over the chair platform

Endoscopic Endonasal Surgery (Fig.  4.2)  Endoscopic endonasal approaches are four-handed binostril techniques with synchronized surgical performance between the rhinologist and the neurosurgeon. In our single-sided technique, both the rhinologist and the surgeon stand on the right side of the patient with the scrub nurse on the opposite side. The video monitor is positioned on the opposite side of the patient head and the navigation monitor at the head of the table [11].

focused illumination and magnification of the surgical field. Modern microscopes can be equipped with augmented reality technology such as navigation, tractography, tumor mapping, indocyanine green (ICG) angiography, fluorescein filters for tumor visualization, and endoscopic assistance. The microscope is the biggest asset to the skull base surgery team. The setup and balancing of the microscope are performed before each surgical procedure. The microscope should be positioned in a way that allows freedom of movement around the head for the surgeon and assistant. The microscope stand should be placed near the foot of the table behind the surgeon. This position also helps bringing the endoscopes to the field in endoscopic-assisted microsurgery. The side tube is placed on the right side of the surgeon for supratentorial procedures and on the contralateral side to the lesion for infratentorial procedures. This setup allows the observer to assist without interfering with the

Surgical Equipment Neurosurgical equipment has evolved over decades of development and refinement. In this day and age, there are basic tools that became indispensable to every skull base surgeon. The Operating Microscope  The operating microscopes have improved visualization through

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scrub nurse’s handling instruments. The main eye piece is adjusted according to the surgeon’s preference. Subsequently, the microscope should be balanced in its final operating position. The Operating Table (Fig.  4.4)  Patient’s positioning is one of the pillars of skull base surgery (Chap. 5). Proper positioning assures patient’s safety and optimum visualization of the surgical exposure in variable skull base procedures. Skull base procedures require wider range of lateral tilt (25° right/left), height change (4500–1100 mm), horizontal sliding, and back tilt, in addition to Trendelenburg/reverse Trendelenburg position. Modern tables allow adjustment of each segment of the table and changes in positioning and rotation in three coordinates with variable speeds. Usually, these adjustments are remotely handled by the anesthesiologist, and in some new-­ generation models (e.g., Mizuho Mobile Microsurgery Table MST-7300 series, Mizuho America Inc., Union City, CA, United States) using a foot pedal. Once the surgeon chooses his/ her preferred table with specific features, it should be standardized in all skull base procedures.

R. Martinez-Perez and A. S. Youssef

undue movements during surgery in addition to the rigid immobilization needed for neuronavigation. The Mayfield® head clamp (Integra LifeSciences Corp, Plainsboro, NJ, United States) allows the attachment of tissue retractor systems. The brain retractors are often needed for deep skull base exposures by securing adequate working space for drilling near critical neurovascular structures [12]. The BUDDE® halo retractor system (Integra LifeSciences Corp, Plainsboro, NJ, United States) is attached to the head clamp prior to opening the dura in order to avoid coarse manipulations near exposed brain tissue. We prefer the BUDDE® halo retractor system (Integra LifeSciences Corp, Plainsboro, NJ, United States) due to ease of application in addition to using the ring to rest the hands near the center of surgical exposure.

Head Clamps and Retractors  The head should be immobilized in a three-point head clamp attached to the operating table. This prevents any

Neuronavigation  Preoperative planning using neuronavigation has proved to be extremely useful to guide precise surgical resection with anatomical and functional preservation. Currently, neuronavigation is an indispensable component of the standard setup of almost any neurosurgical procedure. The development of new systems and three-dimensional (3D) applied technology have allowed incorporation of navigation into microscopic or endoscopic view. The new navigation

Fig. 4.4  Operative table. Modern tables allow adjustment of each segment of the table and changes in positioning and rotation in three coordinates. Superior sliding of the table over the axial plane allows the surgeon to posi-

tion his/her legs under the table and get a position closer to the head, while it also permits the surgeon to pivot around the surgical field to improve the surgical visualization and reduce blind spots

4  The Operating Room

prototypes and virtual reality 3D models can be manipulated in infinite ways to compare various surgical approaches and combinations thereof. The neuronavigation stand is positioned at the foot of the operating table in microsurgery and head of the table in endoscopic endonasal procedures, with the screen monitor on the opposite side of the surgeon. High-Speed Drills  Bone removal by drilling is an essential operative maneuver in all skull base procedures. High-speed drills have transitioned from pneumatic to electric drills. Electric drills are now preferred for being lighter, safe, and easy to use and maintain. Cutting burrs are employed to make burr holes, reduce the bone thickness, and create the bone flap. Drilling using fine diamond is preferred for delicate bone work around neurovascular structures such as optic nerves, or facial and vestibulocochlear nerves. Cooling systems of irrigation have been incorporated in the handpiece by several manufacturers as an attempt to reduce heat spreading when drilling close to neurovascular structures, such as in the optic canal or the internal acoustic meatus [13]. The field may also be cleansed using irrigation systems integrated in the suction system [14]. The surgeon must standardize his preferred drill with the specific drill attachments and bits in his daily practice, which facilitates the setup by the scrub team. Microdissectors  Tissue dissection should be performed with anatomical and functional preservation to neurovascular structures and minimum or no bleeding. Every surgeon has personal preferences to certain microdissector tool sets. The set should be comprehensive and organized in a sequential fashion adapted to the particular task of each instrument in order of use during the procedure (see Chap. 8). Suction Cannulas  Suction tubes are used during neurosurgical procedures to clear surgical sites of fluids and aid in bimanual fine dissection around critical neurovascular structures. While popular sets include Fujita®, Fukushima®, Rhoton®, Frazier®, and other designs, we will

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highlight some of the most relevant features of suction cannulas to skull base procedures. We prefer Fukushima-design® suction tubes (Aesculap Inc., Hazelwood, MO, United States) with fine suction control due to tear-drop-shaped suction interrupter. They can be individually adapted due to bendable material. The conical suction tube design with distal atraumatic tip prevents plugging. The suction cannula set should be versatile with at least four different lengths and eight diameters (3–12 French). Bipolar Forceps  Bipolar forceps are designed to grasp, manipulate, dissect, and coagulate selected tissue. The electric current alternates between the two tips, thus reducing the length of the current’s path and producing a precise, controlled therapeutic effect [15]. We prefer Spetzler-Malis® disposable nonstick bayonet bipolar forceps (Stryker, Kalamazoo, MI, United States) in three different lengths (7, 8, and 9 inches) and tip diameters (0.5, 1, and 1.5 millimeters). The slim ergonomic bayonet design is appropriate for tailored deep surgical approaches. In the case of large vascular tumors such as meningiomas, new bipolar devices using radiofrequency energy and saline irrigation (Aquamantys ™, Medtronic, Minneapolis, MN, United States) provide superior hemostatic control on soft tissues and bone, without producing smoke or char that are encountered with other devices that operate at a higher temperature. In our experience, the Aquamantys device reduces operative time and blood loss in large skull base meningioma surgery. Endoscopic endonasal and keyhole procedures will require different sets of bipolar forceps with slim longer bayonet design in order to maximize unobstructed visibility and prevent scissoring of the tips. In our practice, we use the SILVERglide® bipolar forceps set (Stryker, Kalamazoo, MI, United States) with different lengths and tip sizes for endonasal and keyhole procedures. Microdebriders  The microdebrider system consists of a rotary vacuum dissection that can be used for controlled removal of soft tissues in a cramped surgical field [16]. The advantages of this tool, which can simultaneously remove nasal

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mucosa at a high speed and suction fluids, include decreased operative times and improvement of visualization in the nasal cavity during endonasal procedures [16]. Given the high suction and speed of powered instrumentation, its use must be restricted to the sinonasal stage to avoid injury of the orbit, dura mater, brain tissue, or neurovascular structures.

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and prevents any inadvertent injury [12, 25]. In addition, “brain shift” and changes in anatomy caused by the surgical procedure itself can impair the accuracy of navigation, requiring additional precision tools to confirm the location of relevant vascular structures.

Indocyanine Green Angiogram  Indocyanine green (ICG) is a cyanide fluorescent dye that can Ultrasonic Aspirators  Ultrasonic aspirators use be visualized under near-infrared light and has ultrasonic frequency vibrations generated by a been used in several medical diagnostic modalipiezoelectric element to fragment tissue, while ties. ICG is known to provide high-resolution continuous irrigation emulsifies dissected parti- images in real-time of the arterial, capillary, and cles and aspiration removes them from the field. venous flow of cerebral vasculature. In cerebroSimultaneous use of a selective dissection, irriga- vascular surgery, it is used to assess intraoperation and suction, results in rapid tumor removal tive arterial patency and blood flow, in addition to while preserving the surrounding tissues with a optimum aneurysm clip position under microclean surgical field [17]. Special attachments are scopic view. Its use can be extended to skull base adapted for firm and fibrous tumors such as surgery, to help visualization of vascular strucmeningiomas. More recently, new ultrasonic tures in open and endoscopic procedures for aspirators couple longitudinal vibration with tor- tumors that may involve major arteries. In addisional oscillation [18]. These properties can tion, ICG has been demonstrated to be useful in emulsify bone with a high level of precision, endoscopic transnasal approaches to determine while avoiding temperature increase and subse- the location of important vascular structures (e.g., quent nerve heat injury that is produced by high-­ internal carotid artery or the cavernous sinus), speed drills [19]. Therefore, ultrasonic aspirators assess the viability of the nasoseptal flap, visualhave been employed to safely perform complex ize the superior and inferior hypophyseal arteries, approaches, including transcavernous approach, or differentiate between normal and tumor tissue anterior clinoidectomy, or internal acoustic canal [26–28] decompression [20–24]. Long slim tips facilitate transnasal access through narrow surgical corridors. Recent reports have demonstrated its appli- Conclusion cability for treating lesions in the anterior and middle cranial fossae via expanded endoscopic The skull base operating room includes highly endonasal approaches. specialized staff, advanced technology equipment, and very detailed special surgical instruDoppler Ultrasound  The Doppler ultrasound ments. The coordination between operating room probe helps to determine the blood flow in cere- staff and the different operating teams ensures bral blood vessels and its role has been estab- smooth flow of the procedure, reduces the risk of lished in cerebrovascular surgery. In skull base complications, and hence improves the overall surgery, it is also used to determine proximity to success of the operation. critical vascular structures such as the internal carotid artery in both microsurgical and endo- Disclosure  Funding: This study did not receive any scopic endonasal procedures. For example, in an funding relative to its elaboration. Conflict of interest: ASY is a consultant for Stryker expanded endonasal transpterygoid approach or Corp and has received royalty from Mizuho America. during drilling of the petrous apex through a subEthical approval and informed consent (to participate temporal approach, Doppler ultrasound is used to and for publication): Informed consent and ethical assess the distance to the internal carotid artery approval were not deemed necessary by the local ethics in

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12. Martinez-Perez R, Silveira-Bertazzo G, Carrau RL, Prevedello DM.  The importance of landmarks in endoscopic endonasal reinterventions: the transpterygoid transcavernous approach. Acta Neurochir. 2020;162(4):875–80. 13. Tsuzuki T, Yanai H, Kukita C, Samejima H, Shinyama Y.  Irrigation system equipped on a high-speed air drill--technical note. Neurol Med Chir (Tokyo). References 1998;38(3):173–4. 14. Abe T, Satoh K, Wada A.  Optic nerve decompression for orbitofrontal fibrous dysplasia: recent devel 1. Semel ME, Resch S, Haynes AB, Funk LM, Bader opment of surgical technique and equipment. Skull A, Berry WR, et  al. Adopting a surgical safety Base. 2006;16(3):145–55. checklist could save money and improve the quality of care in U.S. hospitals. Health Aff (Millwood). 15. Greenwood J Jr. Two point coagulation: a follow­up report of a new technic and instrument for elec2010;29(9):1593–9. trocoagulation in neurosurgery. Arch Phys Ther. 2. de Vries EN, Eikens-Jansen MP, Hamersma 1942;23(9):552–4. AM, Smorenburg SM, Gouma DJ, Boermeester 16. Tang D, Lobo BC, D’Anza B, Woodard TD, MA.  Prevention of surgical malpractice claims Sindwani R. Advances in microdebrider technology. by use of a surgical safety checklist. Ann Surg. Otolaryngol Clin N Am. 2017;50(3):589–98. 2011;253(3):624–8. 3. Laws ER, Wong JM, Smith TR, de los Reyes K, 17. Ledderose GJ, Thon N, Rachinger W, Betz CS. Use of an ultrasonic aspirator in transnasal surgery of Aglio LS, Thorne AJ, et  al. A checklist for endonatumorous lesions of the anterior skull base. Interdiscip sal transsphenoidal anterior skull base surgery. JNS. Neurosurg. 2019;18:100545. 2016;124(6):1634–9. 4. Wong JM, Bader AM, Laws ER, Popp AJ, Gawande 18. Vernon D, Lobo BC, Ting JY.  Application of ultrasonic aspirators in rhinology and skull base surgery. AA.  Patterns in neurosurgical adverse events and Otolaryngol Clin N Am. 2017;50(3):607–16. proposed strategies for reduction. Neurosurg Focus. 19. Tarazi N, Munigangaiah S, Jadaan M, McCabe 2012;33(5):E1. JP.  Comparison of thermal spread with the use of 5. McLaughlin N, Carrau RL, Kelly DF, Prevedello DM, an ultrasonic osteotomy device: Sonopet ultraKassam AB. Teamwork in skull base surgery: an avesonic aspirator versus misonix bonescalpel in spinue for improvement in patient care. Surg Neurol Int. nal surgery. J Craniovertebr Junction Spine. 2018 2013;4:36. Mar;9(1):68–72. 6. Chowdhury T, Mendelowith D, Golanov E, Spiriev T, Arasho B, Sandu N, et al. Trigeminocardiac reflex: 20. Campero Á, Tovar L, Ajler P.  Resection of a dumbbell skull base meningioma by a combined two-staged the current clinical and physiological knowledge. J retrosigmoid and transzygomatic transcavernous Neurosurg Anesthesiol. 2015;27(2):136–47. approach. J Neurol Surg B Skull Base. 2019;80(Suppl 7. Starnoni D, Giammattei L, Cossu G, Link MJ, Roche 3):S298–9. P-H, Chacko AG, et al. Surgical management for large 21. Glauser G, Choudhri OA.  Microsurgical clipping vestibular schwannomas: a systematic review, meta-­ of ophthalmic aneurysms in an endovascular era: analysis, and consensus statement on behalf of the Sonopet-assisted intradural clinoidectomy and other EANS skull base section. Acta Neurochir [Internet]. tenets. World Neurosurg. 2019;126:398. 2020 [cited 2020 Sep 27]. Available from: http://link. 22. Henzi S, Krayenbühl N, Bozinov O, Regli L, Stienen springer.com/10.1007/s00701-­020-­04491-­7. MN.  Ultrasonic aspiration in neurosurgery: com 8. Wang EW, Zanation AM, Gardner PA, Schwartz parative analysis of complications and outcome TH, Eloy JA, Adappa ND, et  al. ICAR: endoscopic for three commonly used models. Acta Neurochir. skull-base surgery. Int Forum Allergy Rhinol. 2019;161(10):2073–82. 2019;9(S3):S145–365. 9. Singh H, Vogel RW, Lober RM, Doan AT, Matsumoto 23. Kohlberg GD, Lipschitz N, Tawfik KO, Walters Z, Breen JT, Zuccarello M, et  al. Application of ultraCI, Kenning TJ, et al. Intraoperative neurophysiologisonic bone aspirator for decompression of the internal cal monitoring for endoscopic endonasal approaches auditory canal via the middle cranial fossa approach. to the skull base: a technical guide. Scientifica (Cairo). Otol Neurotol. 2019;40(1):114–20. 2016;2016:1751245. 24. Modest MC, Carlson ML, Link MJ, Driscoll 10. Lehecka M, Laakso A, Hernesniemi J, Çelik Ö. CLW.  Ultrasonic bone aspirator (Sonopet) for Helsinki microneurosurgery basics and tricks. meatal bone removal during retrosigmoid craniM. Lehecka, A. Laakso and J. Hernesniemi: Helsinki; otomy for vestibular schwannoma. Laryngoscope. 2011. 2017;127(4):805–8. 1 1. Castelnuovo P, Pistochini A, Locatelli D.  Different surgical approaches to the sellar region: focusing on 25. Martínez-Pérez R, Hernández-Álvarez V, Maturana R, Mura JM.  The extradural minipterional prethe “two nostrils four hands technique”. Rhinology. temporal approach for the treatment of spheno-­ 2006;44(1):2–7. view of the design of the study. This study does not receive financial support. Availability of data and material (data transparency): This manuscript has not been previously published in whole or in part or submitted elsewhere for review.

70 petro-­ clival meningiomas. Acta Neurochir. 2019;161(12):2577–82. 26. Hide T, Yano S, Shinojima N, Kuratsu J. Usefulness of the indocyanine green fluorescence endoscope in endonasal transsphenoidal surgery. J Neurosurg. 2015;122(5):1185–92. 27. Inoue A, Kohno S, Ohnishi T, Nishida N, Suehiro S, Nakamura Y, et  al. Tricks and traps of ICG endos-

R. Martinez-Perez and A. S. Youssef copy for effectively applying endoscopic transsphenoidal surgery to pituitary adenoma. Neurosurg Rev. 2021;44(4):2133–43. 28. Jeon C, Hong C-K, Woo KI, Hong SD, Nam D-H, Lee J-I, et  al. Endoscopic transorbital surgery for Meckel’s cave and middle cranial fossa tumors: surgical technique and early results. J Neurosurg. 2018;1:1–10.

5

Surgical Positioning Robert S. Heller, Siviero Agazzi, and Harry R. Van Loveren

Introduction Summoned in 1987 by the Congress of Neurological Surgeons (CNS), we created the first-ever Sunday Practical Course at the national meeting in Baltimore, Maryland. At the behest of our Chairman and Past-President of the CNS, Dr. John Tew, we focused on one of the “hot topics” of the time and designed Positioning for Success. A Practical Course on Surgical Positioning for Neurosurgery (Fig.  5.1). Our premise was that improper positioning before the start of any operation could negatively affect outcomes. We believed this was particularly true in neurosurgery at that time when increasingly complex positions were being devised for cranial procedures for brain tumors and vascular lesions. Surgeons needed to be facile in positions beyond just supine that included prone, park-bench, sitting, Concorde, and true-lateral. During our first Positioning for Success course, we laid the foundation for our philosophy on position that later evolved to become the Rule of Five. In this chapR. S. Heller · S. Agazzi Department of Neurosurgery and Brain Repair, University of South Florida, Tampa, FL, USA e-mail: [email protected]; [email protected] H. R. Van Loveren (*) Department of Neurosurgery and Brain Repair, University of South Florida Health Morsani College of Medicine, Tampa, FL, USA e-mail: [email protected]

ter, we present these time-tested strategies in a building-block concept to create an approach to patient positioning that remains highly adaptable for today’s procedures. Fast-forward more than 30 years and surgical positioning has become more complex and nuanced than ever. New surgical approaches and technologies encompass the entire spectrum of skull base surgery. Techniques such as endoscopy, tubular surgery, frameless guidance, and intraoperative imaging have made surgical positioning even more complex and critical. Meanwhile, the tolerance for complications and poor outcomes has dramatically diminished. The Rule of Five comprises the five components of surgical positioning that address position of the patient’s head and body on the table, the surgeon, personnel in the room, and the equipment.

Concepts in Surgical Positioning Surgeons often err by focusing on the “one” position as the head and the trajectory to target. However, our strategy to assess final position before draping ensures that both the patient and the surgeon are in a position of comfort before beginning the operation. Preceding a discussion on optimal positioning, one must ask: How does surgical positioning affect outcome? The answer to this question is more definitive in the negative

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 A. S. Youssef (ed.), Contemporary Skull Base Surgery, https://doi.org/10.1007/978-3-030-99321-4_5

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Fig. 5.1  Positioning for Success: A Practical Course on Surgical Positioning for Neurosurgery. Booklet from the first practical course at the Annual Meeting of the Congress of Neurological Surgeons (Baltimore, MD, 1987)

with risks of injury, compromise of the target, and surgeon fatigue. First, poor patient positioning can result in injury to the patient. Torsion and compression of the jugular veins in the neck can increase venous pressure and lead to catastrophic brain swelling when not recognized and handled properly. Excessive neck flexion and head rotation can lead to acute sialadenitis [1]. Improper neck position

can result in spinal cord compression and injury in patients with unrecognized cervical spine disease. Inadequate padding of pressure points on the body can result in nerve palsies or pressure sores. Tension on joints can result in ligamentous injuries. Inadequate eye coverage can lead to corneal abrasions or burns from sterilization solutions. Although most of these injuries are considered minor, they constitute a major source

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of malpractice litigation because they are unusual, unexpected, and easily avoided—some of the main criteria for a claim of malpractice. Second, more direct effects on outcomes occur when surgical positioning compromises exposure of the surgical target. For example, when the head is insufficiently flexed at the neck, exposure of lesions in the foramen magnum becomes nearly impossible. When the head is rotated too far beyond 45°, clinoidectomy becomes increasingly difficult. Positioning of the head in relation to the thorax is crucial: level of head below the heart increases venous bleeding whereas thorax elevated too high increases the risk of air embolism. Third, the most insidious risk of poor surgical positioning is surgeon fatigue. Poor trajectory and poor exposure of the surgical target force the surgeon into uncomfortable positions. The physical demands on the surgeon in these positions can lead to cervical strain, back strain due to leaning over the field, and muscular fatigue associated with outstretched arms or hands elevated above the elbows. In a survey of neurosurgeons, 73% reported work-related musculoskeletal disorders partially attributable to long procedures and operating with poor ergonomics [2]. Maintaining proper ergonomics is crucial to optimizing microsurgical performance [3]. In long cases, these surgeon discomforts insidiously affect judgement and decision making. Tired surgeons are more likely to cut corners, take risks, or prematurely abort tumor resection. Our mantra when assessing the final position before draping ensures that both the patient and the surgeon are in a position of comfort before the procedure begins. For early career neurosurgeons, the nuances of appropriate head position can be found in textbooks, atlases of neurosurgery, and online expert videos. As experience is gained both in the cadaveric laboratory and operating room, the surgeon can simply close their eyes to envision the head position and surgical approach and forecast with relative accuracy the success of the patient position. Time in the dissection laboratory is not critical to every approach, but is an accelerant to experience, and therefore is valuable to the surgeon and patient.

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The Rule of Five To achieve the goal of successful patient positioning, we attend to the five components of surgical positioning: (1) the patient’s head and (2) body on the table, (3) the surgeon, (4) personnel in the room, and (5) the equipment.

Component 1: The Patient’s Head The head should be rotated to the side necessary to expose to the pathology. Adjust the head position relative to each axis in the X, Y, and Z planes. Ensure that its position keeps the craniotomy elevated for drainage of venous blood. There should be a direct line of sight from the surgeon’s eyes through the microscope to the target, while maintaining the surgeon’s head in comfortable position. When applying the three-pin head holder, always seat the two pins of the rocker arm first. If the single pin side is seated first, it can seat itself at high torque pressure with only one of the pins on the rocker arm. The resulting “floating pin” effect on the rocker arm can be a risk for intraoperative change in head position or significant scalp laceration. Additionally, place at least one pin below the horizon of the head so that the weight of the head rests on the head holder and is not suspended from it. To optimize pin position, we use to the “sweatband” technique (Fig. 5.2). By visualizing where a sweatband would be worn around the patient’s head, the surgeon can create a mental image of where to position each pin. Placement above the sweatband near the vertex of the head risks the pins being seated at an angle that could later slip whereas pins placed below the sweatband risks traversing too much tissue and not being fully seated into the skull.

Component 2: The Body Check the tension of the patient’s neck. If relaxed, no adjustments are required. If a bit of tension is found in the neck, a shoulder roll may be needed.

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a

b

Fig. 5.2 Positioning the Mayfield skull clamp. (a) Visualize a sweatband around the patient’s head. (b) Position the dependent side of head on the two-pin rocker arm. Align the single pin to bisect the intersection of sagittal and coronal axis of the skull. All three pins should be in the sweatband zone. Turn the single pin clockwise until

pressure gauge shows three tension rings (60  lb/in2). Avoid placing single pin in middle of forehead to reduce the risk of penetrating the frontal sinus. Watch for signs of perforation of the thin temporal squama bone. (Printed with permission Mayfield Clinic)

If neck rotation is outright restricted, consider changing the patient position to one that allows more rotation (e.g., lateral position). Ensure thumbs are facing forward when tucking arms and hands. Keep the elbows and knees slightly flexed to avoid long hours of extension. The torso should lie in a slight reverse Trendelenburg position to facilitate venous drainage, but the head should not be more than 20 cm above the heart.

Adjust the microscope so that the eyepieces are at the level of your eyes looking comfortably forward. Do not let the microscope take your head position hostage; your eyes and head must be in comfortable working position for many hours. If you use a mouthpiece, it should rest comfortably in your mouth when looking through the eyepieces. You should be able to depress the mouthpiece without moving your head away from the eyepieces. Adjust the handles of the microscope so they are easily gripped without fatigue or outstretched arms to avoid unbalancing the microscope [4]. Determine where to rest your hands during the operation. We more often use the Budde Halo retractor as a place to rest our hands than for actual retraction. Steadying your hand and wrist on the patient or retractor system has been shown to reduce surgeon tremor [5].

Component 3: The Surgeon Have a proper adjustable chair. Sit on it comfortably. Test the chair before you begin the operation. Bring the operating table to your level of comfort; do not bring yourself to the level of the table. Operate at the level of your elbows or below to avoid arm fatigue.

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Component 4: The Personnel in the Room Position your assistant on your right side. Your left hand often holds a suction in a relatively fixed position resting on the head and blocks entry of the assistant’s hands to the operative field. Therefore, placing the assistant on your right avoids this conflict. Switch these positions if you are left-handed. Generally, station the scrub nurse on your right at the patient’s chest. This position allows the nurse to directly place instruments into your extended hand; this eliminates your need to look away from the microscope or to cross your field of vision with your hand or forearm. Position anesthesia at the foot of the table away from the field. Position monitoring personnel in the corner of the room.

Component 5: The Equipment Place your equipment so that it is simultaneously out of the way of the operation and accessible when needed. The base of the microscope should be at or near 90° with the vertical articulating arm to maintain maximum microscope maneuverability. Cautery, suction, and ultrasonic aspirators should be placed on the opposite side of the patient from the scrub nurse. Navigation screens should be easily seen by just a slight turn of the head. Foot pedals should be within comfortable reach while operating without the need to change position in your chair. Communicate with the staff in the operating room so you have access to the pedals you need and move unnecessary pedals away.

Modeling the Rule of Five Through Case Examples The general principles of surgical positioning for skull base surgery are best demonstrated by modeling specific cases. Given the nature of

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skull base neurosurgery, one or more components of patient positioning may be more important than others to a particular patient or surgery. We chose these cases because the intricacies of each enable discussion of positioning in many of its nuances.

Pre-sigmoid Approach In “building-block” vernacular, the pre-sigmoid approach consists of a temporal craniotomy with anterior and posterior petrosectomies and can include sacrifice of the semicircular canals. The patient’s head (Component 1) should be rotated opposite the side of the tumor to the degree that the sagittal sinus is parallel to the floor. The head is also tilted down toward the floor until the zygomatic root is the highest point in the field (Fig. 5.3). This creates a comfortable line of sight through the subtemporal approach to Kawase’s triangle to the posterior fossa, medial to the temporal lobe, and through the mastoid space into the cerebellopontine angle. Failure to rotate the sagittal sinus parallel to the floor leads to the surgeon extending their own neck to look upward through the mastoid space. Insufficient tilting of the head will compromise the view medial to the temporal lobe. When assessing the body (Component 2), start by checking the tension of the neck. If relaxed, no adjustments are required. If a bit of tension is found in the neck, an ipsilateral shoulder roll is required. If neck rotation is outright restricted, the patient must be placed park-bench. Tuck the arms but ensure the thumbs are up, which is the position of comfort for arms. Place a pillow under the knees to avoid hours of hyperextension. Elevate the thorax to enhance venous drainage but not more than 20 cm above the heart, which risks air embolism. Make sure you, as the surgeon, will be comfortable during the surgery (Component 3). Adjust both the height of your chair and the position of the arm rests. Bring the operating table to a level that is appropriate.

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a

b

Fig. 5.3 Patient positioned supine for pre-sigmoid approaches. (a) Rotate the head contralateral to the tumor until the sagittal sinus is parallel to the floor. Place a shoulder roll beneath the ipsilateral shoulder to decrease tension on the neck. If tension cannot be relieved, the patient is placed in the “park-bench” position. (b) Tilt the head toward the floor so that the zygomatic root becomes

the highest point in the operative field. Note that the bottom pin on the three-pin head clamp is below the horizon and weight-bearing. Tuck the arms at the patient’s side with their hands in the natural “thumbs-up” position. (Modified from Tew et  al. [6]. Used with permission © Mayfield Clinic)

Next, plan the location of the personnel in the room (Component 4). This discussion is for a right-handed surgeon. Position your assistant on your right side (Fig. 5.4). A surgeon on your right is a good assistant. A surgeon on your left is an observer. Generally, place the scrub nurse on your right at the patient’s chest. Position anesthesia at the foot of the table away from the field. Finally, plan the location of the equipment in the room (Component 5). The microscope should come from your left so you and your assistant can comfortably work. The cautery sources and suction should be out of the way, either at the foot of the bed or opposite the scrub nurse.

In this approach, we prefer the patient’s head resting on a horseshoe headrest or soft donut-­ shaped head holder rather than a rigid pin-­fixation system (Fig.  5.5). The head (Component 1) is slightly extended and slightly rotated toward the primary surgeon for better line of sight. The body (Component 2) is supine with all pressure points padded, arms tucked at their side, and hands in a thumbs-up position. The right-handed surgeon (Component 3) will stand at the patient’s right shoulder (Fig.  5.6). Elevation of the table so that the surgeon can work with forearms at or below the level of the elbows will help to avoid arm fatigue and facilitate manual dexterity. The surgeon should not need to lean over the patient to comfortably work. Position the assistant (Component 4) at the patient’s left shoulder or head to drive the endoscope while the surgeon operates with bimanual technique. When positioned at the patient’s left side next to the assistant, the scrub nurse can pass instruments to the surgeon’s right hand with a simple rotation of the surgeon’s arm laterally

Endonasal Endoscopic Approach Another common approach in skull base surgery is the endonasal endoscopic approach. In “building block” vernacular, this can include transsphenoidal, expanded transsphenoidal, transoral, transmaxillary, and transpterygoid components.

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Fig. 5.4  Operating room (OR) setup for a skull base procedure using the surgical microscope and neuronavigation. Personnel are stationed to ensure the most advantageous and efficient positions for the surgeon, assistant, and scrub nurse. For an unobstructed line of

sight for image-guided surgery (IGS), place the monitors and infrared arrays are alongside the bed for clear viewing by surgeon. Anesthesia and neuromonitoring are located at the foot of the bed. (Modified from Tew et al. [6]. Used with permission © Mayfield Clinic)

from the field of view. Position anesthesia at the foot of the bed and neuromonitoring personnel in the corner of the room.

Both the surgeon and the assistant should have endoscope monitors in their direct line of sight at eye level (Component 5). The stereotactic-­

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a

b

Fig. 5.5  Patient position for a skull base endoscopic procedure. (a) The head rests comfortably on a padded horseshoe headrest with a frameless-stereotactic reference placed on the forehead. (b) The head is cocked away from

and simultaneously rotated toward the surgeon for a comfortable line of sight. (Modified from Tew et al. [6]. Used with permission © Mayfield Clinic)

guidance monitor can be positioned between the two endoscope monitors. All other equipment cords come to the field from the foot of the bed.

thought process in attending to the five components of surgical positioning. Rather than giving a blueprint for every possible skull base surgery case, we showed how the Rule of Five can be applied as a scaffold in select cases. This surgical positioning strategy facilitates the physical ­comfort of every member of the team, protects the patient, and avoids complications of either direct injury or indirect consequences of surgeon fatigue.

Summary Patient positioning in skull base surgery is paramount to a successful operation. Our goal in this chapter was to give the reader insights about our

5  Surgical Positioning

Fig. 5.6  OR room setup for a skull base endoscopic procedure. The surgeon stands at the patient’s right shoulder and the assistant at the left shoulder. The scrub nurse stands at the patient’s left side just below the assistant for ease of handing instruments to the surgeon’s right hand.

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Both surgeon and assistant have endoscope monitors in their direct line-of-sight at eye level. Ferromagnetic emitter on edge of bed. (Modified from Tew et al. [6]. Used with permission © Mayfield Clinic)

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References 1. Naylor RM, Graffeo CS, Ransom RC, Carlstrom LP, May MM, Carlson ML, et al. Acute sialadenitis after skull base surgery: systematic review and summative practice recommendations. World Neurosurg. 2021. https://doi.org/10.1016/j.wneu.2021.02.124. 2. Gadjradj PS, Ogenio K, Voigt I, Harhangi BS. Ergonomics and related physical symptoms among neurosurgeons. World Neurosurg. 2020;134:e432– e41. https://doi.org/10.1016/j.wneu.2019.10.093. 3. Belykh E, Onaka NR, Abramov IT, Yagmurlu K, Byvaltsev VA, Spetzler RF, et  al. Systematic review of factors influencing surgical performance: practical recommendations for microsurgical procedures in

R. S. Heller et al. neurosurgery. World Neurosurg. 2018;112:e182–207. https://doi.org/10.1016/j.wneu.2018.01.005. 4. Shimizu S, Kuroda H, Mochizuki T, Kumabe T.  Ergonomics-based positioning of the operating handle of surgical microscopes. Neurol Med Chir (Tokyo). 2020;60(6):313–6. https://doi.org/10.2176/ nmc.rc.2020-­0018. 5. Fargen KM, Turner RD, Spiotta AM.  Factors that affect physiologic tremor and dexterity during surgery: a primer for neurosurgeons. World Neurosurg. 2016;86:384–9. https://doi.org/10.1016/j. wneu.2015.10.098. 6. Tew JM, van Loveren HR, Keller JT. Atlas of operative microneurosurgery, vol. 2. London: W.B.  Saunders; 2001.

6

Cranial Nerve Functional Preservation: Tricks of the Trade Rafael Martinez-Perez

Introduction When facing a skull base lesion, the choice of the most appropriate surgical approach depends on several factors including pathology, goal(s) of treatment, patient’s anatomy, and surgeon’s skills/experience, in addition to preoperative clinical and functional status [1]. The ideal surgical approach is one that maximizes surgical efficiency while minimizing the manipulation of important neurovascular elements and thus, postoperative morbidities [2]. We like to emphasize the concept of approach selection in functional preservation skull base surgery as the rule of 3-S: safe, simple, and straightforward. • Safe: Avoidance of narrow working corridors and crossing relevant neurovascular structures. • Simple; simplicity over complexity: Combining and/or staging two simple

R. Martinez-Perez Department of Neurological Surgery, University of Colorado School of Medicine, Aurora, CO, USA e-mail: [email protected] A. S. Youssef (*) Department of Neurological Surgery, University of Colorado School of Medicine, Aurora, CO, USA Department of Otolaryngology, University of Colorado School of Medicine, Aurora, CO, USA e-mail: [email protected]

and A. Samy Youssef

approaches is sometimes superior to one major complex approach. • Straightforward: Selection of the most direct approach that enhances the surgical exposure and maneuverability [3].

 asic Microsurgical Principles B of Functional Preservation The skull base surgeon should develop and master fine microsurgical skills in order to navigate complex anatomy and dissect challenging pathologies around critical neurovascular structures. An important basic principle is the maintenance of a clean and bloodless surgical field [4] to enable good visualization of important anatomical structures. However, diathermy coagulation should be minimized near cranial nerves in order to avoid heat spreading and thermal injury to adjacent cranial nerves; in such case, irrigating bipolar forceps can be used. In addition, hemostasis can be achieved using hemostatic materials including oxidized cellulose, Floseal (Baxter, Deerfield, IL, USA) or fibrin glue, (Beriplast® ZLB Behring, King of Prussia, PA, USA) in case of cavernous sinus surgery. We find the Delicot ultrathin cotton patties (American Surgical Company, Salem, USA) very useful in covering and protecting cranial nerves during dissection and diathermy bipolar coagulation. Irrigation with warm lactated Ringer’s solution should be employed to flush out

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 A. S. Youssef (ed.), Contemporary Skull Base Surgery, https://doi.org/10.1007/978-3-030-99321-4_6

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any blood clots from the operative field and enhance visualization. Lactated Ringer’s solution shares more similarities with normal CSF in terms of pH, osmolality, and inorganic ions content. It has been reported by various authors as a safer alternative to normal saline for brain irrigation and safer on neural tissue especially during long procedures [5]. Respecting natural barriers, cleavage planes, and arachnoid layers are all helpful maneuvers. Starting from normal anatomy is always a safe strategy to identify neurovascular structures and guide the microsurgical dissection in an anatomically distorted area by the tumor. Surgical manipulation of cranial nerves should be minimized and retraction should be avoided. Sharp dissection using microscissors or sharp microdissectors in arachnoid planes around cranial nerves is key to minimize such manipulations and preserve nerve microvasculature. Early bony decompression of cranial nerves as in the internal acoustic canal or the optic canal decreases surgical manipulations of already compromised nerves. Similarly, section of dural folds (e.g., external dural ring, oculomotor triangle, tentorial incisura, falciform ligament) expands the working area around tumor/nerve interface and enhances the functional preservation of anatomically intact cranial nerves.

Cranial Nerves by Anatomic Region Neurophysiologic monitoring has become a fundamental part of every skull base procedure (Chap. 7). Cranial nerve monitoring should be done whenever feasible in each anatomic region as a real time measure of functional integrity and prediction of postoperative outcomes.

Anterior Cranial Fossa Olfactory Apparatus Olfaction assessment and preservation should be a major goal in the management of anterior mid-

R. Martinez-Perez and A. S. Youssef

line skull base lesions (Chap. 17). A comprehensive approach that includes a smell test (University of Pennsylvania Smell Identification test - UPSIT) and a tailored surgical approach selection based on olfactory function and tumor size can improve olfactory outcomes [6]. In general, endonasal approaches are not favorable for subfrontal lesions if olfaction is intact due to the risk of damaging the olfactory nerves during surgical exposure. Approach selection: As part of our functional preservation philosophy, we opt for the most direct route that does not transgress the olfactory tracts. A frontolateral approach is generally preferred over the bifrontal craniotomy to achieve olfaction preservation in medium-sized lesions (2–4 cm) [6, 7], as the ipsilateral olfactory tract is visualized early and preserved during dissection, and the contralateral olfactory tract can be dissected free from the tumor after partial debulking. The bifrontal approach provides a wide surgical corridor and allows early tumor devascularization, although it has been associated with lower olfaction preservation outcomes [8]. Large and bilateral lesions (>4 cm) [7] are better approached through a tailored unilateral frontal approach with orbitotomy for minimal retraction on the orbitofrontal cortex and olfactory tracts, in addition to facilitating bilateral cribriform plate access. Small midline extra-axial lesions with unilateral origin of the anterior cranial fossa can be approached through an endoscopic endonasal route. To preserve the olfactory function, we recently described the unilateral transcribriform approach with septal mobilization, in which the contralateral olfactory apparatus is preserved [6, 9]. Dissection technique: Sharp extra-arachnoidal dissection is highly encouraged, in order to preserve nerve blood supply while untethering the olfactory tract from the tumor capsule. Direct retraction on the olfactory tracts should be avoided and gentle protection with moist cotton patties is done instead. If the tumor involves both cribriform plates, near total resection with unilateral preservation of the dominant olfactory bulb (right side) should be considered in order to preserve olfaction.

6  Cranial Nerve Functional Preservation: Tricks of the Trade

Optic Nerve A complete ophthalmological examination that includes visual acuity, visual fields, and optical coherence tomography (OCT) is warranted for all parasellar lesions. The optic nerve can be compressed in the optic canal from below and medial aspect, such as in a pituitary adenoma, tuberculum sellae meningioma and in most of craniopharyngiomas; from below and the lateral aspect, such as in carotid-ophthalmic aneurysms; from the lateral aspect, such as in some sphenoid ridge meningiomas; or along its entire circumference, such as in type C clinoidal meningiomas [10], optic sheath meningiomas, or fibrous dysplasia affecting the optic canal [10–13]. Approach selection: Removal of the anterior clinoid process through a transcranial approach is usually performed to decompress the optic canal [14]. However, recent development of endoscopic techniques has prompted the utilization of other approaches to decompress the optic canal, such as the transorbital or endonasal routes [15–17]. Although the transcranial approach has demonstrated to provide the widest surgical maneuverability and decompression of the optic canal in three quarters of the circumference (as opposed to other routes that only allow decompression of 180°) [18], the endoscopic endonasal optic decompression has demonstrated to be successful when the optic nerve compression is mainly over the medial aspect of the nerve, such as in small tuberculum sellae meningiomas. Lesions that are primarily below the optic apparatus such as pituitary adenomas and craniopharyngiomas should be approached through an endonasal route. When the optic nerves are superiorly compromised due to an anterior midline skull base lesion, a unilateral transcranial approach on the worse vision side is preferred to avoid manipulations of the better functioning optic nerve. In such case, using the drill, ipsilateral optic nerve decompression by unroofing the optic canal is performed prior to tumor resection. Midline lesions inferomedial to the optic nerves can be approached endonasally, with bilateral optic canal decompression by drilling the medial wall.

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Dissection technique: There should be minimal if any manipulations to the optic apparatus. Blood supply from the anterior communicating complex to the dorsum of optic chiasm and from the superior hypophyseal arteries to the inferior surface should be preserved in transcranial and endonasal procedures, respectively. Drilling of the optic canal and anterior clinoid process should be performed with diamond burs under continuous irrigation to avoid thermal injury. The falciform ligament should be sharply severed medial and lateral to the optic nerve for decompression. Whenever possible, the arachnoid layer should be preserved during dissection near the optic apparatus.

Middle Cranial Fossa  culomotor, Trochlear, Trigeminal, O and Abducens Nerves Along their course from the brain stem to the middle fossa and superior orbital fissure, the oculomotor, trochlear, abducens, and trigeminal nerves can be approached through a middle [19–23] or a posterior fossa approach [24, 25]. Recently, Kassam et  al. described the anterior route to approach the Meckel’s cave through an endoscopic endonasal expanded approach [26]. Also, endonasal approaches provide good exposure of the medial and anterior compartments of the cavernous sinus [27–29]. Approach selection: Cavernous sinus lesions directly impact cranial nerves III, IV, and VI in addition to the first division of the trigeminal nerve (V1) that are located in the lateral cavernous sinus. Lesions involving the cavernous sinus can be holocavernous, lateral cavernous sinus wall, or extracavernous with secondary cavernous sinus invasion such as pituitary adenomas or meningiomas. Holocavernous meningiomas infiltrate cranial nerves and the ICA adventitia and surgery does not have a major role beyond cavernous sinus decompression. In cases where lesions extend above the petrous ridge and infiltrate the superior aspect of the Meckel’s cave,

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these are commonly approached through a middle fossa approach. In middle fossa lesions involving the Meckel’s cave, such as trigeminal schwannomas, we perform early decompression of the trigeminal nerve, by performing an anterior petrosectomy, ligation, and division of the superior petrosal sinus through an extradural subtemporal corridor [30, 31] (Chap. 27). For lesions extending to the cavernous sinus with visual symptoms derived from the compression of the optic nerve such as petroclival-sphenocavernous meningiomas, a pretemporal approach can be employed [20, 32]. Petroclival and cerebellopontine angle (CPA) meningiomas, or schwannomas are usually intimate to the trigeminal nerve, and may compress the abducens, trochlear, and even oculomotor nerve. A transcranial lateral skull base approach offers the most direct access for early cranial nerve decompression through a large shallow corridor for superior surgical maneuverability [33, 34]. Distal V2 and V3 lesions in the retromaxillary or infratemporal regions are directly approached through and endoscopic endonasal approach (Chap. 54). Dissection technique: Mobilization of the outer layer of the lateral wall of the cavernous sinus is performed in a blunt fashion after sharply splitting the temporo-periorbital ligament except at 4 points: the oculomotor trigone, trochlear, and V2 and V3 nerves where sharp dissection is warranted. This maneuver offers decompression of cranial nerves III–VI.  Dissection of the superior orbital fissure should be avoided in order to avoid nerve injury. The blood supply of the cranial nerves comes the cavernous ICA from the medial side and any devascularizing maneuvers should be avoided especially from a medial approach. Similar to the lateral wall of the cavernous sinus, the Gasserian ganglion and trigeminal nerve roots have two layers of dura on their dorsolateral surface: an inner or visceral layer from the posterior fossa dura propria that constitutes the dorsolateral wall of the cavernous sinus and Meckel’s cave, and an outer or parietal layer from the dura propria of the middle fossa. The cleavage plane between these two layers continues distally as the cleavage plane between the epineural sheaths of the trigem-

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inal divisions and the dura propria of the middle fossa [21]. Such plane is accessed after sharply severing the periosteal dura at the foramen ovale and foramen rotundum. This is followed by blunt dissection along V2 and V3 upward and posteriorly toward Meckel’s cave. In a middle fossa approach, dural elevation is performed from posterior to anterior in order to avoid avulsion injury of the greater superficial petrosal nerve (GSPN). During tentorial splitting, the superior petrosal sinus (SPS) is ligated and sharply severed between two hemoclips. Care must be taken not to cut through the trigeminal nerve root, which is directly below the sinus. When the cut is extended medially to the tentorial hiatus, the trochlear nerve should be identified and preserved before making the final cut. Preserving the arachnoid layer of the middle incisural space is key to anatomically preserving the trochlear nerve.

Posterior Cranial Fossa Facial and Vestibulocochlear Nerves The facial and vestibulocochlear nerves are mostly impacted by CPA tumors. The most common CPA tumors are vestibular schwannomas [30] and meningiomas [31]. The level of difficulty in cranial nerve dissection is determined by several factors such as tumor consistency, vascularity, and adherence to neurovascular structures. Tumor consistency varies widely, and although some studies suggest that this feature can be predicted preoperatively [32], tumor characteristics are usually an intraoperative finding. Approach selection: Small intracanalicular vestibular schwannomas (Koos I) [33] with normal or mildly impaired hearing function are approached through a middle fossa approach (Chap. 30). Tumors extending to the CPA cistern (Koos II–IV) [33] are approached through a translabyrinthine or a retrosigmoid approach (Chap. 38). Preserving cochlear nerve function remains a challenging enterprise in patients with normal hearing function. The retrosigmoid approach is versatile and can be used for hearing/cochlear

6  Cranial Nerve Functional Preservation: Tricks of the Trade

nerve preservation in tumors of all sizes. When hearing preservation is not the goal, a translabyrinthine approach has the advantage of early exposure and preservation of the facial nerve in tumors that are above the jugular bulb with significant anterior prepontine extension. The cochlear nerve can be anatomically p­ reserved for possible hearing restoration with cochlear implant [34]. Meningiomas of the CPA are directly approach through a retrosigmoid exposure. For petroclival meningiomas, see Chap. 37. Dissection technique: Intraoperative cranial nerve monitoring includes direct electrical stimulation of the facial nerve with the Nerve Integrity Monitor (NIM) nerve monitoring system (Medtronic, Minneapolis, MN, USA) and continuous monitoring of the auditory brainstem responses. We use a vestibular nerve preservation technique which minimizes direct manipulations on the facial and cochlear nerves. Starting at the inferomedial part of the tumor, the cochlear nerve is identified and we sharply establish a plane between the tumor and the vestibular nerve using microscissors and sharp-angled microdissectors. Medial to lateral dissection is pursued in the already established plane between the tumor and the vestibular nerve fascicles. The vestibular nerve is used as an insulating layer to protect the underlying cochlear and facial nerves. As the vestibular nerve starts to separate distally into the superior and inferior divisions, the tumor nerve starts to be thinned out. In contrast, the uninvolved nerve is relatively preserved. The facial nerve is always visualized at the brainstem and distally identified using electrical stimulation of the premeatal segment after tumor debulking [35]. We achieved higher hearing preservation rate (70%) and normal facial nerve function (100%) using this technique in small to medium sized vestibular schwannomas [36]. The nervus intermedius can be identified at the brain stem between the superior vestibular and facial nerves. Preservation of this nerve should be attempted as post-treatment dysfunction in the form of dry eye, excessive lacrimation, or loss of taste has been reported in up to 75% of cases [37–40]. We believe that the vestibular nerve preservation

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technique is helpful in anatomically preserving the nervus intermedius with better functional preservation in our practice [37]. If dissection of the last part of the tumor from the facial nerve is not feasible due to extreme adherence to the nerve, near total resection by leaving a minimal residual tumor on the nerve should be decided before losing the nerve conduction signal [38]. In a middle fossa approach, the dissection is performed from medial to lateral on the inferior vestibular nerve. The most adherent points between tumor and nerves are recognized and handled last under direct vision when there is sufficient space to allow manipulation of the tumor [39]. In the rare event of the facial nerve being severed, nerve graft procedures are attempted during the same operation [39, 40].

 ower Cranial Nerves: L Glossopharyngeal, Vagus, Accessory, and Hypoglossal Lower cranial nerves are involved in tumors of the jugular foramen region such as paragangliomas, meningiomas, schwannomas, chondrosarcomas, and chordomas [1, 41]. Lower cranial nerves dysfunction can cause significant disability from vocal cords and swallowing dysfunction, which may result in long stay in the intensive care unit, acute care, and rehabilitation facilities [42, 43]. A thorough preoperative examination of lower cranial nerves is warranted. Formal swallowing evaluation and laryngoscopic vocal cord assessment should be performed to determine the pretreatment functional status [42, 44]. Approach selection: The surgical approach should be determined not only by the functional status of lower cranial nerves, but also the position of the tumor in relation to the neural structures [37]. Where the tumor is pointing most, determines the access approach. Thus, the endonasal far medial approach is favored for tumors that are anteromedial to the lower cranial nerves and pointing to the nasopharynx; while a posterolateral approach is preferred for tumors that are pointing posterolaterally to the temporal bone

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and/or posterior fossa [2]. In the case of extensive tumors surrounding the lower cranial nerves, a multiportal staged approach combining the aforementioned techniques is favored in order to avoid transgressing the cranial nerves which leads to postoperative nerve dysfunction [2]. Dumbbell-­ shaped jugular schwannomas with intact lower cranial nerves function are ideally treated with resection of the intracranial component and surveillance with possible radiotherapy of the ­ extracranial component. Paragangliomas are successfully treated with radiotherapy when the lower cranial nerves function is intact [38, 39]. Large lesions may require maximum safe resection for cranial nerve preservation with postoperative adjuvant radiotherapy for residual tumors [40] (Chap. 48). Dissection technique: Preservation of the arachnoid planes whenever feasible is extremely relevant in posterior fossa surgery. Sharp dissection is needed to avoid traction injury to the lower cranial nerves. Minimizing the use of the diathermy bipolar coagulation and gentle irrigation are keys to avoid thermal injury.

Conclusion Cranial nerve function is a major determinant of post-treatment quality of life in patients with skull base tumors. Pretreatment cranial nerve assessment and intraoperative neurophysiologic monitoring should be performed. Functional preservation should be the goal in the selection of treatment modality, surgical approach, and intraoperative technical maneuvers. Disclosure  Funding: This study did not receive any funding relative to its elaboration. Conflict of interest: ASY is a consultant for Stryker Corp and has received royalty from Mizuho America. Ethical approval and informed consent (to participate and for publication): Informed consent and ethical approval were not deemed necessary by the local ethics in view of the design of the study. This study does not receive financial support. Availability of data and material (data transparency): This manuscript has not been previously published in whole or in part or submitted elsewhere for review.

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6  Cranial Nerve Functional Preservation: Tricks of the Trade 1 3. Drake CG, Vanderlinden RG, Amacher AL.  Carotid-ophthalmic aneurysms. J Neurosurg. 1968;29(1):24–31. 14. Martinez-Perez R, Albonette-Felicio T, Hardesty DA, Carrau RL, Prevedello DM.  Outcome of the surgical decompression for traumatic optic neuropathy: a systematic review and meta-analysis. Neurosurg Rev. 2021;44(2):633–41. 15. Horiguchi K, Murai H, Hasegawa Y, Mine S, Yamakami I, Saeki N.  Endoscopic endonasal trans-­ sphenoidal optic nerve decompression for traumatic optic neuropathy--technical note. Neurol Med Chir (Tokyo). 2010;50(6):518–22. 16. Goldberg RA, Steinsapir KD.  Extracranial optic canal decompression: indications and technique. Ophthalmic Plast Reconstr Surg. 1996;12(3):163–70. 17. Di Somma A, Andaluz N, Gogela SL, Cavallo LM, Keller JT, Prats-Galino A, et al. Surgical freedom evaluation during optic nerve decompression: laboratory investigation. World Neurosurg. 2017;101:227–35. 18. Gogela SL, Zimmer LA, Keller JT, Andaluz N.  Refining operative strategies for optic nerve decompression: a morphometric analysis of transcranial and endoscopic endonasal techniques using clinical parameters. Oper Neurosurg (Hagerstown). 2018;14(3):295–302. 19. Sun DQ, Menezes AH, Howard MA, Gantz BJ, Hasan DM, Hansen MR.  Surgical management of tumors involving Meckel’s cave and cavernous sinus: role of an extended middle fossa and lateral sphenoidectomy approach. Otol Neurotol. 2018;39(1):82–91. 20. Martínez-Pérez R, Hernández-Álvarez V, Maturana R, Mura JM.  The extradural minipterional pretemporal approach for the treatment of spheno-­ petro-­ clival meningiomas. Acta Neurochir. 2019;161(12):2577–82. 21. Youssef S, Kim E-Y, Aziz KMA, Hemida S, Keller JT, van Loveren HR.  The subtemporal interdural approach to dumbbell-shaped trigeminal schwannomas: cadaveric prosection. Neurosurgery. 2006;59(4 Suppl 2):ONS270–277; discussion ONS277–278. 22. Tripathi M, Deo RC, Suri A, Srivastav V, Baby B, Kumar S, et  al. Quantitative analysis of the Kawase versus the modified Dolenc-Kawase approach for middle cranial fossa lesions with variable anteroposterior extension. J Neurosurg. 2015;123(1):14–22. 23. Peris-Celda M, Perry A, Carlstrom LP, Graffeo CS, Driscoll CLW, Link MJ.  Key anatomical landmarks for middle fossa surgery: a surgical anatomy study. J Neurosurg. 2018:1–10. 24. Yang A, Kunigelis K, Youssef AS.  Retrosigmoid approach to the posterior fossa trigeminal schwannoma. J Neurol Surg B Skull Base. 2018;79(Suppl 5):S393–4. 25. Kunigelis KE, Craig D, Yang A, Gubbels S, Youssef AS.  Presigmoid approach to dumbbell trigeminal schwannoma. J Neurol Surg B Skull Base. 2018;79(Suppl 5):S391–2.

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26. Kassam AB, Prevedello DM, Carrau RL, Snyderman CH, Gardner P, Osawa S, et  al. The front door to Meckel’s cave: an anteromedial corridor via expanded endoscopic endonasal approach- technical considerations and clinical series. Neurosurgery. 2009;64(3 Suppl):ons71–82; discussion ons82–83. 27. Cohen-Cohen S, Gardner PA, Alves-Belo JT, Truong HQ, Snyderman CH, Wang EW, et  al. The medial wall of the cavernous sinus. Part 2: selective medial wall resection in 50 pituitary adenoma patients. J Neurosurg. 2018;131(1):131–40. 28. Ferrareze Nunes C, Lieber S, Truong HQ, Zenonos G, Wang EW, Snyderman CH, et al. Endoscopic endonasal transoculomotor triangle approach for adenomas invading the parapeduncular space: surgical anatomy, technical nuances, and case series. J Neurosurg. 2018:1–11. 29. Martinez-Perez R, Hardesty DA, Silveira-Bertazzo G, Carrau RL, Prevedello DM.  Bony landmarks in the endoscopic endonasal transoculomotor approach. Neurosurg Rev. 2021;44(5):2717–25. 30. Starnoni D, Giammattei L, Cossu G, Link MJ, Roche P-H, Chacko AG, et  al. Surgical management for large vestibular schwannomas: a systematic review, meta-­analysis, and consensus statement on behalf of the EANS skull base section. Acta Neurochir [Internet]. 2020 [cited 2020 Sep 27]. Available from: http://link.springer.com/10.1007/ s00701-­020-­04491-­7. 31. Harvey SA, Haberkamp TJ.  Pitfalls in the diagnosis of CPA tumors. Ear Nose Throat J. 1991;70(5):290–8, 303–4. 32. Rizk AR, Adam A, Gugel I, Schittenhelm J, Tatagiba M, Ebner FH.  Implications of vestibular schwannoma consistency: analysis of 140 cases regarding radiologic and clinical features. World Neurosurg. 2017;99:159–63. 33. Koos WT, Day JD, Matula C, Levy DI.  Neurotopographic considerations in the microsurgical treatment of small acoustic neurinomas. J Neurosurg. 1998;88(3):506–12. 34. Dahm V, Auinger AB, Honeder C, Riss D, Landegger LD, Moser G, et al. Simultaneous vestibular schwannoma resection and cochlear implantation using electrically evoked auditory brainstem response audiometry for decision-making. Otol Neurotol. 2020;41(9):1266–73. 35. Aref M, Kunigelis K, Cass SP, Youssef AS.  Retrosigmoid approach for vestibular schwannoma. J Neurol Surg B Skull Base. 2019;80(Suppl 3):S271. 36. Labib MA, Inoue M, Banakis Hartl RM, Cass S, Gubbels S, Lawton MT, et  al. Impact of vestibular nerve preservation on facial and hearing outcomes in small vestibular schwannoma surgery: a technical feasibility study. Acta Neurochir [Internet]. 2021 [cited 2021 Jan 7]. Available from: http://link.springer. com/10.1007/s00701-­020-­04678-­y.

88 37. Ditzel Filho LFS, Prevedello DM, Dolci RL, Jamshidi AO, Kerr EE, Campbell R, et  al. The endoscopic endonasal approach for removal of petroclival chondrosarcomas. Neurosurg Clin N Am. 2015;26(3):453–62. 38. Patel AK, Rodríguez-López JL, Hirsch BE, Burton SA, Flickinger JC, Clump DA. Long term outcomes with linear accelerator stereotactic radiosurgery for treatment of jugulotympanic paragangliomas. Head Neck. 2021;43(2):449–55. 39. Fatima N, Pollom E, Soltys S, Chang SD, Meola A. Stereotactic radiosurgery for head and neck paragangliomas: a systematic review and meta-analysis. Neurosurg Rev. 2021;44(2):741–52. 40. Borba LAB, Araújo JC, de Oliveira JG, Filho MG, Moro MS, Tirapelli LF, et  al. Surgical management of glomus jugulare tumors: a proposal for approach selection based on tumor relationships with the facial nerve. J Neurosurg. 2010;112(1):88–98.

R. Martinez-Perez and A. S. Youssef 41. Ramina R, Maniglia JJ, Fernandes YB, Paschoal JR, Pfeilsticker LN, Neto MC, et  al. Jugular foramen tumors: diagnosis and treatment. Neurosurg Focus. 2004;17(2):E5. 42. Mesquita Filho PM, Ditzel Filho LFS, Prevedello DM, Martinez CAN, Fiore ME, et  al. Endoscopic endonasal surgical management of chondrosarcomas with cerebellopontine angle extension. Neurosurg Focus. 2014;37(4):E13. 43. Mohyeldin A, Prevedello DM, Jamshidi AO, Ditzel Filho LFS, Carrau RL.  Nuances in the treatment of malignant tumors of the clival and petroclival region. Int Arch Otorhinolaryngol. 2014;18(Suppl 2):S157–72. 44. Raza SM, Gidley PW, Kupferman ME, Hanna EY, Su SY, DeMonte F.  Site-specific considerations in the surgical management of skull base chondrosarcomas. Oper Neurosurg (Hagerstown, MD). 2018;14(6):611–9.

7

Neurophysiologic Monitoring Rafael Martinez-Perez, Angela Downes, and A. Samy Youssef

Introduction Intraoperative neurophysiologic monitoring has played an important role in modern skull base surgery. For instance, facial nerve electromyogram (EMG) not only helps to identify the nerve anatomically, but can also preserve the functional integrity and helps to predict the postoperative functional outcome of the nerve [1]. The introduction of routine intraoperative facial nerve monitoring has significantly reduced the incidence of facial nerve paresis in a postoperative anatomically intact nerve [1–3]. Brain stem auditory evoked potentials (BAEPs) are widely used

R. Martinez-Perez Department of Neurological Surgery, University of Colorado School of Medicine, Aurora, CO, USA e-mail: [email protected] A. Downes Department of Neurological Surgery, University of Colorado, Aurora, CO, USA e-mail: [email protected] A. S. Youssef (*) Department of Neurological Surgery, University of Colorado School of Medicine, Aurora, CO, USA Department of Otolaryngology, University of Colorado School of Medicine, Aurora, CO, USA e-mail: [email protected]

for intraoperative monitoring of cochlear nerve and brain stem function, and as a means of hearing preservation during cerebellopontine angle surgery. Likewise, the different patterns of BAEP have been identified and correlated with postoperative hearing outcome [4]. Somatosensory (SSEP) and motor evoked potentials (MEPs) have been useful to anticipate poor functional outcomes in patients undergoing surgery of the craniocervical junction, such as in foramen magnum meningiomas [5]. With the refinement of microsurgical techniques and surgeons accumulating more experience with skull base surgery, the goals of surgery are set at a higher level. Postoperative anatomical and functional integrity of cranial nerves during skull base surgery is the ultimate goal of intraoperative nerve monitoring. The early collaboration of neurophysiologists, neurotologists, and neurosurgeons in skull base surgery blossomed the introduction of comprehensive intraoperative neurophysiologic monitoring, which significantly increased the likelihood of successful preservation of cranial nerve function. In this chapter we aim to review the different techniques of intraoperative neurophysiologic monitoring in skull base surgery, and identify the clinical impact of certain pathognomonic patterns on postoperative outcomes. We will highlight the role of postoperative medications in improving delayed cranial nerve dysfunction in the different reported series.

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 A. S. Youssef (ed.), Contemporary Skull Base Surgery, https://doi.org/10.1007/978-3-030-99321-4_7

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Basic Requirements for Intraoperative Monitoring

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sensitive to olfactory stimulant [8, 9]. The olfactory stimulant (H2S or CO2) is delivered with a constant concentration applied by means of a Anesthesia-induced physiological variations may computer-controlled air-dilution olfactometer [7, affect intraoperative monitoring reliability. EMG 10]. The outlet of the stimulator is placed in the monitoring and all different subtypes of evoked nostril, while the exhaust gas is evacuated through potentials are vulnerable to physiological changes a medical scavenging vacuum pipeline. OEPs are since baseline-recoding patterns (amplitude and then recorded using scalp electroencephalogram latency of the action potential) may be affected or (EEG) electrodes. Reference and ground elecaltered. Long-acting paralytics and certain neuro-­ trodes are placed at the vertex. Raw recordings active inhaled anesthetics should be minimized, are finally filtered and processed to translate into as they may inhibit EMG and cochlear nerve the actual reading. Although still in the experiaction potentials (CNAPs) recording. Nitrous mental phase, delays in latency and decrease in oxide and Isoflurane may be used at low doses signal amplitude have been proposed as potential with concomitant narcotics to maintain anesthe- predictors of hyposmia [11]. The detection of sia [6]. Induction should be performed with fast OEPs is accessible to monitoring, but technically acting neuro-muscular blockade agents. Twitch challenging: the complex setup and the long monitor during EMG monitoring is used to con- duration it takes to obtain a meaningful stimulafirm reversal of paralysis prior to intraoperative tion limit its use in skull base surgery on a regular monitoring of cranial nerves. Although paralytics basis. may improve SSEP recordings by reducing muscle artifact, neuromuscular blockade should allow, at least, 2 out of 4 twitches to provide a Optic Nerve (II) reliable reading of the EMG during the MEP, or Open and endoscopic anterior skull base surgery other cranial nerves monitoring. procedures are generally performed close to the pre-chiasmatic visual apparatus, and clear strategies for detecting and handling visual pathway Cranial Nerve Monitoring damage are essential. To overcome the limitation of a missed clinical examination during skull Olfactory Nerve (I) base surgery, flash visual evoked potentials have Intraoperative monitoring of chemical sense been proposed by some as a technique to intraop(smell and taste) has been recently introduced as eratively assess the integrity of the visual pathpart of the surgical armamentarium of anterior way [12–14]. cranial fossa lesions and skull base malignancies [7]. Although its efficacy to preserve olfactory Visual Evoked Potentials (VEPs) function is yet to be proven, intraoperative olfac- VEPs are the electrical potentials initiated by tory evoked potentials have shown promising visual stimuli recorded from electrodes on the results in terms of feasibility for neuromonitoring scalp over the visual cortex [15]. VEP recording of olfactory function during neurosurgical proce- requires three electrodes: the mid-occipital elecdures. Monitoring this sense could be very use- trode is placed right above the external occipital ful, notably for the preservation of olfactory protuberance (inion), and the lateral occipital function during anterior cranial fossa approaches electrodes are placed 4 cm laterally on each side of the mid-occipital electrode [16]. VEP wavein which the olfactory tracts are manipulated. forms are extracted by averaging electroencephalogram (EEG) signals. The primary role of VEP Olfactory Evoked Potentials (OEPs) OEPs are elicited by chemical stimulation of the has been in the diagnosis of demyelinating dishuman olfactory and respiratory nasal mucosa, eases [12]. VEP have been proved to provide

7  Neurophysiologic Monitoring

some benefit in protecting the visual pathways during tumor removal in the anterior cranial fossa and parasellar region [17–19]. Both the reduction in amplitude of the evoked potentials and the increase in latency have been proposed as indicators of a reduction in function [13, 14, 20]. However, its efficacy and reliability to correlate intraoperative findings and postoperative visual outcomes have remained questionable by several authors [12, 15, 21–23]. Among the limitations of using VEP in an operative setting, extreme variability of VEP responses ­secondary to the waveforms’ sensitivity to volatile anesthetics, the interpersonal variability, and inefficient delivery of the stimulus, are the biggest concerns [15, 17, 24]. To date, development of new techniques to deliver the visual stimulus is still required to implement the routine use of VEP in skull base surgery.

 culomotor, Trochlear, Trigeminal, O and Abducens Nerves (III, IV, V, and VI) The functional and structural integrity of the trigeminal and extraocular cranial nerves is at risk during skull base surgery of the cavernous sinus, superior orbital fissure, or petroclival region [25, 26]. Likewise, the trigeminal and abducens nerves can be monitored during surgery of the cerebellopontine angle, such as in some large vestibular schwannomas with anterior and superior extension [27]. The use of intraoperative monitoring of trigeminal and extraocular cranial nerves has been widely used for treating lesions with intracavernous extension [27–29]. Beyond the aesthetic concerns, diplopia following intraoperative injury to extraocular cranial nerves will have a serious impact on the patient’s quality of life, since it often results in monocular vision and loss of stereoscopic vision. Likewise, functional preservation of the trigeminal nerve is key to prevent some complications such as sensory deficits, most seriously corneal ulceration, dysesthesia, trigeminal neuralgia, or mastication dysfunction [30, 31].

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 lectromyogram (EMG) E Intraoperative EMG monitoring of the muscles innervated by the extraocular cranial nerves (the levator palpebrae superioris muscle and/or inferior rectus muscle for the purpose of oculomotor nerve monitoring, the superior oblique muscle to monitor the trochlear nerve, and the lateral rectus muscle to monitor the abducens nerve) is a very efficient measure to avoid postoperative transient or permanent diplopia caused by cranial nerve injuries [32]. Monitoring of the trigeminal nerve is performed through EMG monitoring of the masseter muscle, which is innervated by the mandibular nerve (V3) [27]. Electrophysiologic monitoring of the oculomotor, abducens, and trigeminal nerves is relatively easy to perform. However, monitoring of the trochlear nerve is technically cumbersome due to the anatomic difficulty of applying a recording electrode to the superior oblique muscle [27]. Continuous EMG monitoring of spontaneous muscle activity and intraoperative direct stimulation of cranial nerves by a uni-/bipolar probe are regularly employed to identify the nerve and assess functional status after tumor resection. While neurotonic discharges have a limited value in predicting the postoperative function of extraocular cranial nerves, the onset latency of muscle action potential longer than 2.5 ms after tumor removal is probably relevant to determine the risk of suffering postoperative oculomotor, trochlear, or abducens dysfunction [33, 34]. However, a definite quantitative correlation between the amplitude of the muscle action potential and the postoperative functional outcomes of extraocular cranial nerves has yet to be proven.  lectrooculogram (EOG) E The EOG is a record of the changes in electrical potential between the cornea and the retina as the eye moves between two fixed points [35]. The EOG has been employed for the assessment of oculomotor abnormalities such as nystagmus, strabismus, and supranuclear oculomotor dysfunction [36, 37]. More recently, the EOG has been demonstrated to be a valuable monitoring

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tool to prevent postoperative extraocular motor nerve dysfunction [32]. An EOG is recorded using surface electrodes placed around both eyes. EOG records eye movements by measuring electrical potential differences between two electrodes. This takes advantage of the fact that the human eye is an electrical dipole consisting of a positively charged cornea and a negatively charged retina [38]. When the extraocular motor nerves are stimulated mechanically, directly, or indirectly during surgical intervention, abnormal extraocular ­muscle responses appear on the monitor screen of the EOG [32].

Facial Nerve (VII) The facial nerve (VII) is particularly vulnerable during surgical approaches to cerebellopontine angle (CPA) tumors (i.e., vestibular schwannoma and meningiomas) or microvascular decompression of posterior fossa cranial nerves. The facial nerve can be anatomically preserved in most cases; however, 20–70% of patients suffer from postoperative facial nerve functional deficits [1]. Resection of CPA lesions, in particular vestibular schwannoma, has benefitted the most from facial nerve monitoring. Facial nerve paresis significantly impairs patients’ quality of life and may require several cosmetic and palliative procedures (Chap. 13).

 lectromyogram (EMG) E Parallel pairs of non-insulated needle electrodes are placed percutaneously in the lateral orbicularis oculi, the nasal muscle, and the orbicularis oris, thus providing continuous EMG monitoring. Two different modalities of EMG signals can be monitored: first, the spontaneous muscle activity; and second, compound muscle action potentials (CMAPs) that is obtained during facial nerve stimulation. The facial nerve stimulation probe is used for direct electrical stimulation to localize the nerve prior to tumor resection and intermittently during dissection of tumor from the nerve. Responses are conveyed on a digital oscilloscope and by a loudspeaker projected

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across the operating room to the surgeon [39]. After resection of a vestibular schwannoma, the probe is used to elicit the stimulation threshold at the brain stem and medial to the tumor dissection. The lowest stimulation threshold can be elicited by first applying 0.05 mA and increased in 0.05-mA increments until response amplitude is obtained. The response amplitude achievable with the lowest stimulation threshold is recorded and can be used as a predictor of postoperative facial nerve function [40–42]. There is a wide variability between intraoperative EMG monitoring protocols reported in the literature. The type of stimulating device (monopolar vs. bipolar, insulated vs. non-insulated) will generate distinct CMAP waveforms, with variations in amplitude and morphological features [43]. Thus, the reported value of the response amplitude predicting the functional outcome is inconsistent across the literature [43–45]. However, a drop-off in the response amplitude between the brain stem and the porus acusticus of less than 70% has been demonstrated to be a good predictor of facial functional recovery (House Brackman 10% increase in the latency compared with the baseline wave [74].  otor Evoked Potentials (MEPs) M MEP are generated by transcranial electrical stimulation using surface or subdermal needle electrodes on the scalp, or direct electrical stimulation on the brain [66]. MEP monitor motor pathways as transcranial electrical stimulation

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elicits excitation of corticospinal projections at multiple levels [66]. Electrodes are placed percutaneously at the midline and both sides of the vertex over the motor cortex. Evoked potentials are measured over the spinal cord below the surgery level or in the muscle of interest. Distally, the evoked response is measured in the abductor hallucis for the lower extremities and bilateral thenar muscles in the upper extremities [75]. Compound muscle action potentials are routinely measured due to their sensitivity, specificity, and minimal invasiveness [66]. Abnormal MEP is defined as the disappearance of the wave [73–75].

 edical Management to Enhance M Functional Preservation Ambitions to improve skull base surgery functional outcomes are more than simply predicting such outcomes [1]. In addition to surgical tricks and technical nuances, several drugs have been suggested to have a positive effect in the long-­term functional status of cranial nerves. Animal studies have proven the neuroprotective effect of the calcium channel blocker nimodipine as it improves nerve resprouting, promotes axonal growth and remyelination, and reduces polyneuronal innervations of the target muscles [76–78]. Clinical trials based on medicating patients who develop reversible pathognomonic intraoperative BAEP patterns or A-train EMG activity with a combination of intravenous hydroxyethyl starch (HES), a vasoactive agent, and nimodipine, have shown improvement in long-term functional results [79–82]. Delayed postoperative functional decline of the facial nerve can commonly occur in the setting of intact intraoperative recordings. Some authors have proposed that this complication is due to viral reactivation, suggesting the treatment with antiviral agents, such as acyclovir or valacyclovir [45]. In our practice, we use 7-days perioperative Valtrex (oral Valacyclovir) in all vestibular schwannoma cases. Also, edema and microvascular constriction have been suggested as an etiology for delayed dysfunction.

Perioperative steroids and calcium channel blockers (i.e., nimodipine) have been shown to improve outcomes in some studies. Scheller et al. [81, 83] suggest that prophylactic administration of a combination of neuroprotective vasoactive agents (nimodipine + HES) is superior to their intraoperative administration. However, most recent clinical trials do not show a clear benefit, and the administration of a short course of postoperative steroids has been a standard practice by most surgeons [84, 85].

Conclusion Preservation of neural function while obtaining maximal grade of tumor resection has become the main goal in skull base surgery. Neurophysiologic intraoperative monitoring enhanced functional preservation and improved postoperative outcomes. The identification of different pathognomonic patterns in the EMG, EEG, or evoked potentials has been correlated with postoperative functional outcomes. Along with the intraoperative monitoring, perioperative administration of nimodipine, steroids, or HES has shown promising results to improve functional outcomes in skull base surgery, although further clinical trials are still needed to standardize their use. Disclosure  Funding: This study did not receive any funding relative to its elaboration. Conflict of interest: ASY is a consultant for Stryker Corp and has received honorarium from Mizuho America. Ethical approval and informed consent (to participate and for publication): Informed consent and ethical approval were not deemed necessary by the local ethics in view of the design of the study. This study did not receive financial support. Availability of data and material (data transparency): This manuscript has not been previously published in whole or in part or submitted elsewhere for review.

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98 a treatment algorithm to avoid long-term morbidity. Acta Neurochir. 2018;160(4):681–8. 32. Kawamata T, Ishii N, Amano K, Namioka T, Hori T, Okada Y.  A novel simple real-time electrooculographic monitoring system during transsphenoidal surgeries to prevent postoperative extraocular motor nerve dysfunction. Neurosurg Rev. 2013;36(3):371–6. 33. Hariharan P, Balzer JR, Anetakis K, Crammond DJ, Thirumala PD.  Electrophysiology of extraocular cranial nerves: oculomotor, trochlear, and abducens nerve. J Clin Neurophysiol. 2018;35(1):11–5. 34. Li Z-Y, Li M-C, Liang J-T, Bao Y-H, Chen G, Guo H-C, et  al. Usefulness of intraoperative electromyographic monitoring of oculomotor and abducens nerves during skull base surgery. Acta Neurochir. 2017;159(10):1925–37. 35. Müller JA, Wendt D, Kollmeier B, Brand T. Comparing eye tracking with electrooculography for measuring individual sentence comprehension duration. PLoS One. 2016;11(10):e0164627. 36. Ingster-Moati I, Bui Quoc E, Pless M, Djomby R, Orssaud C, Guichard JP, et  al. Ocular motility and Wilson’s disease: a study on 34 patients. J Neurol Neurosurg Psychiatry. 2007;78(11):1199–201. 37. Melek NB, Blanco S, Garcia H. Electro-oculography of smooth pursuit and optokinetic nystagmus eye movements in type I Duane’s retraction syndrome. Binocul Vis Strabismus Q. 2006;21(1):37–44. 38. Wendt D, Kollmeier B, Brand T. How hearing impairment affects sentence comprehension: using eye fixations to investigate the duration of speech processing. Trends Hear. 2015;19:2331216515584149. 39. Schmitt WR, Daube JR, Carlson ML, Mandrekar JN, Beatty CW, Neff BA, et  al. Use of supramaximal stimulation to predict facial nerve outcomes following vestibular schwannoma microsurgery: results from a decade of experience. J Neurosurg. 2013;118(1):206–12. 40. Fenton JE, Chin RY, Fagan PA, Sterkers O, Sterkers JM.  Predictive factors of long-term facial nerve function after vestibular schwannoma surgery. Otol Neurotol. 2002;23(3):388–92. 41. Troude L, Boucekine M, Montava M, Lavieille J-P, Régis J-M, Roche P-H.  Predictive factors of early postoperative and long-term facial nerve function after large vestibular schwannoma surgery. World Neurosurg. 2019;127:e599–608. 42. Ren Y, MacDonald BV, Tawfik KO, Schwartz MS, Friedman RA.  Clinical predictors of facial nerve outcomes after surgical resection of vestibular schwannoma. Otolaryngol Head Neck Surg. 2020;164:194599820961389. 43. Kartush JM, Niparko JK, Bledsoe SC, Graham MD, Kemink JL. Intraoperative facial nerve monitoring: a comparison of stimulating electrodes. Laryngoscope. 1985;95(12):1536–40. 44. Nissen AJ, Sikand A, Curto FS, Welsh JE, Gardi J.  Value of intraoperative threshold stimulus in predicting postoperative facial nerve function after acoustic tumor resection. Am J Otol. 1997;18(2):249–51.

R. Martinez-Perez et al. 45. Magliulo G, Zardo F. Facial nerve function after cerebellopontine angle surgery and prognostic value of intraoperative facial nerve monitoring: a critical evaluation. Am J Otolaryngol. 1998;19(2):102–6. 46. House JW, Brackmann DE. Facial nerve grading system. Otolaryngol Head Neck Surg. 1985;93(2):146–7. 47. Neff BA, Ting J, Dickinson SL, Welling DB.  Facial nerve monitoring parameters as a predictor of postoperative facial nerve outcomes after vestibular schwannoma resection. Otol Neurotol. 2005;26(4):728–32. 48. Mandpe AH, Mikulec A, Jackler RK, Pitts LH, Yingling CD. Comparison of response amplitude versus stimulation threshold in predicting early postoperative facial nerve function after acoustic neuroma resection. Am J Otol. 1998;19(1):112–7. 49. Goldbrunner RH, Schlake HP, Milewski C, Tonn JC, Helms J, Roosen K.  Quantitative parameters of intraoperative electromyography predict facial nerve outcomes for vestibular schwannoma surgery. Neurosurgery. 2000;46(5):1140–6; discussion 1146–1148. 50. Acioly MA, Liebsch M, Carvalho CH, Gharabaghi A, Tatagiba M.  Transcranial electrocortical stimulation to monitor the facial nerve motor function during cerebellopontine angle surgery. Oper Neurosurg. 2010;66(suppl_2):ons354–62. 51. Matthies C, Raslan F, Schweitzer T, Hagen R, Roosen K, Reiners K.  Facial motor evoked potentials in cerebellopontine angle surgery: technique, pitfalls and predictive value. Clin Neurol Neurosurg. 2011;113(10):872–9. 52. Dong CCJ, MacDonald DB, Akagami R, Westerberg B, AlKhani A, Kanaan I, et  al. Intraoperative facial motor evoked potential monitoring with transcranial electrical stimulation during skull base surgery. Clin Neurophysiol. 2005;116(3):588–96. 53. Ling M, Tao X, Ma S, Yang X, Liu L, Fan X, et al. Predictive value of intraoperative facial motor evoked potentials in vestibular schwannoma surgery under 2 anesthesia protocols. World Neurosurg. 2018;111:e267–76. 54. Eggermont JJ.  Auditory brainstem response. Handb Clin Neurol. 2019;160:451–64. 55. Legatt AD.  Mechanisms of intraoperative brain stem auditory evoked potential changes. J Clin Neurophysiol. 2002;19(5):396–408. 56. Colletti V, Bricolo A, Fiorino FG, Bruni L. Changes in directly recorded cochlear nerve compound action potentials during acoustic tumor surgery. Skull Base Surg. 1994;4(1):1–9. 57. Møller AR, Jannetta PJ.  Preservation of facial function during removal of acoustic neuromas. Use of monopolar constant-voltage stimulation and EMG. J Neurosurg. 1984;61(4):757–60. 58. Winzenburg SM, Margolis RH, Levine SC, Haines SJ, Fournier EM. Tympanic and transtympanic electrocochleography in acoustic neuroma and vestibular nerve section surgery. Am J Otol. 1993;14(1):63–9. 59. Schlake HP, Goldbrunner RH, Milewski C, Krauss J, Trautner H, Behr R, et al. Intra-operative electromyo-

7  Neurophysiologic Monitoring graphic monitoring of the lower cranial motor nerves (LCN IX-XII) in skull base surgery. Clin Neurol Neurosurg. 2001;103(2):72–82. 60. Husain AM, Wright DR, Stolp BW, Friedman AH, Keifer JC.  Neurophysiological intraoperative monitoring of the glossopharyngeal nerve: technical case report. Neurosurgery. 2008;63(4 Suppl 2):277–8; discussion 278. 61. Loftis CM, Traynelis VC.  Intraoperative monitoring techniques in neurosurgery. New York: McGraw-Hill; 1994. 62. Topsakal C, Al-Mefty O, Bulsara KR, Williford VS.  Intraoperative monitoring of lower cranial nerves in skull base surgery: technical report and review of 123 monitored cases. Neurosurg Rev. 2008;31(1):45–53. 63. Møller AR.  Intra-operative neurophysiologic monitoring. Harwood Academic: Luxembourg; 1995. 64. Doyle DJ, Mark PW. Reflex bradycardia during surgery. Can J Anaesth. 1990;37(2):219–22. 65. Duane DT, Howard SJ, Kraayenbrink M.  Incidence and predictors of bulbar palsy after surgery for acoustic neuroma. J Neurosurg Anesthesiol. 1997;9(3):263–8. 66. Ghatol D, Widrich J. Intraoperative neurophysiological monitoring. In: StatPearls [Internet]. StatPearls Publishing: Treasure Island; 2020 [cited 2021 Jan 26]. Available from: http://www.ncbi.nlm.nih.gov/books/ NBK563203/. 67. Biscevic M, Biscevic S, Ljuca F, Smrke BU, Ozturk C, Tiric-Campara M. Motor evoked potentials in 43 high risk spine deformities. Med Arch. 2014;68(5):345–9. 68. Deletis V, Sala F.  Intraoperative neurophysiology: a tool to prevent and/or document intraoperative injury to the nervous system. In: Quinones-Hinojosa A, editor. Schmidek and Sweet: operative neurosurgical techniques e-book: indications, methods and results. 6th ed. Elsevier; 2014. p. 30–45. 69. Legatt AD, Laarakker AS, Nakhla JP, Nasser R, Altschul DJ.  Somatosensory evoked potential monitoring detection of carotid compression during ACDF surgery in a patient with a vascularly isolated hemisphere. J Neurosurg Spine. 2016;25(5):566–71. 70. Kerkhof FI, van Schaik J, Massaad RA, van Rijswijk CSP, Tannemaat MR.  Measuring CMAPs in addition to MEPs can distinguish peripheral ischemia from spinal cord ischemia during endovascular aortic repair. Clin Neurophysiol Pract. 2021;6:16–21. 71. Banga PV, Oderich GS, Reis de Souza L, Hofer J, Cazares Gonzalez ML, Pulido JN, et  al. Neuromonitoring, cerebrospinal fluid drainage, and selective use of iliofemoral conduits to minimize risk of spinal cord injury during complex endovascular aortic repair. J Endovasc Ther. 2016;23(1):139–49. 72. Koht A, Sloan TB. Intraoperative monitoring: recent advances in motor evoked potentials. Anesthesiol Clin. 2016;34(3):525–35.

99 73. Gonzalez AA, Jeyanandarajan D, Hansen C, Zada G, Hsieh PC. Intraoperative neurophysiological monitoring during spine surgery: a review. Neurosurg Focus. 2009;27(4):E6. 74. Lee JJ, Hong JT, Kim IS, Kwon JY, Lee JB, Park JH.  Significance of multimodal intraoperative monitoring during surgery in patients with craniovertebral junction pathology. World Neurosurg. 2018;118:e887–94. 75. Deletis V, Sala F.  Intraoperative neurophysiological monitoring of the spinal cord during spinal cord and spine surgery: a review focus on the corticospinal tracts. Clin Neurophysiol. 2008;119(2):248–64. 76. Axon PR, Ramsden RT.  Assessment of real-time clinical facial function during vestibular schwannoma resection. Laryngoscope. 2000;110(11):1911–5. 77. Guntinas-Lichius O, Martinez-Portillo F, Lebek J, Angelov DN, Stennert E, Neiss WF.  Nimodipine maintains in vivo the increase in GFAP and enhances the astroglial ensheathment of surviving motoneurons in the rat following permanent target deprivation. J Neurocytol. 1997;26(4):241–8. 78. Mattsson P, Aldskogius H, Svensson M.  Nimodipine-­induced improved survival rate of facial motor neurons following intracranial transection of the facial nerve in the adult rat. J Neurosurg. 1999;90(4):760–5. 79. Sekiya T, Yagihashi A, Asano K, Suzuki S. Nimodipine ameliorates trauma-induced cochlear neuronal death. Neurol Res. 2002;24(8):775–80. 80. Strauss C. The facial nerve in medial acoustic neuromas. J Neurosurg. 2002;97(5):1083–90. 81. Scheller C, Strauss C, Fahlbusch R, Romstöck J.  Delayed facial nerve paresis following acoustic neuroma resection and postoperative vasoactive treatment. Zentralbl Neurochir. 2004;65(3):103–7. 82. Coleman JK, Dengerink HA, Wright JW.  Effects of hydroxyethyl starch, nimodipine, and propylene glycol on cochlear blood flow. Otolaryngol Head Neck Surg. 1991;105(6):840–4. 83. Scheller C, Rampp S, Leisz S, Tatagiba M, Gharabaghi A, Ramina KF, et  al. Prophylactic nimodipine treatment improves hearing outcome after vestibular schwannoma surgery in men: a subgroup analysis of a randomized multicenter phase III trial. Neurosurg Rev. 2021;44(3):1729–35. 84. Aronzon A, Ruckenstein MJ, Bigelow DC. The efficacy of corticosteroids in restoring hearing in patients undergoing conservative management of acoustic neuromas. Otol Neurotol. 2003;24(3):465–8. 85. Bozorg Grayeli A, Ferrary E, Tubach F, Bernat I, Deguine O, Darrouzet V, et  al. Effect of corticosteroids on facial function after cerebellopontine angle tumor removal: a double-blind study versus placebo. Audiol Neurootol. 2015;20(4):213–21.

8

Microdissection Tools A. Samy Youssef

Introduction Neurosurgery was revolutionized by the advent of the surgical microscope. With the development of microsurgery came the art of tissue dissection which necessitated the development of precise well-crafted instruments. The novel tools were called micro-instruments and were developed by pioneer neurosurgeons [1–4], who were not only skilled in surgery but also talented in designing and developing what was missing from their toolbox. In skull base surgery, tissue dissection is safely performed around critical neurovascular structures such as cranial nerves, vessels, and brain stem. Dissection should be gentle, precise, and often sharp in order to avoid traction injury, especially to cranial nerves, or avulsion of minute vessels. Also, dissection goes from superficial to deep, which may require different sets of instruments with variable lengths and tips. Microdissectors are generally crafted and adapted for easy use under the microscope or endoscope. Dissector tips vary in shape, size, sharpness, and bend angle in the case of angled tip dissectors. Different dissectors serve particular roles in order of utilization in the surgical procedure. A. S. Youssef (*) Department of Neurological Surgery, University of Colorado School of Medicine, Aurora, CO, USA Department of Otolaryngology, University of Colorado School of Medicine, Aurora, CO, USA e-mail: [email protected]

There are many options for microdissector sets [1, 5–9]; however, the surgeon should standardize a personalized comprehensive set rather than having multiple sets. A standard comprehensive set will enable the surgeon to become familiar with the different instruments and their particular roles, in addition to facilitating setting up the operating room by the scrub nursing team.

A Comprehensive Microdissectors Set Combined skull base procedures require the use of different instrument sets between neurosurgery and otolaryngology, which requires opening multiple surgical trays and adds more time to the setup. Oftentimes, only few instruments are needed from each tray; however, the entire tray has to be sterilized afterward which is wasteful. We developed a comprehensive cranial nerve microdissectors set, the Youssef Cranial Nerve Dissector Set (Mizuho America Inc., Union City, CA, United States), that should suffice in performing the great majority of skull base procedures with the following main features (Fig. 8.1): • Lower profile instruments to accommodate minimally invasive procedures. • Uniquely designed tips to mitigate cranial nerve trauma.

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Fig. 8.1  A comprehensive skull base microsdissectors set

A. S. Youssef

Fig. 8.2  Spatula dissectors 2.4 mm, 2.2 mm, 1.8 mm tip sizes

• Vividly colored handles for better instrument identification. • Extended working length to adapt to a full range of skull base procedures. • 20  cm (±5  mm) overall length, and 10  cm (±3 mm) working length. • Straight design to be rotated 360° under the microscope and be used in endoscopic endonasal procedures as well. The instruments are arranged in a sequential fashion in the order of utilization in the procedure as follows: 1. Spatula dissectors (Fig.  8.2): These are used for initial semi-sharp dissection to establish arachnoidal/cisternal planes around skull base tumors in a bloodless fashion. 2. Round dissectors (Fig. 8.3): These are used to extend dissection planes into deep corners or around neurovascular structures. 3. Putter dissectors (Fig. 8.4): 60° left and right angles are used to conform to the tumor surface during dissection off cranial nerves such as dissecting vestibular schwannomas off the facial and vestibulocochlear cranial nerve complex. The curve and semi-sharp tip permit precise and gentle dissection without violating the nerve integrity. 4. Teardrop probes (Fig. 8.5): The blunt ball tip is safe in dissecting around blood vessels and cranial nerves in already established dissection planes. 5. Needle dissectors (Fig.  8.6): We specifically developed these dissectors for sharp dissection of tumor tissue off neurovascular structures. For example, these dissectors can be

Fig. 8.3  Round dissectors 2 mm size tip with 30 and 45 angles

Fig. 8.4  Putter dissectors: 60° left and right angles

Fig. 8.5  Teardrop probes: 15°, 45°, and 90° angles with ball tip

8  Microdissection Tools

Fig. 8.6  Needle dissectors: Straight, 45° and 90° angles

Fig. 8.7  Teardrop backcutting knife, 90° up with ball tip

used to peel the most adherent last piece of tumor off the pre-meatal segment of the facial nerve. We start with the straight dissector to establish the dissection plane, followed by the curved tip dissectors as dissection progresses between the tumor and the nerve. In addition, the sharp tip is anchored to the tumor tissue which allows delicate tumor mobilization with minimal traction or manipulation on the nerve. 6. Teardrop back cutting knife (Fig. 8.7): This is used to sharply split the dural sheath around cranial nerves while protecting the underlying nerve due to the ball tip and blunt edges design. For example, it is used to spilt the falciform ligament above the optic nerve, the dural sleeve around the nerve in the oculomotor trigone in cavernous sinus surgery, and the dura of the internal auditory canal (IAC) in vestibular schwannoma surgery.

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Fig. 8.8  Cup curettes in 45°, 90° angles and 100° reverse angle

7. Cup micro-curettes (Fig.  8.8): The cup curettes in different sizes and angles are used during the final stages of tumor resection in narrow corridors such as bony canals around cranial nerves. For example, they can be used in meningiomas extending into the optic canal or to gently dissect vestibular schwannomas from the fundus of the IAC in retrosigmoid or middle fossa approaches.

Microscissors Microscissors are a crucial component of the microsurgery and endoscopy tool set. For endoscopic endonasal procedures, we prefer bayonet single shaft scissors. They are appropriate for keyhole and endonasal procedures as they have a slim design and don’t obstruct the view. They come with both straight and curved blades, with the curved blades configured in both horizontal and vertical planes. For microsurgical procedures, different types of microscissors can be used in different phases of the same procedure; from strong to more delicate (ultrafine) as tumor resection advances closer to neurovascular structures, and from short to long (18–24 cm) as the depth of field increases. In order to minimize the number of instrument trays to be opened, we combined variable bayonetted microscissors in one tray arranged in order of utilization in the procedure (Fig. 8.9).

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Ethical approval and informed consent (to participate and for publication): Informed consent and ethical approval were not deemed necessary by the local ethics in view of the design of the study. This study did not receive financial support. Availability of data and material (data transparency): This manuscript has not been previously published in whole or in part or submitted elsewhere for review.

Fig. 8.9  Microscissors tray arranged in order of utilization. Meningioma and strong round handle microscissors are used for the dural opening and firm tumors resection (Mizuho America Inc., Union City, CA, United States). More delicate (ultrafine) microscissors (Charmant Inc., Sabae-city, Fukui, Japan) are used for fine dissection along cranial nerves and vessels during later stages of the procedure

Conclusion In summary, the surgeon should try to limit the number of instruments used and be familiar with the particular role of each instrument in order of utilization in the procedure. The selection and standardization of a comprehensive tool set facilitates the setup of the instruments which boosts efficiency and decreases operative time. Safe cranial nerve dissection is key to functional preservation with significant positive impact on postoperative course and outcomes, and duration of hospital stay. Disclosure  Funding: This study did not receive any funding relative to its elaboration. Conflict of interest: ASY is a consultant for Stryker Corp and has received royalty from Mizuho America.

References 1. Sugita K, Kobayashi S.  Microneurosurgical atlas [Internet]. Springer Berlin Heidelberg: Berlin, Heidelberg; 1985 [cited 2020 Nov 28]. Available from: https://doi.org/10.1007/978-­3-­642-­61669-­3. 2. Freer OT.  The window resection operation for the correction of deflections of the nasal septum. JAMA. 1903;XLI(23):1391. 3. Penfield W. No man alone: a neurosurgeon’s life. 1st ed. Boston: Little, Brown; 1977. 398 p. 4. Hernesniemi J, Niemelä M, Dashti R, Karatas A, Kivipelto L, Ishii K, et al. Principles of microneurosurgery for safe and fast surgery. Surg Technol Int. 2006;15:305–10. 5. Rhoton AL.  Operative techniques and instrumentation. In: Cranial anatomy and surgical approaches. 1st ed. Neurosurgery. The Congress of Neurological Surgeons: Schaumburg; 2003. p. 1–28. 6. Lehecka M, Laakso A, Hernesniemi J, Çelik Ö. Helsinki microneurosurgery basics and tricks. Helsinki: M. Lehecka, A. Laakso and J. Hernesniemi; 2011. 7. Fukushima T.  Manual of skull base dissection. Pittsburgh: AF Neuro Video; 1996. 8. Todeschini AB, Otto BA, Carrau RL, Prevedello DM. The Angelina dissectors: a novel design of dissectors for endoscopic endonasal approaches. J Neurol Surg B Skull Base. 2020;81(3):295–300. 9. Jha DK. Frugal malleable microdissectors and arachnoid knives for microneurosurgery. World Neurosurg. 2018;112:148–52.

9

Neuroimaging Precision Tools and Augmented Reality Torstein R. Meling and Maria-Isabel Vargas

Introduction Imaging of the central nervous system has developed significantly in recent decades and now offers a wide range of tools. Furthermore, modern neuroimaging has a sub-millimetric spatial resolution with isotropic voxels that permits creation of multiplanar reconstructions in all three planes. In this chapter, we describe and illustrate different neuroimaging precision tools in the context of skull base tumor surgery. Imaging has three phases, namely the preoperative, the intraoperative, and the postoperative phases. The main goal of pre- and intraoperative visualization is to facilitate a maximal resection with functional preservation, whereas the goal of postoperative visualization is to document the resection grade and the presence or absence of “collateral damage.” It is beyond the framework T. R. Meling (*) Department of Neurosurgery, Geneva University Hospitals, Geneva, Switzerland Faculty of Medicine, University of Geneva, Geneva, Switzerland Department of Neurosurgery, Carlo Besta Neurological Institute, Milano, Italy M.-I. Vargas Faculty of Medicine, University of Geneva, Geneva, Switzerland Department of Neuroradiology, Geneva University Hospitals, Geneva, Switzerland e-mail: [email protected]

of this chapter to discuss preoperative imaging used for diagnostics, patient information, and staging (i.e., TNM tumor classification in case of malignant tumors). Neuroimaging is based on three main modalities, namely CT, MRI, and angiography (DSA, digital subtraction angiography). However, there is a plethora of various techniques based on these modalities that there might be an overlap as to the information yielded (e.g., CT and MR venography), and there are important differences in their utility and practicality during the three different phases. In this chapter, we will not discuss each in great detail, but rather highlight some techniques that we find particularly valuable. Finally, imaging can be experienced in three realities, namely standard reality (i.e., real world), virtual reality (VR), and augmented reality (AR). While standard reality (SR) needs no further explanation, VR is a simulated experience and can be defined as an artificial environment which is experienced through sensory stimuli (such as vision) provided by a computer. Yet at its infancy, VR will unquestionably gain increasing importance in the preoperative phase, both with respect to patient information, training, surgical planning, and virtual rehearsals [1–3]. However, it is outside the scope of this chapter to discuss VR further. In contrast, we will discuss the use of AR, defined as an enhanced version of reality created by the use of technology to overlay digital information on an image of something being viewed

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through a device (such as a microscope), during the intraoperative phase [4–6].

Preoperative Visualization The goal of preoperative visualization is to obtain detailed information regarding the lesion itself and its surroundings, as well as along the route that gets us there, in order to plan the surgical intervention so as to avoid or reduce the risk of any intraoperative surprises. We systematically seek information regarding lesion characteristics like localization, size, delineation (presence or absence of a capsule), consistency (e.g., hard vs. soft vs. fluid, presence or absence of calcifications), homogeneity, vascularity, contrast enhancement, perfusion and diffusion characteristics, and relations to its surrounding structures (e.g., adhesions, encapsulations, compressions, deviations, edema), as well as information regarding its surroundings per se, that is, the CNs, arteries, and veins/sinuses, brain or brain stem, the CSF spaces (surgical corridor, hydrocephalus, risk of postoperative CSF leak), dura mater and skull base bone, as well as the adjacent soft tissues (e.g., muscles, skin) and the air–tissue interface (e.g., nasal mucosa, mastoid cells) (Table 9.1). The complexity of the surrounding structures varies with lesion location and extension. In general, the corridor extending from the orbital apex/ superior orbital fissure, through the cavernous sinus/Meckel’s cave down through the petro-­ Table 9.1  Key points to analyze in preoperative imaging of skull base lesions Lesion characteristics: Localization Size Delineation Consistency (cystic, solid, calcifications) Homogeneity Vascularity Enhancement Perfusion/diffusion Relations to surroundings

Lesion surroundings: Cranial nerves Arteries/perforators Veins/venous sinuses Brain and/or brain stem CSF spaces Dura mater Skull base bone Soft tissues (muscles/ skin) Air–tissue interface

Fig. 9.1  “The devil’s path” of tumor location or extension, that is, the corridor extending from the orbital apex/ superior orbital fissure, through the cavernous sinus/ Meckel’s cave down through the petro-meatal area and ending at the jugular foramen

meatal area and ending at the jugular foramen (Fig. 9.1), which we call “the devil’s path,” represents the most complicated zone. Here, most lesions will have complex contacts with multiple cranial nerves, cerebral arteries/perforators, and draining veins/venous sinuses. Furthermore, most lesions will be in contact with surrounding brain or brain stem tissue, dura mater, and the cranial base bone.

Computed Tomography CT in the context of skull base surgery is used principally with bone reformatting as a differential diagnostics tool and to depict the presence or absence of bone destruction, hypertrophy, sclerosis or tumor invasion in different localizations such as the lamina cribrosa, anterior clinoid process, petrous apex, inner ear, and jugular tubercle. Also, it is used to look for any signs of instability of the spine in case of chordomas or

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osteolytic metastases, for example. Hyperostosis of the skull base most often indicates the tumor epicenter and origin (Fig. 9.2). CT scans can also detail the sinus anatomy, the presence of skull base erosions, or enlargements of the superior orbital fissure, sella turcica, Meckel’s cave, internal auditory canal (IAC), or jugular foramen. Finally, intratumoral calcifications may give indications as to tumor type, tumor growth dynamics, and the difficulties a resection will entail. CT is still an important modality both for the planning phase and during the execution phase of skull base surgery. Consider endoscopic endonasal surgery, where preoperative high-resolution CT is essential to identify potential high-risk anatomical variations of the sphenoid sinus, like anterior clinoid process pneumatization or dehiscence over the optic nerve or the internal carotid artery (ICA) [7]. Furthermore, as CT imaging provides more precise geometrical localization than MRI, CT-based neuronavigation is more accurate than MRI-based procedures [8]. Consequently, three-dimensional (3D) CT/MRI co-registration (see below) can improve the stereotactic image accuracy while preserving the superior resolution of the MRI. As another example, consider the planning of a transpetrosal approach, where a CT scan is mandatory to properly evaluate mastoid and petrous apex pneumatization, petrous apex hyperostosis, the position and depth of the cochlea, and semicircular canals with respect to the superior and posterior petrous bone surfaces, dehiscence over the geniculate ganglion, the course of the facial nerve, the course of the ICA, and any dehiscence of the petrous carotid canal and superior semicircular canal.

Computed Tomography Angiography (CTA) CT angiography (CTA) is a 3D iodine-contrast technique with sub-millimetric resolution that provides information about cranial or cervical vessels. CTA in the context of skull base surgery is used principally to depict relationships between tumors, vessels, and bone structures to guide the

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preoperative planning of skull base tumors. Dynamic CTA or 4D CTA offers excellent spatial and temporal resolution for assessment of vascular lesions [9, 10].

Computed Tomography Venography (CTV) CT venography (CTV) as it relates to skull base surgery is used to analyze the status of veins and venous sinuses, such as the vein of Labbé, the superior petrosal sinus, and the sigmoid sinus [11, 12]. The dual-source CT technique allows for digital removal of adjacent bone structures to improve the visualization. It can be used for CTA and CTV and is particularly useful to the preoperative planning for skull base tumors.

Magnetic Resonance Imaging The role of MRI in the context of skull base surgery is to detect and characterize lesions, as well as to evaluate adjacent neurovascular structures and bone involvement. Although CT is superior in assessing intratumoral calcifications and the skull base bone, MRI is indispensable for skull base lesions, as there is a wide range of pathologies that can arise in the skull base, ranging from non-neoplastic lesions to tumors, which means that there is a large variety in disease and lesion appearances that may complicate the radiological diagnosis. Fortunately, there is an abundance of different MRI techniques, be it field-strength, sequences, or postprocessing techniques, that can be used to triangulate to find the correct diagnosis [13–15]. Furthermore, MRI permits an exquisite and precise analysis of complex skull base anatomy and pathology, helping to create a preoperative neuronavigation road-map for the surgeon [16–19]. MRI may be useful in an intraoperative setting for complex cases, to guide and detect residual tumor after surgery [20, 21]. Finally, MRI is used postoperatively to document the resection grade [22–25], the presence or absence of complications [26, 27], and in the follow-up of patients [28, 29].

T

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Fig. 9.2  A giant parasagittal meningioma which involves the superior sagittal sinus and the sinus confluence and extends to the posterior fossa and tentorium cerebelli. (a) Axial CT shows a large supratentorial mass that is partially calcified, surrounded by edema, and infiltrates the bone. Note the thickened and spiculated appearance of the bone. (b) Axial T2 illustrates a heterogenous, infiltrating mass at level of the fronto-parietal lobes surrounded by edema (b). The mass displaces the splenium of the corpus callosum in red anteriorly and inferiorly (b). (c) Volume rendering of an MRV shows an occlusion of the posterior part of the sagittal sinus and sinus confluence, with the development of collateral networks in the subcutaneous

tissue. A venous phase DSA shows the same occlusion of sagittal sinus but is easier to interpret because of its higher selectivity (visualizes only the intracranial veins) and gives a better understanding of the flow dynamics, both with respect to velocity and direction as compared to the MRV. (d) Sagittal T1 Gd shows a large, heterogeneous and enhancing mass infiltrating bone, the superior sagittal sinus, the sinus confluence, the rectus sinus, and soft cutaneous tissue. The mass displaces the cerebellum, corpus callosum, brain stem, rectus sinus, and internal cerebral veins (a). Cinematic posttreatment imaging at the same level (b)

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Fig. 9.2 (continued)

MRI Field Strength

postoperative controls of patients in the ICU, when they have several devices, infusion pumps, With respect to field strength, 1.5 T standard field tracheal tubes, etc. MRI is the “work-horse” in all of the three above-­ The 3 T high-field MRI has a higher temporal mentioned phases. First, it is widely available. and spatial resolution, offering a high signal-to-­ Second, the numerous sequences and postpro- noise ratio and 3D and vascular sequences with cessing techniques developed for this field sub-millimetric resolution that makes 3 T particstrength makes it very versatile. Third, the 1.5 T ularly valuable for skull base tumors and vascular is useful in patients with metallic devices, as the lesions. Finally, the 7  T ultra-high field MRI artifacts are less important at 1.5  T than at might prove to be a great tool, but is currently not 3 T. The 1.5 T scanner is also advantageous for used in daily practice.

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MRI Sequences With respect to MRI sequences, we can group them into three categories, namely conventional or morphological, vascular, and advanced sequences.

 onventional or Morphological C Sequences T1-weighted sequences without and with contrast are used for localization and characterization of lesions and their relationships with the surrounding structures (Table 9.2). T1 sequences

Table 9.2  MR sequences useful to skull base surgery Sequence Morphologic T1 and T2 weighted 2D and 3D sequences T1 with Gadolinium (Gd) contrast agent

Clinical utility Depiction, delineation, characterization, and presurgical planning 3D useful for neuronavigation Delineation of lesion and depiction of areas of high grade Helpful for neuronavigation and detect arterial and venous structure Fat-sat sequence (T1, Fat-sat sequence T2 (STIR) useful to depict high T2) signal of optic nerves or infiltration of cavernous sinus Fat-sat sequence T1, used after administration of contrast, to visualize enhancement in areas rich in fat Delineation and depiction of cyst wall T2 high resolution (CISS, FISP, FIESTA, Visualization of nervous structures useful in case of vascular conflict or to determine origin, infiltration, T2 SPACE) and limits of tumors FLAIR Detection of lesions Depiction of blood, calcifications, normal and Susceptibility pathologic arterial and venous structures sequences (SWI, GET2) 3D TOF

MRV

DTI / DWI

DCE/DSC (MR perfusion)

MRS

Tips and tricks Knowledge of artifacts and normal anatomic structures to avoid misinterpreting these as lesions Indispensable to perform noncontrast T1 to depict spontaneous high signals that can mimic enhancements Highly sensible to metal devices, these cause loss of signal and susceptibility artifacts

Due to a small field of view, a good placement of slices is necessary to properly cover lesions

Low specificity Overestimation of bleeding due to blooming effect Air may also provoke blooming artifact Depiction of displacement, stenosis, encasement and Analyze of native slices and relationship with tumors reformatting Warning: VR increase falsely stenosis Illustration of displacement, stenosis, infiltration, Analyze of native slices and encasement, and relationship with lesions reformatting Warning: VR increase falsely stenosis Technical indispensable parameters DTI—presurgical planning and in IA situation to 30 directions, 3 T and b1000 detect white matter tracts DWI—indispensable to make the difference between Susceptibility artifacts due to air/ tissue interface and blood tumors and abscesses, and to delineate epidermoid cyst Loss of signal and susceptibility Characterization, depiction of tumor areas of high artifacts in postchirurgical phase grade and help to make differential diagnosis (blood, air) and close to air/tissue vis-a-vis complications of radiotherapy treatment interface Susceptibility artifacts in case of Characterization of lesions, and help to make bleed, and close of air/tissue differential diagnosis with complications of interface radiotherapy treatment

Abbreviations: T Tesla, 2D 2-dimensional, 3D 3-­dimensional, Fat sat fat-saturated, CISS constructive interference in steady state, FISP fast imaging with steady-state precession, FIESTA fast imaging employing steady-state acquisition, SPACE sampling perfection with application-optimized contrasts using different flip-angle evolution, FLAIR fluid attenuated inversion recovery, SWI susceptibility-weighted imaging, TOF time-of-flight, MRV magnetic resonance venography, VR volume rendering, DWI diffusion-weighted imaging, DTI diffusion tensor imaging, DCE dynamic contrast enhanced, DSC dynamic susceptibility contrast, MRS MR spectroscopy

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Fig. 9.3  A cavernous hemangioma of the right orbit is shown on a STIR sequence as a round, well-delineated, hyperintense lesion (H in a) that displaces the superior rec-

tus muscle (arrow in a) and has a homogeneous contrast-­ enhancement on a T1 Gd fat-sat sequence (H in b)

have a low specificity to detect lesions without contrast, as most of the lesions have low signals that are similar to an edema. However, fat-sat T1 Gd sequences are particularly useful at the level of the orbit and the cavernous sinus, since these regions are rich in fat (e.g., after fat graft reconstruction) and the suppression of the fat signal enhances the detection of contrast enhancement (Figs. 9.3, 9.4, and 9.5). T2-weighted sequences are generally valuable for tumor detection, delineation, characterization, and differentiation between common skull base pathologies (Table 9.2). A cavernous meningioma, for example, will have a low T2 signal (Fig. 9.6) whereas a hemangioma of the cavernous sinus will have a high T2 signal (Fig.  9.7). Furthermore, T2-weighted sequences may also inform tumor resectability. For example, the presence of brain stem edema on T2 is a sign of disrupted or non-existing arachnoidal plane of dissection between tumor and brain stem and complicated postoperative course should dissection is attempted [30] whereas the presence of an arachnoidal cleavage plane, defined as a rim of low intensity on T1 and high intensity on T2-weighted images has been shown to correlate well with tumor resectability [31]. Heavily T2-weighted high-resolution sequences such as CISS, FISP, FIESTA, and T2 SPACE are 3D, isotropic, and millimetric sequences that provide an excellent contrast between cranial nerves, vessels, tumors, and cerebrospinal fluid (CSF) and may yield precious

information about cranial nerve positions (Table 9.2). They are often used to depict neurovascular conflict, especially if co-registered with a 3D TOF sequence. Furthermore, they are useful to show relations between cranial nerves and adjacent structures, as well as displacement of nerves or schwannomas (Figs. 9.8 and 9.9). Susceptibility-weighted imaging (SWI) is a 3D sequence used to detect bleeds, even millimetric, as well as differentiate calcification from bleeds and iron, thanks to it being very sensitive to differentiate tissue susceptibility. SWI can also depict normal and pathologic arterial and venous structures, including sinus thrombosis (Table 9.2).

Vascular Sequences MR angiography (MRA) with a 3D, millimetric, isotropic TOF sequence is one of the most important non-contrast methods for visualization of intracranial vessels and hypervascular skull base tumors. A 3D image of the vessels analogous to conventional 3D angiography is generated using a reconstruction technique such as Maximum Intensity Projection (MIP) or Volume Rendering (VR). In MR venography (MRV), a TOF sequence or contrast-enhanced series can be used to visualize veins (Fig.  9.2). Non-contrast enhanced (phase-contrast) sequences are useful in cases where patients have a contraindication to contrast or in pregnant women, but the resolution is lower compared to contrast-enhanced sequences and one of the most common with pitfalls phase-­

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Fig. 9.4  Axial T2, coronal STIR, and axial and coronal fat-sat T1 Gd (a–d) illustrate infiltration of left cavernous sinus, ocular muscles, and intraorbital veins by a pituitary macroadenoma. Note the enlarged veins (asterisk in a–c) and displacement of the left optic nerve (arrow in b). The

fat-sat T1 Gd sequence highlights the contrast enhancement, as well as the infiltration of the left trigeminal nerve that appears thickened and enhances (asterisk and arrow in c and d)

contrast is that it can mimic false strictures or occlusions when the venous flow is slow. Dynamic angiographic sequences like TWIST (time-resolved angiography with interleaved stochastic trajectories), 4D TRICK (time-resolved imaging of contrast kinetics), 4D TRAK (4D timeresolved angiography using keyhole), and TRAQ

(Time-Resolved AcQuisition) are MRA techniques with very high temporal and spatial resolutions (sub-millimeter), which permit the capture of multiple arterial, mixed, and venous phase images during the passage of the contrast agent through the vessels. Such sequences provide information about narrowing or displacement of vascular struc-

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Fig. 9.5  An axial T2 sequence shows a melanoma metastasis which is seen as a hypointense lesion around the left anterior clinoid process (arrow in a). Note tumor infiltration of the adjacent dura and rectus superior muscle

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(arrows in b and c). The strong linear enhancement of the left oculomotor nerve corresponding to neural dissemination (black arrows in d)

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Fig. 9.6  An axial T2 sequence shows a right-sided cavernous sinus meningioma as a hypointense, infiltrating lesion in both the right and left cavernous sinuses, the

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Gasserian ganglion, cisternal segment of the right trigeminal nerve (arrows in a, b, and d). Note the stenosis of the ICA (asterisk in a and arrow in c)

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Fig. 9.7 A left-sided intracavernous hemangioma is shown as a hyperintense lesion on axial T2 sequence (arrows). The tumor crosses the midline and encases and displaces the right ICA (a, b). Fat-sat T1 sequences illus-

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trate well the limits between the hemangioma (white asterisk in c) and normal pituitary gland (black asterisk in d)

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Fig. 9.8  A round, well-delineated schwannoma of the left oculomotor nerve (asterisk in a and b) is nicely shown with a CISS sequence (arrow in a). Also note the right

oculomotor nerve as a hypointense linear structure (arrow in a). A homogenous enhancement of the lesion is noted on the axial T1 Gd sequence (black asterisk in b)

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Fig. 9.9  An axial CISS (a) shows a right-sided vestibular schwannoma (asterisk) with the cochlear nerve posterior and the facial nerve anterior to the tumor. In the coronal plane, note the facial nerve anterior to mass (b). An axial

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T1 Gd sequence shows a well-delineated and homogeneously enhancing lesion corresponding to a schwannoma (asterisk in c)

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Fig. 9.10  Axial view of a tractography showing normal cisternal segment of both trigeminal nerves (a) and sagittal view of right and left trigeminal nerves (b, c)

tures caused by tumors, vascularization of tumors, nidus of vascular malformations, and arteriovenous fistulas. Dynamic angiographic sequences can also be used for MRV.

Advanced Sequences Diffusion tensor imaging (DTI) is a non-invasive, in vivo method to show the architecture of white matter and nervous structure (Fig.  9.10). In the context of skull base lesions, DTI and tractography can be used to track cranial nerves and has been mostly used for the facial nerve in vestibular schwannomas, or the trigeminal and facial nerves in cerebellopontine angle (CPA) meningiomas [18, 32, 33]. However, in large or giant lesions where it would have the greatest impact, the reliability of DTI deserves further clinical studies as DTI can be unreliable in complex lesions or where crossing fibers appear within one voxel. Diffusion spectrum imaging (DSI) is an alternative technique to visualize fiber crossings in optic chiasm and brain stem [34]. Recently, it has been suggested that probabilistic algorithms yield more accurate depiction of cranial nerve trajecto-

ries [32] and that a full tractography approach could enable routine enhanced surgical planning for skull base tumors [35]. Diffusion-weighted imaging (DWI) extends MRI from depiction of neuroanatomy to function and physiology. It is based on measuring the random Brownian motion of water molecules within a voxel of tissue and when the tissue is highly cellular or has cellular swelling, the apparent diffusion coefficient (ADC) is reduced. DWI can reveal pathology in cases where conventional MRI remains unremarkable and is particularly useful in tumor characterization and cerebral ischemia. Areas with a high diffusion will have a high ADC value and appear consequently hyperintense on ADC maps (e.g., CSF), whereas areas with restricted diffusion (e.g., acute ischemia) will appear hypointense. For example, an epidermoid cyst in the CP angle can often mimic CSF-­ filled arachnoid cysts on CT and MRI, but mean ADC values will be significantly lower than that of CSF.  Furthermore, the skull base is a review area where metastases are often missed and where DWI might be useful.

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Fraction of anisotropy (FA) is often used in DWI where it is thought to reflect fiber density, axonal diameter, and white matter myelination. Based on a combination of anisotropy and diffusion directions, the voxels can be colored to provide a fiber direction map in order to visualize compressions, invasions, or displacements of fiber tracts (Fig. 9.2). Perfusion-weighted imaging (PWI) is a non-­ invasive MRI technique to examine cerebral perfusion with assessment of various hemodynamic parameters, such as cerebral blood volume (CBV), cerebral blood flow (CBF), mean transit time (MTT), and time to peak (TTP). Although mostly used in stroke neurology and vascular neurosurgery, it can be useful in primary or secondary vascular stenosis showing regions at risk of hypoperfusion in skull base tumors (Fig. 9.11). For PWI, we use either a T1-based dynamic contrast enhanced (DCE), or a susceptibility T2 dynamic susceptibility contrast (DSC) sequence.

Multiplanar Reformatting and Postprocessing Many of the sequences mentioned above require substantial and advanced multiplanar reformatting or postprocessing in order to be useful as the number of cuts can reach 5000 or even more. These include the 3D sequences (Standard 3D T1, fat-sat T1, high-resolution T2), as well as the vascular sequences (3D TOF MRA/MRV and dynamic MRA/MRV) and all the advanced sequences (DTI, DSI, DWI, ADC, FA, and PWI) (Table 9.2). Co-registration of different MRI sequences and even with CT images facilitates the analysis of lesion characteristics and lesion surroundings (Table  9.1) in order to make the diagnosis and plan the surgical intervention during the preoperative phase, to execute the intervention with intraoperative aids like neuronavigation and AR, and to evaluate the result of surgery both in the short term and long term postoperatively. Reformatting in MIP or multiplanar reformatting (MPR) is used for 3D morphologic sequences and VR is used for vascular sequences. Reformatting in MIP, MPR, and VR with super-

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position of arterial and venous phases is particularly useful to understanding skull base pathologies. Finally, DTI and DCS often need significant work effort and dedicated software designed to process this type of imaging. In multiplanar reformatting or postprocessing, an in-depth knowledge of the traps is indispensable for a good clinical interpretation and to avoid creating false images, wherefore it should ideally be performed by an experienced neuroradiologist or, if in a high-volume environment, by a dedicated postprocessing laboratory to assure the quality [36].

Digital Subtraction Angiography Catheter-based, digital subtraction angiography (DSA) with intravenous contrast is less commonly used in modern skull base surgery, often being replaced by CT or MR angiographies (CTA or MRA, respectively) which can produce 3D images through less invasive methods. However, DSA still offers two main advantages, namely, superior visualization of the flow dynamics and selectivity. With respect to flow dynamics, modern DSA equipment delivers X-ray energy for fluoroscopy and CINE in pulses that can give up to 30 frames per second, thus enabling the visualization of feeding arteries and the characterization of the venous drainage pattern, capturing both velocity and flow direction. With respect to selectivity, super-selective angiography allows detailed endovascular exploration of various feeding arteries by, thereby “editing out” unimportant vessels in the analysis. Finally, there are certain scenarios where DSA is indispensable, as we will discuss below.

DSA and Diagnostics—Arteries We do not routinely perform DSA for meningiomas, as CTA or MRA scans generally afford the information necessary to safely plan the surgery, although the ability of said techniques to depict small arteries is somewhat limited. On the other hand, DSA gives excellent visualization of small

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Fig. 9.11  An axial T1 Gd shows a left-sided cavernous sinus meningioma (asterisk in a) that was treated with gamma knife radiosurgery 3 years before she started having episodes with aphasia during exercise. The MRI (arrow in a) and angiographic studies showed a significant stenosis of the intracavernous ICA and preoperative CT

perfusion maps showed a significant deficit in the left MCA territory (CBF in b, CBV in c, MTT in d, and TTP in e). She had a STA-MCA bypass and became complete asymptomatic. Postoperative CT perfusion studies showed a near normalization of the left MCA territory (CBF in f, CBV in g, MTT in h, and TTP in i)

feeders and branches arising from posterior circulation arteries that represent major surgical risks for instance in large or giant petroclival meningiomas where there is suspected engulfment of the basilar artery (BA) and its terminal

branches. In such cases, DSA is useful to better define the dural attachment area, to identify feeding arteries and adapt the surgical approach accordingly [37], and to evaluate the value and feasibility of preoperative embolization [38, 39].

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Another scenario, albeit rare, where diagnos- dural time, and/or to occlude surgically inaccestic DSA may play a role, is in arterial stenosis sible arterial tumor feeders. However, tumor caused by tumors such as skull base m ­ eningiomas; hypervascularity per se does not justify the added for example, iatrogenic ICA stenosis after gamma risk of preoperative embolization if the tumor is knife treatment of cavernous sinus meningiomas small, the major blood supply is superficial or [40]. DSA in combination with balloon test readily accessible early at surgery, and/or if the occlusion (BTO) is valuable to evaluate collateral extra blood loss anticipated without embolization circulation and the need for extracranial-­ is not excessive and would be physiologically Intracranial (EC-IC) bypass in case of ICA sacri- well-tolerated by the patient [45]. fice [41, 42] or severe ICA stenosis (Chap. 12). In skull base meningiomas, preoperative embolization may offer a relatively avascular field, thus reducing the need for bipolar coagulaDSA and Diagnostics—Veins tion in the vicinity of critical neurovascular structures, and should not be disregarded. However, A careful preoperative evaluation of the venous the major feeding arteries are often superficial or channels is essential when planning a skull base readily accessible during the craniotomy stage of intervention (Fig. 9.2). For example, the vein of surgery and the non-negligible complication rate Labbé is frequently responsible for intraoperative associated with preoperative embolization leave brain swelling and postoperative venous infarcts open questions concerning the risk/benefit ratio after petrosal approaches, but attention should be [39]. On the other hand, in case of large or giant given also to the superior petrosal vein and the petroclival meningiomas, preoperative embolizasuperior and inferior petrosal sinuses [43]. The tion should always be considered if it can be pervenous anatomy may be very complex [44], but formed by an experienced team. usually MR or CT venography (MRV and CTV, In certain hypervascular tumors, preoperative respectively) suffice. However, for tumors in the embolization can reduce the surgical risk and proximity of the vein of Galen, for example, the blood loss [45]. Juvenile angiofibromas arise at pineal and tentorial incisural spaces, or the the sphenopalatine foramen and a DSA can accupetrous apex, there might be collateral anastomo- rately demonstrate the vascular supply, typically ses between the petrosal veins and the Galenic distal branches of the internal maxillary artery, system that are important to preserve during sur- that can be used as “portes d’entrée” to embolize gery to avoid venous complications involving the the tumor and subsequently be occluded [46]. brain stem and the cerebellum. These are still Likewise, in jugulotympanic paragangliomas, best evaluated by DSA. Other examples include DSA allows for a better understanding of tumor evaluations of venous drainage dynamics in cer- vascularization and its relationship with surtain large sphenoid wing meningiomas when the rounding vessels, in addition to preoperative frontoparietal sinus or sylvian veins are compro- embolization [45]. mised or the venous outflow dynamics at the jugular foramen level in large glomus tumors.

DSA and Therapeutics—ICA Stenting

DSA and Therapeutics—Embolization The goal of DSA with preoperative tumor embolization is the selective obliteration of pathologic intratumoral vessels while preserving the normal blood supply to surrounding tissues. It can be used to decrease intraoperative blood loss, to improve visualization at surgery, decrease proce-

Finally, DSA may play a role in large jugular paragangliomas that encase the ICA and infiltrate its walls. Sanna et  al. [47], among others, have used presurgery ICA stenting as a precautionary measure to obtain a safe dissection plane so as to reduce the risk of ICA injury in extensive tumors that involve the petrous and cervical ICA segments.

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Intraoperative Visualization The goal of intraoperative visualization is to have detailed information regarding the surgical route, the perilesional surroundings, as well as the lesion itself, while performing the intervention so as to avoid or reduce the risk of any intraoperative complications. Visualization tools are routinely used to improve intraoperative SR visibility during executions of interventions (e.g., loops, microscopes, endoscopes, and exoscopes) and the latter three tools can be used to present an augmented version of reality to further improve visibility [5, 48, 49]. Other intraoperative visualization tools include neuronavigation [50, 51] and AR [2, 4, 6, 52–56], fluorescence for tumor visualization [57–59], fluorescence for vessel visualization [60–62], and intraoperative ultrasonography [63– 65]. Finally, visualization can be achieved using intraoperative CT [66], MRI [67–69], or DSA [70]. In this section, we focus on the use of neuronavigation, AR, and fluorescence in skull base surgery.

Neuronavigation Neuronavigation is based on 3D imaging datasets that reproduce the exact geometrical situation of the normal and pathological tissues and struca

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tures. In neuronavigation, preoperative CT and/or MRI image sequences are brought into spatial alignment with the anatomical reference space through patient registration and two or more image series can be aligned so that corresponding voxels representing the same objects may be integrated or fused through image co-registration. Neuronavigation allows for a frameless stereotactic tracking of surgical instruments in relation to the patient’s anatomy as shown on images of the patient. As a result, the surgeon can use the system to “navigate” the location of an instrument and helps to be more accurate and safer when performing surgical procedures [51]. Trackable surgical instruments not only include pointers, dissectors, bipolars, but also microscopes, endoscopes, and more recently drills. Reflective markers attached to the microscope enable visual tracking of the optical axis and focal point, and combined with neuronavigation-­ controlled robotic movements, the microscope itself may auto-align to any navigated instrument, planned trajectory, or anatomical landmark. Modern high-end microscopes allow full robotic alignment of microscope in three degrees (Leica ARveo) or even six degrees of freedom (ZEISS KINEVO 900) with automated positioning to predefined anatomical landmarks based on preoperative planning or used to follow and focus on a navigated instrument tip. This is particularly useful in brain stem or skull base surgery with b

Fig. 9.12  Sagittal T2 MRI showing a large cavernoma in the mesencephalon preoperative (a) and postoperative (b)

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deep and narrow surgical corridors, as it eliminates the need for surgeons to reposition the microscope by hand (Fig. 9.12). The main disadvantage of neuronavigation is the lack of direct view of the surgical field [71]. The surgeon has to interpret 2D images on an external screen and correlate the tip position of the navigated pointer to a real-life 3D images [72].

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according to intracranial reference structures is possible whenever required. These recalibrations represent an important evolution of neuronavigation systems as they reduce the registration error and error inherent in the surgery [5].

AR in Skull Base Surgery

The AR technology is particularly useful for skull base tumors [2, 4, 6, 56, 72, 79]. It can be Augmented Reality combined with endoscopic techniques for endonasal skull base surgery [48, 61, 75], as well as AR refers to the projection of virtual objects on for microscopic surgery [4, 6, 49, 79]. Consider a real-world structures and is based on the neuro- chordoma of the clivus in a young female navigation discussed above. In AR-based sur- (Fig. 9.14) where a resection via extended endogery, preregistered overlaid images and virtual scopic endonasal approach (EEA) is the preferred objects are projected on real-world anatomical approach [80–82]. During an EEA, it is essential structures and can be visualized using surgical to confirm several important anatomical landmicroscopes, endoscopes, exoscopes, head-­ marks, including the medial and lateral optico-­ mounted binocular displays, glasses, iPads, and carotid recesses and the carotid protuberances iPhones [2, 4, 6, 52, 54, 56, 71, 73–78]. Whereas [83]. If the sphenoid sinus is not well-­ some techniques are only useful for surgical pneumatized, drilling is required to remove the planning and training, microscope, endoscope, cancellous bone to expose the compact bone of and exoscope AR navigation visualizes the the skull base. Likewise, bone in front of the cavplanned surgical target and surrounding struc- ernous sinus and the internal carotid artery can be tures as semi-transparent volumes mixed with the skeletonized using a high-speed microdrill. These real anatomy. maneuvers can be greatly facilitated by using AR In microscope AR navigation, the data is (Fig. 9.14). injected directly into the microscope ocular, proAR is also a particularly valuable tool to viding the surgeon meaningful context and spatial improve safety of the anterior petrosectomy [49]. orientation throughout the procedure without los- Although the extradural anterior petrosectomy ing focus on the surgical field. Apart from the provides access to the upper petroclival region in lesion itself, preregistered objects range from the cases of petroclival meningiomas [84–87] and skin (used to fine-tune the standard neuronavigation-­ trigeminal schwannomas Samii types B and C based co-registration), the skull bone, intraosseous [88, 89], the floor of the middle fossa contains structures such as the cochlea, arteries, and veins or few anatomical landmarks to guide the surgeon. venous sinuses (Fig. 9.13), as well as finer details Drilling of Kawase’s triangle must be performed such as cranial nerves. carefully to avoid iatrogenic injury to critical AR and modern recalibration tools can be neurovascular structures like the cochlea, internal used to maintain neuronavigation accuracy dur- carotid artery, and the contents of the internal ing surgery [5]. Anatomy shift after craniotomy acoustic canal (Fig. 9.13). or tumor resection can be visualized by comparThe same is true for the retrosigmoid intraduing cortical vessels with their corresponding MIP ral suprameatal approach (RISA) [90–93] and the or cerebral sulci with their corresponding T2. The more recently described retrosigmoid intradural surgeon can then update the patient registration inframeatal petrosectomy (RESIP) [79, 94]. As by matching the actual signature vessels and MIP an example, consider a grade II chondrosarcoma on the navigation screen. Repeated update of the petrous temporal bone in a young female

9  Neuroimaging Precision Tools and Augmented Reality

a

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b

c

d

Fig. 9.13  Chondrosarcoma in the right petrous bone. Axial T2 MRI shows the lesion (a). A subtemporal approach exposed the petrous bone under the microscope (b). Using AR, the critical structures, that is, the petrous

ICA (red), the cochlea (yellow), and the trigeminal nerve (green) were injected into the surgeon’s ocular (c), so as to avoid them when drilling (d)

(Fig.  9.15) where a resection via the RESIP afforded limited drilling and minimal disruption of perilesional anatomical structures [79]. When using AR for a RESIP, the transverse sinus (TS) and sigmoid sinus (SS) are outlined for the retrosigmoid craniotomy. For the planned intradural drilling and tumor resection, mastoid bone pneumatization, the caudal cranial nerves (CNs), the jugular bulb, the superior and inferior petrosal sinuses (SPS, IPS), the acoustico-facial bundle, the internal acoustic canal (IAC), the petrous apex, the carotid canal, and Eustachian tube must be outlined. Finally, the tumor is delineated (Fig. 9.15).

Fluorescence Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation and is a form of luminescence. It can be used to visualize structures that are hidden from view under conventional white light and fluorescence-guided surgery is a cost-­ effective method to visualize certain neoplastic tissues in real-time [95, 96]. 5-Aminolevulinic-acid (5-ALA) is an FDA-­ approved prodrug that leads to selective accumulation of a fluorophore in malignant cells, which can then be visualized intraoperatively as pink

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a

b

c

d

Fig. 9.14  Chordoma in the clivus and posterior clinoid process. Axial T2 MRI shows the lesion (a). The preoperative plan (b), marking the carotid arteries (red), the pituitary gland and stalk (green), the optic nerves (blue),

the oculomotor nerves (light green), and the tumor (orange). After a wide sphenoidotomy, the surgical site can be seen without (c) and with AR (d)

areas when using blue-light excitation (400– 410 nm) [97]. First used in neurosurgery in 1998 by Stummer et al. [97], subsequent studies demonstrated that use of 5-ALA improved resection rates and prolonged progression-free survival in patients with high-grade gliomas [96]. Later, 5-ALA has been shown to have potential benefits in other intracranial tumors, such as meningiomas and pituitary adenomas [98, 99]. However, protoporphyrin-IX, the metabolic product of 5-ALA that accumulates in neoplastic cells, fluo-

resces in the visible-light range, which suffers from limited tissue penetration and significant auto-fluorescence [95]. The fluorescent dye indocyanine green (ICG) is an FDA-approved fluorescent agent in the near-infrared window (700–900 nm). It was first used in neurosurgery by Raabe et  al. [100] in 2003 to develop ICG video-angiography as a simple method for intraoperative blood flow assessment in aneurysm surgery (Fig. 9.16). ICG video-angiography provides information on the

9  Neuroimaging Precision Tools and Augmented Reality

a

b

d

f

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c

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g

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Fig. 9.15  A large chondrosarcoma in the left petrous bone. MRI shows the lesion in axial T2 (a), coronal T1 with contrast (b), and sagittal T1 with contrast (c). Using AR, the transverse and sigmoid sinuses were projected onto the skin (d) before incision and onto the dura after the craniotomy (e). After the dura was opened, the brain stem (right) and the extradural tumor (shown in orange) could be seen (f). Further dissection revealed the

acoustic-­facial bundle (shown in green) and the tumor (shown in orange) (g). The tumor (shown in orange) could be reached by incising the dura and drilling of the bone inferior to the acoustic-facial bundle, that is, a retrosigmoid intradural inframeatal petrosectomy (RESIP). Care was taken to avoid injury to the cochlea (shown in blue), the facial nerve (shown in green) and the ICA (shown in red) (h)

patency of arteries and veins, including small and perforating arteries (25 kg/m2 [18]. Copeland et al. noted between 2.5 and sixfold increase in CSF leak rates for BMI >25  kg/m2 and BMI >40m2 [19].

“Watertight” Closure The dogma that has been established for skull base reconstructions is to attempt a watertight dural closure prior to augmentation [20, 21]. Though this should be attempted when possible, it is often not possible and sometimes can contribute to pseudomeningocoele development. Theoretically, a near complete dural closure could result in a ball-valve effect, trapping CSF in the epidural space [22, 23]. Hence, some authors would prefer dural sutures for approximation only, with dural overlay and use of other agents. This is typical in endonasal reconstructions as well as translabyrinthine approaches [9, 24–26]. Separately, CSF rhinorrhea needs to be avoided by sealing any exposed air cells. This is commonly performed with paraffin bone wax [24, 27]. In a highly septated region that requires sealant, a diamond bit drill is used to shave down the septations and develop a larger, more contiguous region. This allows the wax to be easily compressed, minimizing the chance of a small air cell that may not have received adequate wax. It is helpful, while the bone is being drilled, that the surgical technician primes the wax by frequently kneading it and keeping it warm in a water bath. This allows the wax to remain soft and easily applicable. One should remember the adage, “Wax in, wax out and wax again…”.

Dural Onlays There are many options available to the surgeon that would function as a dural overlay. Autologous

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tissue options include tensor facia lata (TFL) graft, free pericranial graft, temporalis fascia, or omentum [9, 28–32]. In some locations, pericranial graft is readily available, and would likely be utilized as a vascularized layer either in conjunction or instead of a dural onlay [33]. The subfrontal approach often utilizes this technique. However, free pericranial graft is a great option even when not in the direct field (e.g., craniocervical junction). Its pliable tissue allows for form-­ filling and can achieve watertight closure when sewn in, if so desired. Muscle fascia, alternatively, can be somewhat more difficult to manipulate, but has increased tensile strength, which makes it a more attractive only option at inferior skull defects (e.g., anterior cranial fossa). The classic fascia used is tensor fascia lata, but this requires a separate lateral thigh incision which can be somewhat tender for active patients, especially if the remaining fascia is overzealously closed [28]. This is also a very visible scar and not very desirable by patients [34, 35]. In an endonasal approach, free mucosal grafts can be harvested if necessary (and a pedicled flap is not possible or necessary). A common source of a free graft is the native nasal septum mucosa as well as middle turbinate or inferior turbinate mucosa [36, 37].Given that a pedicled nasoseptal flap is a mainstay in endonasal skull base reconstruction, using this tissue as a free flap should only be considered if it has already been devascularized. Conversely, middle turbinectomy is a common procedure to gain access to the lateral sphenoid recess, and pterygopalatine and infratemporal fossae. Hence, the middle turbinate is an ideal free-graft option. It is important to remember that both middle and inferior turbinates can also be sources of pedicled grafts, if the vascular supply is maintained [38–40]. Hence, these closure options should not be squandered if other free graft options remain. Fat graft (abdominal, peritoneal, or local) is also a great autograft that can be used in conjunction or in the place of a dural onlay [41]. It adds bulk to a closure, which helps fill the “dead space” that remains after tumor resection or bony exposure [42]. Some surgeons consider the value

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of adipose stem cells in the wound-healing process, encouraging fibroblast migration and scar formation [43]. Abdominal fat graft harvesting can be done through a 2–3  cm incision, can be easily hidden by undergarments or other clothing, and heals quite well with low infection and hematoma rates (~1% complication rate) [43]. In very thin or malnourished patients, with very low body-fat composition, there may not be enough fat available, necessitating a longer incision to harvest the graft. At our center, we routinely use autologous fat grafts for most endonasal skull base cases, for supraorbital cases when the frontal sinus is breached, and in retrosigmoid approach if there is major entry into mastoid air cells (to augment bone waxing of air cells). Our CSF leak rates for these three approaches are very low (endonasal endoscopic 1%; supraorbital 10% growth per year and peritumoral T2 earlier diagnosis in the natural course of the dis- hyperintensity (suggestive of vasogenic edema/ ease. Advances in neuroscience, better health-­ pial invasion) on magnetic resonance imaging care standards, and widespread availability of (MRI). These patients had a 92% chance of sympscreening in the era of preventive medicine have tomatic progression in the future. In contrast, led to more frequent and earlier diagnosis of patients with lesions 20 Gy (63.1%) than with a 90%), but there was 53% tumor shrinkage in the SRS group vs. 29% tumor shrinkage in the FSRT group. One review of retrospective series on the use of FSRT to treat CSM demonstrated 5- and 10-year PFS rates of 89–94% and 76–94%, respectively [84]. In the included studies, the average radiation dose was 50–55 Gy in 30 fractions of 1.6–1.9  Gy [84]. Other studies have reported even better 10-year PFS rates ranging from 81 to 96%, with an average follow-up of 3.4–9 years [83, 85, 86]. For example, Maguire et al. [85] observed 8-year actuarial OS and PFS of 96% and 81%, respectively, in a series of 28 predominantly benign CSM tumors (21 primary, 7 recurrent) in which 22 lesions were subtotally resected and 6 lesions were deemed unresectable. Metellus et al. [83] used FSRT with an average dose of 52.9 ± 1.8 Gy over 29.4 ± 1.0 fractions in a series of CSM patients with a mean tumor volume of 11.7  ml. They observed tumor progression in only 4% of patients. FSRT apparently has an acceptable complication profile as well [83, 85, 86]. With a median dose of 53.1 Gy, Maguire et  al. [85] noted that the radiation dose did not correlate with delayed radiation toxicity, while in the study by Metellus et al. [83], RRC included a 2% rate of new-onset diplopia. The literature on the use of fractionated proton beam therapy in CSM tumors is very limited, but a theoretical advantage is posited because of the sharp dose fall-off with proton beams over conventional photon or electron beam therapies. One

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study described a 99% local tumor control rate at 5-year follow-up with the use of fractionated proton beam therapy for 72 CSM patients with a mean tumor volume of 27.6 ml [87]. RRCs in that study included visual loss and hypopituitarism each in 4% of patients and radiation necrosis requiring radical decompression in 1%.

Multimodality Therapy Although some surgeons favor aggressive surgical removal or primary SRS, others prefer a limited extracavernous resection with adjuvant SRS/ FSRT [37]. These decisions for selecting a particular treatment strategy should be based on the clinical, radiological, and pathological findings for each patient, and the pros and cons of each approach should be discussed with the patient in detail before finalizing the treatment plan. The three principal options in the management strategy for patients with symptomatic lesions are surgery alone, RT alone or delayed adjuvant RT, and surgery with upfront adjuvant RT, which have similar long-term tumor control rates: 86.8– 90% for surgery alone (mean follow-up 24–100 months) [30, 31, 37, 43–45]; 90.5–98% for RT alone/delayed adjuvant RT (mean follow­up 30.5–109.2  months) [69, 70, 74, 79, 80, 88, 89]; and 81–94.1% for surgery with upfront adjuvant RT (mean follow-up 40–73 months) [46, 47, 85, 90]. We have presented a practical algorithm that can serve as a road map for the management of CSM tumors (Fig. 23.1). The management strategy for patients with these tumors is governed by tumor-associated symptoms, the anatomic localization, size and extent, and pathology of the CSM, the age and functional status of the patient, and tumor recurrence. A purely extracavernous approach leaving the CS proper unopened is optimal for most CSM tumors because it reduces the risk of iatrogenic complications without significantly increasing the risk of tumor progression/ recurrence [16, 17]. There are benefits to the brain stem and cranial nerves transgressing the lateral wall of the CS with this approach, especially when extracavernous resection is supple-

mented with partial decompression of the cranial nerves at their entry points into the CS.  Decompression and dissecting the tumor away from the optic apparatus are also critical when adjuvant SRS/FSRT is to be used. A minimum distance of 3–5 mm between residual tumor and optic apparatus allows safer institution of SRS/FSRT [52, 91]. Lastly, surgery also helps in diagnosis and grading of the lesion for optimizing further treatment options. For small intracavernous asymptomatic CSMs, either conservative management with close radiological surveillance or primary SRS/FSRT is a feasible option. However, because neurovascular complication rates increase as the surgical exploration of CS proper is expanded, the optimal strategy for a large CSM with extracavernous components includes maximal safe surgical exploration and adequate resection of the extracavernous component of the tumor, decompression of vital cranial nerves and brain stem, and adjuvant RT if required. Factors predicting resectability of CSM include the extent of ICA involvement/encasement, tumor consistency, increased adhesions or loss of anatomic planes from prior RT, and degree of extracavernous extensions involving orbital apex, SOF, and petroclival dura mater. Aggressive attempts at radical tumor resection or CSE are reserved for patients with atypical or malignant meningioma or complete loss of vision and extraocular movements or if previous RT (SRS or FSRT) has failed.

Newer Modalities Adjuvant treatment with chemotherapeutic agents like bevacizumab and sunitinib can be used to treat CSM tumors that grow despite surgical resection and adjuvant RT if the patient is not a suitable candidate for aggressive CS resection [92–94]. Other newer modalities of treatment for CSM, such as hydroxyurea (a ribonucleotide reductase inhibitor), mifepristone (an antiprogesterone agent), diltiazem and verapamil (calcium channel blockers), interferon-alpha, and somatostatin [95–99], have not been fully investigated, but ongoing research on genetic and signaling

23  Cavernous Sinus Meningioma

pathways of meningioma will likely have a significant impact on the treatment paradigm in future.

Conclusion As enhanced screening tools enable early identification of CSM, the clinical dilemma of whether to observe or intervene first for asymptomatic lesions offers a challenge to neurosurgeons. In the case of symptomatic CSM tumors, multimodality management is warranted, with either primary SRS/FSRT or maximally safe surgical resection with close radiological surveillance with or without delayed radiotherapy. These choices depend on the size, location, extent, and WHO grade of the tumor. The choice to undertake radical resection with exploration of the CS proper has been supplanted in many cases by a more conservative surgical strategy that involves resection of the extracavernous component of the tumor without exploration of the CS proper, which is often associated with a more favorable functional outcome. Acknowledgments  This chapter was adapted from a previous work by the senior author [17, 28, 58]. We thank Kristin Kraus, MSc, our medical editor, for her contribution to manuscript editing.

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23  Cavernous Sinus Meningioma gioma for preservation and improvement of cranial nerve function. Technical note. J Neurosurg. 2006;105(1):148–52. 53. Nakamura M, Krauss JK. Image-guided resection of small lesions in the cavernous sinus and Meckel’s cave. Eur J Surg Oncol. 2010;36(2):208–13. 54. Sughrue ME, Rutkowski MJ, Aranda D, Barani IJ, McDermott MW, Parsa AT. Factors affecting outcome following treatment of patients with cavernous sinus meningiomas. J Neurosurg. 2010;113(5):1087–92. 55. Mathiesen T, Lindquist C, Kihlstrom L, Karlsson B.  Recurrence of cranial base meningiomas. Neurosurgery. 1996;39(1):2–7; discussion 8–9. 56. Hirsch WL, Sekhar LN, Lanzino G, Pomonis S, Sen CN.  Meningiomas involving the cavernous sinus: value of imaging for predicting surgical complications. AJR Am J Roentgenol. 1993;160(5):1083–8. 57. Sekhar LN, Linskey ME, Sen CN, Altschuler EM. Surgical management of lesions within the cavernous sinus. Clin Neurosurg. 1991;37:440–89. 58. Couldwell WT, MacDonald JD, Taussky P. Complete resection of the cavernous sinus-indications and technique. World Neurosurg. 2014;82(6):1264–70. 59. George B, Ferrario CA, Blanquet A, Kolb F. Cavernous sinus exenteration for invasive cranial base tumors. Neurosurgery. 2003;52(4):772–80; discussion 80–2. 60. Liu JK, Couldwell WT. Interpositional carotid artery bypass strategies in the surgical management of aneurysms and tumors of the skull base. Neurosurg Focus. 2003;14(3):e2. 61. Pieper DR, LaRouere M, Jackson IT.  Operative management of skull base malignancies: choosing the appropriate approach. Neurosurg Focus. 2002;12(5):e6. 62. Yang T, Tariq F, Chabot J, Madhok R, Sekhar LN.  Cerebral revascularization for difficult skull base tumors: a contemporary series of 18 patients. World Neurosurg. 2014;82(5):660–71. 63. Lawton MT, Spetzler RF. Internal carotid artery sacrifice for radical resection of skull base tumors. Skull Base Surg. 1996;6(2):119–23. 64. Mendelowitsch A, Taussky P, Rem JA, Gratzl O.  Clinical outcome of standard extracranial-­ intracranial bypass surgery in patients with symptomatic atherosclerotic occlusion of the internal carotid artery. Acta Neurochir. 2004;146(2):95–101. 65. Couldwell WT, Cole CD, Al-Mefty O.  Patterns of skull base meningioma progression after failed radiosurgery. J Neurosurg. 2007;106(1):30–5. 66. Hasegawa T, Kida Y, Yoshimoto M, Koike J, Iizuka H, Ishii D.  Long-term outcomes of Gamma Knife surgery for cavernous sinus meningioma. J Neurosurg. 2007;107(4):745–51. 67. Kano H, Park KJ, Kondziolka D, Iyer A, Liu X, Tonetti D, et  al. Does prior microsurgery improve or worsen the outcomes of stereotactic radiosurgery for cavernous sinus meningiomas? Neurosurgery. 2013;73(3):401–10.

363 68. dos Santos MA, de Salcedo JB, Gutierrez Diaz JA, Calvo FA, Samblas J, Marsiglia H, et al. Long-term outcomes of stereotactic radiosurgery for treatment of cavernous sinus meningiomas. Int J Radiat Oncol Biol Phys. 2011;81(5):1436–41. 69. Iwai Y, Yamanaka K, Ishiguro T.  Gamma Knife radiosurgery for the treatment of cavernous sinus meningiomas. Neurosurgery. 2003;52(3):517–24; discussion 23–4. 70. Lee JY, Niranjan A, McInerney J, Kondziolka D, Flickinger JC, Lunsford LD.  Stereotactic radiosurgery providing long-term tumor control of cavernous sinus meningiomas. J Neurosurg. 2002;97(1):65–72. 71. Morita A, Coffey RJ, Foote RL, Schiff D, Gorman D. Risk of injury to cranial nerves after Gamma Knife radiosurgery for skull base meningiomas: experience in 88 patients. J Neurosurg. 1999;90(1):42–9. 72. Nicolato A, Foroni R, Alessandrini F, Bricolo A, Gerosa M.  Radiosurgical treatment of cavernous sinus meningiomas: experience with 122 treated patients. Neurosurgery. 2002;51(5):1153–9; discussion 9–61. 73. Pollock BE, Stafford SL.  Results of stereotactic radiosurgery for patients with imaging defined cavernous sinus meningiomas. Int J Radiat Oncol Biol Phys. 2005;62(5):1427–31. 74. Roche PH, Regis J, Dufour H, Fournier HD, Delsanti C, Pellet W, et al. Gamma Knife radiosurgery in the management of cavernous sinus meningiomas. J Neurosurg. 2000;93(Suppl 3):68–73. 75. Shin M, Kurita H, Sasaki T, Kawamoto S, Tago M, Kawahara N, et  al. Analysis of treatment outcome after stereotactic radiosurgery for cavernous sinus meningiomas. J Neurosurg. 2001;95(3):435–9. 76. Kimball MM, Friedman WA, Foote KD, Bova FJ, Chi YY. Linear accelerator radiosurgery for cavernous sinus meningiomas. Stereotact Funct Neurosurg. 2009;87(2):120–7. 77. Metellus P, Regis J, Muracciole X, Fuentes S, Dufour H, Nanni I, et  al. Evaluation of fractionated radiotherapy and Gamma Knife radiosurgery in cavernous sinus meningiomas: treatment strategy. Neurosurgery. 2005;57(5):873–86; discussion -86. 78. Skeie BS, Enger PO, Skeie GO, Thorsen F, Pedersen PH. Gamma Knife surgery of meningiomas involving the cavernous sinus: long-term follow-up of 100 patients. Neurosurgery. 2010;66(4):661–8; discussion 8–9. 79. Spiegelmann R, Cohen ZR, Nissim O, Alezra D, Pfeffer R.  Cavernous sinus meningiomas: a large LINAC radiosurgery series. J Neuro-Oncol. 2010;98(2):195–202. 80. Kuo JS, Chen JC, Yu C, Zelman V, Giannotta SL, Petrovich Z, et  al. Gamma Knife radiosurgery for benign cavernous sinus tumors: quantitative analysis of treatment outcomes. Neurosurgery. 2004;54(6):1385–93; discussion 93–4. 81. Stafford SL, Pollock BE, Foote RL, Link MJ, Gorman DA, Schomberg PJ, et  al. Meningioma radiosur-

364 gery: tumor control, outcomes, and complications among 190 consecutive patients. Neurosurgery. 2001;49(5):1029–37; discussion 37–8. 82. Pollock BE, Stafford SL, Link MJ, Garces YI, Foote RL.  Single-fraction radiosurgery of benign cavernous sinus meningiomas. J Neurosurg. 2013;119(3):675–82. 83. Metellus P, Batra S, Karkar S, Kapoor S, Weiss S, Kleinberg L, et al. Fractionated conformal radiotherapy in the management of cavernous sinus meningiomas: long-term functional outcome and tumor control at a single institution. Int J Radiat Oncol Biol Phys. 2010;78(3):836–43. 84. Shrieve DC, Hazard L, Boucher K, Jensen RL. Dose fractionation in stereotactic radiotherapy for parasellar meningiomas: radiobiological considerations of efficacy and optic nerve tolerance. J Neurosurg. 2004;101(Suppl 3):390–5. 85. Maguire PD, Clough R, Friedman AH, Halperin EC.  Fractionated external-beam radiation therapy for meningiomas of the cavernous sinus. Int J Radiat Oncol Biol Phys. 1999;44(1):75–9. 86. Nutting C, Brada M, Brazil L, Sibtain A, Saran F, Westbury C, et al. Radiotherapy in the treatment of benign meningioma of the skull base. J Neurosurg. 1999;90(5):823–7. 87. Slater JD, Loredo LN, Chung A, Bush DA, Patyal B, Johnson WD, et al. Fractionated proton ­radiotherapy for benign cavernous sinus meningiomas. Int J Radiat Oncol Biol Phys. 2012;83(5):e633–7. 88. Kondziolka D, Nathoo N, Flickinger JC, Niranjan A, Maitz AH, Lunsford LD.  Long-term results after radiosurgery for benign intracranial tumors. Neurosurgery. 2003;53(4):815–21; discussion 21–2. 89. Liscak R, Kollova A, Vladyka V, Simonova G, Novotny J Jr. Gamma Knife radiosurgery of skull base meningiomas. Acta Neurochir Suppl. 2004;91:65–74. 90. Goldsmith BJ, Wara WM, Wilson CB, Larson DA. Postoperative irradiation for subtotally resected meningiomas. A retrospective analysis of 140 patients treated from 1967 to 1990. J Neurosurg. 1994;80(2):195–201.

W. T. Couldwell and A. Raheja 91. Pamir MN, Kilic T, Bayrakli F, Peker S. Changing treatment strategy of cavernous sinus meningiomas: experience of a single institution. Surg Neurol. 2005;64(Suppl 2):S58–66. 92. Furtner J, Schopf V, Seystahl K, Le Rhun E, Ruda R, Roelcke U, et al. Kinetics of tumor size and peritumoral brain edema before, during, and after systemic therapy in recurrent WHO grade II or III meningioma. Neuro-Oncology. 2016;18(3):401–7. 93. Kaley TJ, Wen P, Schiff D, Ligon K, Haidar S, Karimi S, et al. Phase II trial of sunitinib for recurrent and progressive atypical and anaplastic meningioma. Neuro-Oncology. 2015;17(1):116–21. 94. Raheja A, Colman H, Palmer C, Couldwell WT. Dramatic radiographic response and paradoxical cerebrospinal fluid rhinorrhea associated with sunitinib therapy in recurrent atypical meningioma. J Neurosurg. 2017;127(5):965–70. 95. Chamberlain MC, Glantz MJ, Fadul CE. Recurrent meningioma: salvage therapy with long-acting somatostatin analogue. Neurology. 2007;69(10):969–73. 96. Grunberg SM, Weiss MH, Spitz IM, Ahmadi J, Sadun A, Russell CA, et al. Treatment of unresectable meningiomas with the antiprogesterone agent mifepristone. J Neurosurg. 1991;74(6):861–6. 97. Kaba SE, DeMonte F, Bruner JM, Kyritsis AP, Jaeckle KA, Levin V, et al. The treatment of recurrent unresectable and malignant meningiomas with interferon alpha-2B. Neurosurgery. 1997;40(2):271–5. 98. Loven D, Hardoff R, Sever ZB, Steinmetz AP, Gornish M, Rappaport ZH, et  al. Non-resectable slow-growing meningiomas treated by hydroxyurea. J Neuro-Oncol. 2004;67(1–2):221–6. 99. Ragel BT, Couldwell WT, Wurster RD, Jensen RL.  Chronic suppressive therapy with calcium channel antagonists for refractory meningiomas. Neurosurg Focus. 2007;23(4):E10. 100. Alzhrani G, Gozal YM, Sherrod BA, Couldwell WT.  A modified lateral orbitotomy approach to the superior orbital fissure. Oper Neurosurg (Hagerstown). 2019;16(6):685–91.

Pituitary Adenoma

24

Ben G. McGahan, Giuliano Silveira-Bertazzo, Thaïs Cristina Rejane-Heim, Douglas A. Hardesty, Ricardo L. Carrau, and Daniel M. Prevedello

Introduction Almost all pituitary adenomas are benign, but given the critical neurovascular structures around the sella, compression of neighboring structures can cause significant morbidity [1]. Pituitary adenomas are subdivided into microadenomas, less than 1  cm in size, and macroadenomas, greater than 1  cm. Another way to categorize pituitary adenomas is whether they secrete hormones or not, regarded as functional or nonfunctional, respectively. Invasion of pituitary adenomas into

B. G. McGahan Department of Neurological Surgery, The Ohio State University Medical Center, Columbus, OH, USA e-mail: [email protected] G. Silveira-Bertazzo Department of Neurological Surgery, The Ohio State University Medical Center, Columbus, OH, USA Department of Pediatric Endocrinology and Neurosurgery, Jeser Amarante Faria Children’s Hospital, and Neurological and Neurosurgical Clinic of Joinville, Joinville, SC, Brazil T. C. Rejane-Heim Department of Neurological Surgery, The Ohio State University Medical Center, Columbus, OH, USA Department of Pediatric Endocrinology and Neurosurgery, Jeser Amarante Faria Children’s Hospital, and Neurological and Neurosurgical Clinic of Joinville, Joinville, SC, Brazil Department of Pediatric Endocrinology, Nationwide Children’s Hospital, Columbus, OH, USA

the cavernous sinus leads to difficulty removing these tumors and is an independent risk factor for subtotal resection and recurrence [2–7]. The residual pituitary adenoma in these cases requires multimodal treatment with medical and radiation therapies to achieve the goals of treatment while preserving critical neurovascular structures. The cavernous sinus is a plexus of trabeculated venous channels separating the meningeal and periosteal layers of the dura just lateral to the sella in the coronal plane. The adjacent and intertwined neurovascular components make

Department of Pediatric Endocrinology, Federal University of Santa Catarina, Florianopolis, SC, Brazil D. A. Hardesty Departments of Neurological Surgery and Otolaryngology-Head and Neck Surgery, The Ohio State University Medical Center, Wexner Medical Center, Columbus, OH, USA e-mail: [email protected] R. L. Carrau Department of Otolaryngology-Head and Neck Surgery, Center for Cranial Base Surgery, The Ohio State University Medical Center, Wexner Medical Center, Columbus, OH, USA e-mail: [email protected] D. M. Prevedello (*) Department of Neurological Surgery, Center for Cranial Base Surgery, The Ohio State University Medical Center, Wexner Medical Center, Columbus, OH, USA e-mail: [email protected]

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 A. S. Youssef (ed.), Contemporary Skull Base Surgery, https://doi.org/10.1007/978-3-030-99321-4_24

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operating in this area complex. The lateral wall of the cavernous sinus contains the oculomotor nerve, the trochlear nerve, and the ophthalmic and sometimes maxillary division of the trigeminal nerve. The abducens nerve runs within the sinus just lateral to the cavernous internal carotid artery. The medial wall of the cavernous sinus is a distinct structure separating the pituitary gland from the cavernous sinus and is often invaded by pituitary adenomas [8].

Epidemiology Pituitary adenomas are a common lesion of the skull base with a wide-ranging prevalence based on radiological and postmortem studies estimated to be 16.7–22.5% [1, 9]. The vast majority of pituitary adenomas are not clinically relevant and are asymptomatic. Pituitary tumors account for approximately 10% of all brain tumors [10]. Pituitary adenoma incidence has a slight female predominance of 3.84 cases per 100,000 per year versus 3.23 in males. There has been described a significantly higher incidence of pituitary tumors, including adenomas, among blacks compared to whites [11]. Pituitary adenomas most commonly present between the ages of 30–50, slightly earlier in females [10]. Pituitary tumors account for the highest proportion of tumors in the central nervous system of children, adolescents, and young adults [9]. Approximately 5% are related to familial syndromes such as familial isolate pituitary adenoma (FIPA) or multiple endocrine neoplasia type I [9, 11].

Pathology Pituitary adenomas are thought to develop through clonal expansion of a single abnormal cell in the adenohypophysis due to somatic mutations or chromosomal abnormalities [11]. Approximately 65% of pituitary adenomas secrete endocrine hormones and are considered functional. All types of pituitary adenomas can invade the cavernous sinus, but the growth

hormone-­secreting adenomas invade the cavernous sinus at roughly twice the rate than the others [3]. Most pituitary adenomas do not grow after detection. Only 10% of microadenomas and 20% of macroadenomas grew over 2–8  years in a meta-analysis of 445 adenomas [12]. Local invasion occurs in approximately 5% of pituitary adenomas and may constitute a unique genetic subset of pituitary adenomas even though being similar on histology [13]. Pituitary carcinomas are rare metastatic lesions with a poor prognosis of approximately 66% survival at 1 year. These entities are typically invasive and secrete either ACTH or prolactin [14].

Diagnosis Presentations of pituitary adenomas can be quite variable, ranging from endocrine abnormalities causing Cushing’s disease, acromegaly, hyperthyroidism, amenorrhea/galactorrhea, and symptoms of mass effect such as headache, visual field loss, or diplopia. They can also present with apoplexy, causing severe headache and visual symptoms. Large macroadenomas can cause the underproduction of pituitary hormones through compression. Growth hormone is the most sensitive to compression and is typically first to become deficient, followed by gonadotropins, TSH, ACTH, and prolactin. Rarely, pituitary adenoma invasion into the sphenoid sinus can present with epistaxis or CSF leak. Pituitary adenomas are also often found incidentally, and many never require surgical intervention. Initial imaging obtained is often a noncontrasted head CT that reveals a sellar iso-­ hypoattenuation. Further evaluation of a patient with a suspected pituitary adenoma includes complete endocrine labs (24 h urinary-free cortisol, prolactin, thyroxine, thyrotropin, ACTH, growth hormone, luteinizing hormone, follicle-­ stimulating hormone, insulin-like growth hormone-­ 1, testosterone for men, estradiol in women) along with detailed ophthalmological testing. Imaging evaluation includes MRI with and without gadolinium along with delayed

24  Pituitary Adenoma

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Fig. 24.1  Original artistic depiction showing the Knosp grade system and other used new/modified anatomy-­ based classifications for pituitary tumors with cavernous sinus invasion. The modified Knosp system grades the

parasellar extension of the tumor toward the cavernous sinus concerning the intracavernous carotid artery. PG: pituitary gland; ICA: internal carotid artery; EEA: endoscopic endonasal approach

contrast-­enhanced imaging of the sella, which demonstrates delayed enhancement of the pituitary adenoma. In addition, vascular imaging if the cavernous sinus is involved. Formal catheter angiography or a CT angiogram depending on the surgeon’s preference; ours is usually a CT angiogram, which can then be fused with MRI in thin slices for neuronavigation. For patients with a history of significant facial trauma or sinus surgery, we have an endoscopic examination by our otolaryngology colleagues performed before ­surgery to assess the anatomy and viability of various reconstructive flap options. Typically, MRI is used to detect and determine the severity of cavernous sinus invasion, although direct observation intraoperatively is considered the gold standard. The incidence of cavernous sinus invasion by pituitary adenomas is extremely variable, with rates from 9% to 63%, depending on the imaging and observational classification used [15]. The Knosp classification is the most widely used grading of cavernous sinus invasion. It is based on the relationship of the pituitary

adenoma to tangential lines drawn in relation to the medial, midpoint, and lateral cavernous ICA and the supraclinoid ICA (Fig. 24.1). As such, the Knosp grade 0 does not encroach the medial carotid line. The Knosp grade 1 passes the medial tangential line of the carotids but not the intercarotid line, grade 2 passes the intercarotid line but not the line tangential to the lateral carotids, grade 3 passes the lateral border of the ICAs, and grade 4 encompasses the ICA circumferentially within the cavernous sinus. An updated grading system created subtypes 3a and 3b, which is when the pituitary adenoma passes the lateral ICA margin from above (3a) or below (3b) the cavernous ICA [5]. Knosp class 3 and 4 imaging classifications are predictive of cavernous sinus invasion: grade 3b is associated with intraoperative findings of cavernous sinus invasion in 70% of cases, and grade 4 is associated with a 100% rate of invasion [5]. The Knosp classification has been related to the histologically and surgically confirmed invasion of the cavernous sinus and the likelihood of gross total resection and endo-

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crinologic remission [5, 16]. The rate of endocri- sification promises to be useful for identifying nologic remission is inversely related to the invaded compartments of the CS, both preoperaKnosp classification. A study of growth tively and intraoperatively, complementing the hormone-­ secreting adenomas had 82.2% of current imaging-based classifications and anaKnosp class 1 and 2 achieving biochemical tomical studies of the CS from an endonasal perremission following surgery, 42.9% of class 3, spective [20]. and only 25% of class 4 [4]. Although the Knosp Furthermore, in an attempt to better underclassification scale has been found to be overall stand and standardize the management for pitureliable, the middle-­grade interrater reliability is itary adenomas, several classifications were relatively weak. When dichotomized to two clin- proposed, such as Hardy’s classification based on ically useful categories unlikely to have cavern- radiological parameters, histological classificaous sinus involvement and likely to have tions of adenohypophyseal tumors proposed by cavernous sinus involvement, the reliability was WHO in 2004 and 2007, as well as size-based again strong [17]. classifications for pituitary tumors, such as the Wilson’s system has been devised to classify threshold of 1  cm used for microadenomas or the invasiveness of pituitary adenomas [18]. In macroadenomas, as aforementioned, or the Wilson’s system, the extension is classified as (0) threshold of 4  cm for giant pituitary adenomas. no extension, (A) into the suprasellar cistern, (B) Also, for these giant tumors, Goel established into the anterior recess of the third ventricle, (C) four-grade rating based on their anatomic extendisplacement of the floor of the third ventricle, sions and the nature of their meningeal coverings (D) parasellar extension intracranially, and (E) [21]. These grades reflected an increasing order into or beneath the cavernous sinus. Invasion and of invasiveness of adjacent dural and arachnoidal spread are also classified as (I) sella normal or compartments in which a radical transsphenoidal focally expanded, (II) sella enlarged, (III) local- resection would be indicated for pituitary tumor ized perforation of the sellar floor, (IV) diffuse grades I–III. In contrast, for those transgressing destruction of the sellar floor, and (V) distant the diaphragma sellae boundary and encasing the spread through the CSF or blood. arteries of the Circle of Willis (grade IV), a radiThus, some authors do not consider this cal resection would be difficult, and the surgical radiologic-­based classification useful for predict- aim can be tumor biopsy followed by radiation ing the invasion of the medial wall of the cavern- therapy. ous sinus (MWCS). As such, Fernandez-Miranda et  al. recently proposed a new anatomy-based classification of cavernous sinus invasion based Management Strategies on modifications of the previous three-­ compartment CS classification described by Treatment options for pituitary adenomas include Harris and Rhoton in 1976 [19]. It divides the CS observation, medical therapy, open craniotomy, into four compartments based on their relation- minimally invasive microsurgical or endoscopic ship with the cavernous ICA’s horizontal seg- endonasal transsphenoidal approaches, and radiament, where the superior and inferior tion therapy. The goals of treatment should be compartments, corresponding to grades 3a and clearly defined to decide the optimal treatment 3b of the modified Knosp classification, are paradigm for each patient. Generally, the goal of located above and below this level (horizontal pituitary adenoma treatment should be to resolve cav-ICA). Likewise, the posterior compartment, symptoms by achieving tumor control and induce not previously detailed by the Knosp classifica- biochemical remission (if functional adenoma) tion, would be located posteriorly to the horizon- while preserving critical neurovascular structures tal cavernous ICA, and the lateral compartment and avoiding iatrogenic injury. Other considerwould be lateral to the anterior genu and horizon- ations include obtaining tissue for diagnosis if the tal segment of the ICA. This anatomy-based clas- preoperative imaging and/or history are unclear

24  Pituitary Adenoma

on the differential diagnosis of the tumor (such as a patient with known metastatic disease from another organ site). Also, pathological investigation of an adenoma will yield some information on the risk of recurrence via proliferative index. Debulking of a tumor considered unresectable based on preoperative imaging in preparation for radiation might also be considered [22]. The endoscopic endonasal transsphenoidal approach has been increasingly accepted as the mainstay of surgical treatment for pituitary adenomas with cavernous sinus invasion. The microsurgical endonasal approach had previously been restricted medially to the sella due to restrictions in hand freedom of movement as well as illumination and line of sight. However, the development of expanded coronal plane approaches and angled endoscopes has opened more lateral visualization and access into the cavernous sinus. Currently, the use of open craniotomy for pituitary adenomas with cavernous sinus invasion is only in the rare case when the tumor expands beyond the reaches of an endonasal approach, far laterally into the middle fossa. Compared to the prior use of the operative microscope, the use of the endoscope has also demonstrated a greater extent of resection and biochemical remission in functional pituitary adenomas [23, 24]. The different options and techniques for the endonasal endoscopic approach are based on how much lateral access is needed and if the ICA needs to be transposed. For most pituitary adenomas with lower Knosp grades (1 or 2), a standard transsphenoidal approach is usually sufficient, with or without resection of the middle turbinate depending on individual patient anatomy. The more lateral access that is needed with higher Knosp grades (3 or 4) may necessitate the use of a proper expanded endoscopic endonasal approach with middle turbinate resection, the sacrifice of the ipsilateral nasoseptal pedicle, and adoption of an expanded transpterygoid exposure with mobilization and/or sacrifice of the Vidian nerve, as needed, to improve the exposure and working angles within the cavernous sinus. In addition, patient outcomes are expected to be good if gross total resection is achieved safely. In one series of 50 pituitary adenomas Knosp

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class 1–3, there was 100% gross total resection, no radiographic recurrence, and only one biochemical recurrence 2 years postoperatively [25]. There were no deaths or ICA injuries, only once CSF leak. There were four (8%) new cranial nerve palsies that resolved by 3  months. Although the generalizability of these results may be limited due to the fact that this series coming from a highvolume skull base center with an experienced team. Experts have suggested that only experienced surgeons should perform resections into the cavernous sinus due to the risks of devastating neurovascular complications [25, 26]. A metaanalysis of pituitary adenomas with cavernous sinus invasion reports ICA injuries from 0% to 5% and new cranial nerve deficits up to 27% [15]. There are many considerations in weighing the benefits of an aggressive gross total resection against the risk of injuring the critical structures within the cavernous sinus. In our practice, we favor an aggressive resection for functional adenomas as a gross total resection is the best opportunity for biochemical remission [6] and a less aggressive approach to nonfunctioning adenomas as their symptoms can often be controlled with decompression and the majority do not recur or grow. Less aggressive options could include leaving residual tumor behind with the intention of treating with stereotactic radiosurgery upon tumor progression. Instead of resecting the medial wall of the cavernous sinus that is invaded by the tumor, it could be coagulated and shrunken. In case gross total resection is not achievable, there is evidence that subtotal resection remains beneficial. Subtotal resection of GH secreting tumors can improve hormonal control with somatostatins or radiation [27, 28]. Functional pituitary adenomas should be treated with the assistance of an endocrinology team (Chap. 19). Many patients can achieve hormonal and symptomatic control medically either before surgery in the case of prolactinomas or after surgery due to residual or recurrence from growth hormone-­ secreting or ACTH-secreting adenomas. Up to 90% of patients with prolactinomas can achieve hormonal control with dopamine agonists such as bromocriptine or cabergoline. With growth hormone-secreting

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tumors, somatostatin analogs can achieve near 40% hormonal control and up to 90% control with pegvisomant, a growth hormone receptor blocker. The cortisol blocker mifepristone can achieve symptomatic control in up to 87% of those with ACTH-­ secreting adenomas [29]. Radiation therapy techniques allowing focused delivery can also be harnessed for the treatment of pituitary adenomas. Stereotactic radiosurgery (SRS) has traditionally been reserved for patients not able to tolerate surgery or for those with residual tumors after surgery since it is not often able to provide a “cure.” Nevertheless, SRS is effective: nonfunctioning pituitary adenomas with the residual disease have up to a 96% control rate with SRS [30]. Prolactinomas had 27–50% biochemical remission with SRS [31, 32]. Growth hormone-­releasing tumors had an even greater response, with up to 82% having biochemical control after SRS, although this effect is often delayed for 6–18 months after radiation delivery [33]. Although considered safe and often less morbid than surgery, there remains a 30% risk of hypopituitarism after radiation to the sella, as well as rare side effects such as radiation-induced optic neuritis [33]. Along with SRS, there are many treatment options to consider for pituitary adenomas that recur. In patients who are older or asymptomatic, observation is a reasonable approach. Reoperation using the same approach or a different approach can be considered as well. Reoperation has been shown to be as successful as primary operations for growth hormone-secreting tumors [2]. Although, when the cavernous sinus is involved, reoperation is less successful. A multimodal combination of surgery, SRS, medical treatment, and reoperation maybe what is needed to produce the best results for some patients. With ACTH secreting macroadenomas with cavernous sinus invasion, remission increased from 20% to 40% after reoperation, SRS, and medical treatment. Similar results were seen with remission for macro-prolactinomas going from 17.6% to 47%. Growth hormone-secreting macroadenomas up to 52.6% remission with SRS and somatostatins used in combination with surgery compared to just 15.8% with surgery alone [7].

Surgery Described Surgical Technique: Expanded Endoscopic Endonasal Approach to Cavernous Sinus The patient is placed in rigid head fixation with the head turned toward the operator. We utilize neuromonitoring via somatosensory-evoked potentials (SSEPs) and extraocular muscle electromyography (EMG) for tumors that invade the cavernous sinus. Preoperatively, antibiotics are administered. We strongly believe in a team approach for endoscopic skull base surgery consisting of a neurosurgeon and an otolaryngologist head and neck surgeon, each with subspecialty training in endoscopic endonasal surgery. Endoscopic endonasal approaches to the cavernous sinus are detailed in Chap. 22. Figure  24.2 illustrates our endoscopic technique through step-by-step intraoperative photos detailing the expanded endoscopic approach to remove an extensive pituitary adenoma with cavernous sinus invasion. Broad exposure of the sphenoid sinus is performed, and mucosal flaps are preserved for later reconstruction. If a full transpterygoid approach is necessary, a nasoseptal flap pedicled on the contralateral side is fashioned, and the ipsilateral side pedicle is coagulated and cut to allow for the lateral transpterygoid corridor to be opened. A medial maxillary opening is widened, and the contents of the pterygopalatine fossa can be exposed after removal of the posterior wall of the maxillary sinus. Nevertheless, we tend to lateralize the periosteum covering the pterygopalatine contents, thus avoiding damage to neurovascular structures inside the fossa. However, the Vidian nerve can at times be exposed and lateralized but often will be sacrificed if dealing with malignancies. As such, most of the time this nerve is used to map the ICA. Tracing the Vidian canal back to the carotid level allows for safe identification of the carotid level as it turns from the petrous to the clival segment. The sella is drilled with a high-­speed drill, and the dura is wide opened. A micro Doppler is used to confirm and map the ICA; neuronavigation can be helpful but should not be relied on

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Fig. 24.2  Step-by-step intraoperative photos show the tumor debulking technique during the expanded sellar/ parasellar EEA during the removal of an extensive pituitary adenoma with suprasellar extension and left-sided cavernous sinus invasion (a–n). Comparative pre- and

postoperative coronal MRI can be seen in o, demonstrating a near-total resection and adequate skull base reconstruction. CS: cavernous sinus; CR: cavernous recess; *t: tumor; SC: sellar cavity; SD: sellar diaphragm; IHA: inferior hypophyseal artery

without Doppler confirmation when incising the lateral dura to expose tumor within the cavernous sinus. Venous bleeding is controlled with hemostatic agents; cavernous sinus invasion may lead to significant preoperative thrombosis and a relatively dry cavernous sinus due to the mass effect of the tumor. Brisk bleeding may herald the completion of the resection, “unplugging” the remaining sinus. Dissection of the pituitary adenoma is performed circumferentially, preferably extracapsular, although this is often impossible with cavernous invasion, and so debulking is performed as needed. Compared to many pathologies found within the cavernous sinus, many pituitary adenomas are relatively soft, and so, angled dissectors and suction tips may allow for additional resection beyond what boundaries straight dissectors reach. Maximal safe resection is done working from medial to lateral while preserving cranial nerves and the ICA. Alternatively, the medial wall of the cavernous sinus can be resected initially to gain early control of cavern-

ous sinus bleeding [8, 25]. Stimulation with EMG probes here can be helpful in identifying the abducens nerve. Relevant surgical anatomy is demonstrated in Fig.  24.3. After maximal safe resection, the reconstruction is performed with Duragen© matrix inlay/onlay and a vascularized nasoseptal flap, as needed.

Case Illustration A 57-year-old female presented with a history of gradual onset of bilateral blurry vision and tooth pain. Upon examination, the patient was neurologically intact, except for bilateral peripheral vision loss. MRI with contrast was obtained. It demonstrated a large sellar/parasellar mass with a superior suprasellar extension and significant displacement of the optic chiasm, and invasion of all compartments of the cavernous sinus on the left side. After blood samples and ­multidisciplinary team evaluation, in the setting

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a

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Fig. 24.3  Original stepwise cadaveric dissection images showing the anatomical landmarks of the expanded sellar and parasellar approaches to remove pituitary tumors with cavernous sinus invasion. The cavernous sinus and its neurovascular relations are exposed on both sides after bone and dural opening (a–c). III: oculomotor nerve; V1: ophthalmic nerve; V2: maxillary nerve; V3: mandibular nerve; VI: abducens nerve; C: clivus; BA: basilar artery;

SCA: superior cerebellar artery; ICA-Sa: anterior bend of the internal carotid artery–parasellar segment; ICA-Sp: posterior bend of the internal carotid artery–parasellar segment; ICA-C: paraclival segment of the internal carotid artery; ICA-L: lacerum segment of the internal carotid artery; ICA-P: petrous segment of the internal carotid artery; PG: pituitary gland; VN: Vidian nerve

of the normal pituitary panel, and a young patient with progressive visual deterioration due to a sizeable nonfunctional macroadenoma, surgery was indicated. The patient underwent an expanded endoscopic endonasal transsphenoidal sellar/parasellar approaches with left-sided transpterygoid, transcavernous exposures. A step-by-step approach is depicted in Fig.  24.2. There were no intra- or postoperative complications, and the pathology was positive for LH/ gonadotropin tumor (Ki-67 index of 1–2%). The patient remains stable after surgery with no postoperative deficits. Pre-and postoperative MRI can be seen in Figs. 24.2 and 24.4. It demonstrates a near-total resection of the tumor with a minimal residual in the lower aspect of the left cavernous sinus only observed during follow-up.

the pituitary adenoma is not a prolactinoma and is symptomatic with either endocrine or mass effect, then the primary treatment is surgery. If asymptomatic and incidentally discovered, then observation is safe with serial MRIs. One caveat in our practice is if a patient is younger (generally, under age 65) and a pituitary adenoma is abutting or displacing the optic chiasm or prechiasmatic nerve, we offer upfront surgical resection instead of observation to prophylactically decompress and prevent future vision loss. This is a nuanced discussion to have with the patients, and if they choose further observation knowing the risk of future vision loss, that is a viable strategy as well. When pursuing surgery, we prefer the endoscopic endonasal approach unless the pituitary adenoma is expanding outside the sellar region, past the lateral aspect of the cavernous sinus into the temporal fossa. In that case, an open craniotomy or combined or staged endonasal and craniotomy is considered. If an endonasal endoscopic approach is utilized, then evaluating the Knosp grade helps determine the degree of exposure needed. For Knosp grade 1 or 2 tumors, a midline standard endonasal approach should suffice. For Knosp grade 3 or 4 tumors, an attempt is made at gross total resection, then a middle turbinate resection with lateral transpterygoid extension is used; the degree of lateral exposure can be ­tailored to the lesion. For tumors classified as Knosp grade 4, a full transpterygoid approach

Summary Algorithm for Management At diagnosis, endocrine and ophthalmological evaluations are needed to assess for subtle abnormalities. If the pituitary adenoma is a prolactinoma, then primary treatment is with a dopamine agonist and serial imaging to ensure stability or regression. If a prolactinoma is resistant to dopamine agonists and symptoms persist or tumor size increases on imaging, or if the patient cannot tolerate the side effects, then we pursue surgery. If

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Fig. 24.4  Pre- (a–c) and postoperative (d–f) MRI with contrast of the patient whose procedure is described in case 1 can be seen. It demonstrates a near-total resection and adequate skull base reconstruction

with skeletonization and the ICA mobilization can be performed depending on how aggressive resection is planned. The functional status of a pituitary adenoma is taken into account when deciding how aggressive to be with resection. For a functional pituitary adenoma, symptomatic relief is usually only achieved with a gross total resection as lack of biochemical remission is associated in functional adenomas with poor quality-of-life outcomes [34]. Our team thus prefers a more aggressive resection for functional tumors. If the pituitary adenoma is nonfunctioning and symptoms are related to mass effect, symptom control can be achieved with subtotal resection. The goal remains in nonfunctioning tumors to resect as much tumor as safely possible, but typically less aggressively than functional tumors. The residual tumor can be observed or treated with radiation therapy. Functional tumors with residual should be treated with SRS.  This treatment algorithm is summarized in Fig.  24.5. There are a variety of options on how to treat residual tumor or recurrence, including the same surgi-

cal approach, a different surgical approach, radiation therapy, or observation. The treatment should be tailored to the individual patient with an experienced multidisciplinary team.

Conclusion The mainstay of surgical treatment for pituitary adenomas with cavernous sinus invasion is an endoscopic endonasal approach for resection, sometimes with extended approaches, radiation, medical treatment, or open craniotomies. As such, excellent knowledge of surgical anatomy and nuances to remove the natural barriers preventing full access to the paramedian skull base, determines the ease of using the expanded sellar/ parasellar approaches as the main gateway for all the coronal modules during endoscopic endonasal access (EEA) to pituitary tumors with ­cavernous sinus (CS) invasion. Meticulous utilization of operative landmarks and strategies can help avoid and mitigate surgical complications.

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374 Pituitary adenoma

Non prolactinoma

Asymptomatic

Prolactinoma treat with dopamine agonist, and ovserve

Symptomatic

Young with adenoma touching optic nerve

Symptoms resistant to dopamine agonist

Without invasion into middle fossa. EEA

Surgery*

Large invasion to middle fossa. Combination EEA and craniotomy

* More aggressive with functional PA for GTR; Less aggressive with non-functioning, ok leaving some residual tumor if necessary

Observation

Knosp Class 1& 2 Midline EEA

Knosp 3 & 4 transpterygoid approach, consider ICA transposition

Residual or recurrence. If asymptomatic and older patient, could consider observation or consider craniotomy or EEA for reresection +/- radiation

Fig. 24.5  Flow sheet of a treatment algorithm for pituitary adenomas with invasion into the cavernous sinus

Compliance with Ethical Standards

References

Funding No funding was received for this research.

1. Ezzat S, Asa SL, Couldwell WT, Barr CE, Dodge WE, Vance ML, McCutcheon IE. The prevalence of pituitary adenomas: a systematic review. Cancer. 2004. https://doi.org/10.1002/cncr.20412. 2. Almeida JP, Ruiz-Treviño AS, Liang B, Omay SB, Shetty SR, Chen YN, Anand VK, Grover K, Christos P, Schwartz TH.  Reoperation for growth hormone– secreting pituitary adenomas: report on an endonasal endoscopic series with a systematic review and meta-­ analysis of the literature. J Neurosurg. 2018. https:// doi.org/10.3171/2017.2.JNS162673. 3. Hofstetter CP, Shin BJ, Mubita L, Huang C, Anand VK, Boockvar JA, Schwartz TH.  Endoscopic endonasal transsphenoidal surgery for functional pituitary adenomas. Neurosurg Focus. 2011. https://doi.org/10. 3171/2011.1.FOCUS10317. 4. Jane JA, Starke RM, Elzoghby MA, Reames DL, Payne SC, Thorner MO, Marshall JC, Laws ER, Vance ML.  Endoscopic transsphenoidal surgery for acromegaly: remission using modern criteria, complications, and predictors of outcome. J Clin ­ Endocrinol Metab. 2011. https://doi.org/10.1210/ jc.2011-­0554. 5. Micko ASG, Wöhrer A, Wolfsberger S, Knosp E. Invasion of the cavernous sinus space in pituitary adenomas: endoscopic verification and its correlation with an MRI-based classification. J Neurosurg. 2015. https://doi.org/10.3171/2014.12.JNS141083. 6. Nishioka H, Ukuhara N, Horiguchi K, Yamada S.  Aggressive transsphenoidal resection of tumors

Ethical Approval All procedures performed in studies involving human participants were in accordance with the ethical standards of the Ohio State University Wexner Medical Center institutional research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards. Informed Consent Informed consent was obtained from all individual participants included in this study. Conflict of Interest Photographs in this chapter were taken at ALT-VISION at The Ohio State University. This laboratory receives educational support from the following companies: Carl Zeiss Microscopy, Intuitive Surgical Corp., KLS Martin Corp., Karl Storz Endoscopy, Leica Microsystems, Medtronic Corp., Stryker Corp., and Vycor Medical. Dr. Prevedello is a consultant for Stryker Corp., Medtronic Corp., and Integra; he has received an honorarium from Mizuho and royalties from KLS- Martin. Ricardo L. Carrau is a consultant for Medtronic Corp.

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25

Schwannoma Shahed Elhamdani, Vijay A. Patel, and Paul A. Gardner

Introduction While meningiomas compose the bulk of benign cavernous sinus neoplasms, schwannomas compose the second largest group. Nonvestibular intracranial schwannomas are rare and compose a small portion of intracranial tumors, appearing anywhere along the length of the nerve, often involving multiple anatomical compartments. This makes cavernous sinus schwannomas particularly rare as the majority of nonvestibular schwannomas occur outside the cavernous sinus [1–5]. Despite its relative rarity, schwannomas are of particular interest to neurosurgeons if only for the fact that complete resection of the tumor is curative, making schwannomas a surgical disease above all else. Therefore, it is the neurosurgeon’s responsibility to have a detailed understanding of the pathology and anatomical nuances of the cavernous sinus for appropriate S. Elhamdani Department of Neurosurgery, Allegheny Health Network, Pittsburgh, PA, USA e-mail: [email protected] V. A. Patel Department of Otolaryngology, University of Pittsburgh School of Medicine, Pittsburgh, PA, USA e-mail: [email protected] P. A. Gardner (*) Departments of Neurological Surgery and Otolaryngology, University of Pittsburgh School of Medicine, Pittsburgh, PA, USA e-mail: [email protected]

decision-making. A detailed discussion of the surgical approaches to the cavernous sinus will be discussed elsewhere in this book, but it is important to understand the key relationships of these tumors to the intimately intertwined anatomy of the region.

Pertinent Pathology and Anatomy Schwannomas are benign, encapsulated peripheral nerve sheath tumors that are generally slow growing, arising from the neural crest-derived Schwann cells. On histopathology, they are defined by Antoni A and Antoni B patterns, with newer research focused on a deeper understanding of how these biphasic patterns and associated cellular microenvironments affect tumor growth and progression [6–8]. It is important to remember that schwannomas arise from a single nerve fiber; while this makes surgical treatment easier in some ways, it also increases the potential to damage the other nerve fibers and function [6]. The majority of these tumors occur as solitary tumors, with multiple tumors raising concern for disease processes such as neurofibromatosis (NF) or schwannomatosis [6, 9]. Schwannomas can rarely convert into malignant nerve sheath tumors, though the incidence is quite low except in the case of NF-1 and slightly less so NF-2 [6, 10]. Schwannomas can arise intracranially along the tract of any peripheral cranial nerve with ves-

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 A. S. Youssef (ed.), Contemporary Skull Base Surgery, https://doi.org/10.1007/978-3-030-99321-4_25

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tibular schwannomas comprising the overwhelming majority followed distantly by trigeminal schwannomas [1, 11, 12]. Though not the subject of this chapter, surgical and nonsurgical management of nonvestibular schwannomas benefits immensely from the relatively high volume of vestibular schwannoma literature. Understanding the neural structure of origin and thus where the schwannoma lies within the cavernous sinus is the key to planning the surgical approach. A detailed anatomic overview of the cavernous sinus can be found in Chap. 22, but it is essential to highlight a few key points that provide a framework for surgical planning. A useful way of beginning to classify cavernous sinus masses comes from Chotai et  al., in which they organized cavernous sinus masses based on the location of origin. Type I consisted of tumors originating from within the cavernous sinus, type II from the lateral wall of the sinus, and type III extending from outside the cavernous sinus [13]. This is a simple framework for classifying pathology and is useful as an initial way of categorizing the various pathologies. A more surgically focused framework, especially useful for endoscopic approaches, comes from Fernandez-Miranda et  al., where the cavernous sinus was divided into four compartments based on the relationship with the intracavernous internal carotid artery (ICA). The superior compartment consisted of the interclinoidal ligament and potentially the oculomotor nerve, the posterior compartment of the gulfar segment of the abducens nerve and inferior hypophyseal artery, the inferior compartment with the distal abducens nerve, and the lateral compartment with the remaining cavernous cranial nerves and inferolateral trunk of the ICA [14]. This is more useful surgically as it defines a centerpiece, that is, the ICA, within the cavernous sinus, and creates a framework for orientating the neural and vascular structures, particularly important with endoscopic approaches. Both of these systems highlight an important point: most of the cranial nerves and thus cavernous schwannomas run in the lateral compartment of the cavernous sinus. The lateral wall of the cavernous sinus is composed of two dural layers,

an external (meningeal) and internal (endosteal) layer, of which the cranial nerves of the cavernous sinus run within the latter, except for the sixth cranial nerve that runs more freely within the cavernous sinus itself [15–18]. Therefore, the majority of cavernous sinus schwannomas lie outside of the cavernous sinus proper and are separated from the sinus by this dual dural layer. This is a crucial point and will form the basis for much of the decision-making in regards to surgical approaches and reducing morbidity and mortality associated with treatment.

Imaging Like most skull base pathology, the combination of imaging modalities, primarily computed tomography (CT) and magnetic resonance imaging (MRI), is fundamental to diagnosis, preoperative planning, intraoperative navigation, and postoperative monitoring. Particularly in situations where there is no imminent risk to the patient’s life or rapidly deteriorating neurological function, which is almost never the case with cavernous sinus schwannomas, comprehensive imaging is necessary to allow for the most ideal surgical and overall treatment course [19]. While the clinical exam and medical workup are clearly important for the evaluation of these patients, the variety of cavernous sinus pathology and overlap of presenting symptoms make imaging often the best chance at establishing a diagnosis prior to surgical biopsy or resection. Computed tomography (CT) lacks the neural structure fidelity of MRI, but as an initial screening tool and evaluation of skull base bony structures and cavernous vasculature, it is key to the approaches to this region. Patients present to the emergency department or primary care physician with complaints of diplopia, facial pain, or sensory deficits and will usually be triaged with CT for initial evaluation. Particularly in large schwannomas the CT can provide additional information regarding extension into the skull base [3, 5, 20]. If endonasal surgery is being considered, fine-cut CT of the skull base is essential to planning the endoscopic approach [19, 21].

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Furthermore, CT angiography imaging is valuable to document the relationship between the cavernous ICA and the mass prior to surgery. Venous phase imaging can reveal medialization or lateralization of the cavernous sinus itself, suggesting an external mass compressing or extending into the sinus rather than originating from the sinus [22]. Magnetic resonance imaging (MRI) is the gold standard for examining neural structures and thus is the primary tool for preoperative eval-

a

uation, diagnoses, and post-treatment follow-up. Gadolinium-based contrast MRI will reveal an enhancing mass in the cavernous sinus often with extension into the adjacent compartments. There is also a typically heterogeneous but largely hyperintense T2 appearance that can help to guide the diagnosis (Fig. 25.1) [5]. Despite this, the density of neural and vascular structures within the region makes definitive diagnoses difficult if not impossible. If the mass extends outside of the cavernous sinus along the tract of the

b

c

Fig. 25.1  T2-weighted axial (a) T1-weighted postcontrast axial (b) and coronal (c) MRI showing a typical, heterogeneous T2 but enhancing cavernous sinus schwannoma

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nerve, either before or after the cavernous sinus, the extension of the mass would suggest which of the cranial nerves are involved, such as foramen ovale suggesting a trigeminal nerve schwannoma and oculomotor triangle suggesting a third nerve tumor. MRI also serves as the modality of choice for tumor observation prior to intervention and monitoring post intervention [3–5, 19]. Ideally, newer technologies such as high-definition fiber tracking (HDFT) would allow for greater definition of the relationships of tumors and nerves, but the tight space and skull base surrounding the cavernous sinus currently present significant challenges for this technology to accurately define these relationships.

Trigeminal Schwannoma Trigeminal nerve schwannomas are the most common cavernous sinus schwannomas and therefore will compose the bulk of our discussion. While the most common in the cavernous sinus, they are rare among brain tumors in general and within intracranial schwannomas, accounting for less than 1–5% of the latter [1, 3, 4]. In Sarma et al.’s review of 46 nonvestibular schwannomas, they found almost 50% of them to be trigeminal schwannomas [1]. Similar numbers have been seen in other reviews [11, 23–25]. It is important to recall the differentiation between cavernous sinus and Meckel’s cave schwannomas, the latter of which will be covered in a separate section (see section V). Technically, only V1 (ophthalmic branch) trigeminal schwannomas lie in the cavernous sinus while V2 and V3 occur in Meckel’s cave. Therefore, the bulk of these tumors lie outside the cavernous sinus, but these tumors can reside solely within the lateral wall of the cavernous sinus or more commonly as extensions from Meckel’s cave into the cavernous sinus [3, 5, 11]. Classically presenting in the fourth to sixth decade of life, the most common presenting symptoms for these patients are facial pain or paresthesias [3, 20]. Other presenting signs result from compression of the surrounding neu-

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ral structures leading to oculomotor or abducens nerve palsies with patients usually complaining of diplopia [22, 26–28]. Unfortunately, the symptomatology of the tumors that lie within this region can be similar and therefore difficult to distinguish on clinical presentation alone. It is generally assumed that regardless of where they originate along the track of the nerve, trigeminal schwannomas should share the same characteristics in terms of response to therapy and recurrence rates. Interestingly, there is evidence to suggest that trigeminal schwannomas within the cavernous sinus recur more frequently than schwannomas located elsewhere within the trigeminal nerve, with recurrences nearing 80% in a retrospective analysis by Taha et al., significantly higher than extra-cavernous trigeminal schwannomas [29]. This could potentially be due to some genetic or microenvironment differences between the tumors, but also could correlate with the difficulty in obtaining a radical resection in the cavernous sinus. The low incidence makes it difficult to provide any highpowered answer to this question. Nonetheless, this should alert the surgeon to closely follow these patients even after removal, both clinically and radiographically. While more is known about trigeminal schwannomas than the others discussed in this chapter, the rarity of the cases as well as the overall difficulty of operating the cavernous sinus makes this a challenging case even for the experienced surgeon. For tumors that are either incidentally found or causing minimal symptoms, particularly when extraocular motor or masseter weakness is not present, close observation and monitoring is the reasonable and preferable option. There are no long-term studies looking at the growth of this particular subset of schwannoma, but by extrapolation from patterns seen in vestibular and other trigeminal schwannomas, it can be assumed that a subset of these patients will have radiographically inactive tumors and can avoid intervention. However, tumors with progressive symptoms or increasing size on subsequent MRI will likely necessitate treatment, which will be touched on later in the chapter.

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Needless to say, surgery should only be attempted with teams experienced in skull base approaches, preferably both traditional open and endoscopic approaches.

Abducens Schwannoma The extreme rarity of the remainder of the tumors in the chapter makes any detailed analysis difficult, but the general principles remain the same, albeit with important caveats. There have only been several dozen cases reported in the literature of abducens schwannomas, with a recent systemic review by Nakamizo et al. only reporting 29 cases in the English literature [26]. Furthermore, the majority have been reported outside of the cavernous sinus, making cavernous sinus abducens schwannomas particularly rare even among the already rare abducens schwannomas. The most common presentation of abducens schwannoma regardless of location is diplopia secondary to paresis of the nerve, though as with all cavernous sinus pathology, adjacent cranial nerve deficits are possible [27]. As mentioned before, the abducens nerve differs from the other cranial nerves within the cavernous sinus in that it does not run between the dual dural layer on the lateral wall but actually runs within the sinus itself [15–17]. Referring back to our prior anatomical frameworks, the abducens nerve would be in the posterior, inferior, and lateral compartments, whereas all other nerves lie within the lateral compartment [14]. Therefore, surgical resection for a tumor that is entirely within the cavernous sinus requires opening of the cavernous sinus, making surgery particularly challenging. It is important to recognize that even within the cavernous sinus the schwannoma can vary in its location along the abducens nerve, impacting the resectability as well as clinical recovery [26, 30, 31]. In addition, the ophthalmic branch of trigeminal lies immediately lateral to the abducens nerve, making access via a transcranial approach difficult without significant V1 manipulation. Though there is limited data, evaluation of available outcomes data shows that when compared to trigeminal schwannomas, abducens schwannomas have lower rates of com-

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plete recovery following surgical resection with over 60% of patients with paresis or complete loss of function following surgical resection [26, 31]. This is not surprising given that the nerve is small caliber, and resection therefore is likely to damage other fibers.

Oculomotor Schwannoma Like abducens schwannomas, oculomotor schwannomas are extremely rare with no standard treatment approach. They can arise from anywhere along the course of the nerve with a significant proportion either extending into the cavernous sinus or being contained entirely within it [32]. Similar to V1 trigeminal schwannomas, these tumors lie within the dual dural layer of the lateral wall, making resection possible without entering into the cavernous sinus proper [13, 14, 16, 17]. In their review of oculomotor schwannomas, Muhammad et  al. identified 60 cases of oculomotor schwannoma reported in the literature, with 45 treated surgically and 7 treated with radiosurgery. Outcomes were not separated based on location along the nerve. They found that there was a significant risk of worsening postoperative cranial nerve palsy of over 70% with only 22% recovering function. Furthermore, they found that gross total resection was found to have worse postoperative function when compared to subtotal resection. They ultimately concluded that subtotal resection was a reasonable option especially when paired with radiosurgery [32]. Others have argued that gross total resection can be completed with minimal risk, assuming the tumor is contained within the dural layer without significant involvement of adjacent cranial nerves and extension into other compartments of the middle or posterior fossa [33]. Surgical resection can be safely performed whether open or endoscopic, but the risk of permanent or worsening deficit of the oculomotor nerve cannot be ignored. The degree of function on presentation as well as the progression of the tumor will guide the decision whether to resect and what degree of resection should be the goal. In situations where gross total resec-

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tion cannot or should not be performed, radiosurgery offers a reasonable alternative or adjuvant treatment with data showing comparable rates of cranial nerve deficits and, at least in the short term, a high progression-­free interval [23, 32]. Indeed, for smaller asymptomatic tumors, radiosurgery may be the preferred initial treatment option.

I nternal Carotid Artery Plexus Schwannoma Certainly the rarest of the discussed schwannomas, there have been reports in the literature of schwannomas arising from none of the cranial nerves of the cavernous sinus but from what is

Fig. 25.2  Axial T2-weighted and postcontrast T1-weighted MR images showing a medial cavernous sinus schwannoma (red arrow), likely originating from the

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assumed to be the neural plexus of the carotid artery within the cavernous sinus (Fig.  25.2). Less than 10 total intracranial carotid sympathetic plexus schwannomas have been identified in the literature, with the first intracavernous case reported by Ture et al., where on exploration of the cavernous sinus, they found a tumor inferomedial to the cavernous internal carotid artery without any connection to the neighboring cranial nerves [34, 35]. Of the few cases reported available, almost all involved the petrous internal carotid artery (ICA), leaving only two cases of intracavernous tumors with only one surgical resection [2, 36]. Due to its rarity, there is no established treatment strategy for these tumors, surgical or otherwise. In Ture et al., where surgical resection was performed, an open skull base

plexus around the internal carotid artery (ICA). The patient presented with intermittent transient diplopia

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(Dolenc) approach was used with good outcomes in the case [34, 35]. The patient had a partial Horner’s syndrome on the affected side preoperatively that was stable postoperatively with no other deficits noted. This demonstrates that safe and appropriate resection can be performed, but this should still be approached with extreme caution. Each situation will be unique, and with the rarity of this tumor, resection should be planned and performed at institutions with teams experienced in cavernous sinus approaches along with the necessary facilities to manage the possible severe vascular complications that can arise from ICA injury.

Trochlear Schwannoma Trochlear nerve schwannomas are exceedingly rare and seem to exclusively occur outside the cavernous sinus. In the absence of neurofibromatosis, these are unlikely to be seen. The trochlear nerve does reside in the lateral wall of the sinus, and therefore, surgical management and approaches would be similar to oculomotor or trigeminal schwannomas. Interestingly, they also seem to respond quite well to radiation, making them particularly good candidates for radiosurgery [37–39].

Treatment Options It is important to remember the single-fiber origin of schwannoma when choosing treatment. This is amplified in the cavernous sinus where nerve function is so critical and other nerves are also intimately associated with the tumor. Therefore, any treatment has the potential to provide as much morbidity as it does value, as demonstrated by the majority of surgical patients having worsening of function. As a result, for very small tumors, observation or radiosurgery has the lowest risk to current function. However, as tumors enlarge or start to

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impact function, the risk:benefit ratio for surgical treatment shifts. The surgical approach is always a multifactorial decision. This chapter will not focus on a detailed explanation of different techniques, which can be found in their respective chapters in this book but instead will focus on the decision-­ making component in navigating this complex pathology. Ideally, the approach should be selected based on the relationship of the tumor to key neurovascular structures. For example, an oculomotor or V1 schwannoma in the lateral wall would be best approached via a lateral corridor, an open approach. Conversely, endonasal options may provide more direct access to medially originating tumors (sympathetic plexus or abducens), which would displace the lateral wall and nerves laterally. Naturally, patient goals and preferences, as well as surgeon comfort and learning curve with a particular approach, must also be considered in any approach selection.

“Open” Craniotomy Open surgery has proven to be safe for benign tumors of the cavernous sinus with good preservation of extraocular movement, minimal vascular risk, and low morbidity and mortality when approached carefully in experienced hands. The groundwork for open surgical approaches to the cavernous sinus began with the work of Parkinson and later Dolenc [40]. In 1992, Dolenc described a frontotemporal epidural and intradural approach, which would later bear his name, for resection of cavernous sinus masses [41]. Several after him have continued to refine the approach for specific pathologies. The advantage of this approach is that it can be performed almost entirely extradurally, allowing access to the lateral wall of the cavernous sinus, where the overwhelming majority of intracavernous schwannomas are located. Due to these advantages, this approach has become the standard for approaching the cavernous sinus and for patholo-

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gies particularly in the lateral dual dural layer. That being said, several other skull base approaches have been used with good results. Taha et  al. compared traditional approaches vs. skull base approaches to resection of trigeminal schwannomas, including cavernous sinus schwannomas, finding that skull base approaches significantly increased the percentage of tumor resection (65% residual tumor in the conventional group vs. 10% residual tumor in the skull base group) with similar perioperative complications [29]. Eisenberg et al., in their review of 40 cavernous sinus nonmeningioma benign tumors, 13 of which were trigeminal schwannomas, showed that 82.5% total resection was achieved using open approaches, predominantly frontotemporal extradural approach, with stable to improved extraocular motor function in nearly 90%. Interestingly enough, they found that gross total resection and neurological preservation were better than the results obtained with cavernous sinus meningiomas [11]. There is also a “minimally invasive” lateral “open” corridor that can be used for cavernous sinus tumors, but only with significant experience. The lateral orbitotomy approach (Chap. 23), usually performed in conjunction with an oculoplastic surgeon, can provide access from the ante-

a

rior clinoid process down to foramen ovale. In the literature, this has been most commonly used for meningiomas involving the cavernous sinus but can be applied to all cavernous sinus pathologies where predominantly lateral access is needed (Figs. 25.1 and 25.3) [42–45]. This has been used successfully for trigeminal schwannomas in Meckel’s cave as well as cavernous sinus biopsy and may provide a good option for schwannomas in the lateral wall of the cavernous sinus [46, 47]. While intracavernous and medial cavernous schwannomas have been resected using open surgical techniques, as mentioned earlier in the chapter, the open approaches are best suited to lateral wall schwannomas. Therefore, cavernous sinus trigeminal (V1) and oculomotor schwannomas, as well as theoretically trochlear nerve schwannomas, are the most appropriate for open skull base approaches, and ideally, the tumors can be removed without ever entering into the rest of the cavernous sinus. Unfortunately, these tumors often present with relatively large size with extension into adjacent compartments. In the case of trigeminal schwannomas, even if extending posteriorly into Meckel’s cave or laterally into the temporal region, open skull base approaches can still obtain high rates of gross total resection and good surgical outcomes [11,

b

Fig. 25.3  Axial (a) and coronal (b) postcontrast T1-weighted MRI showing complete resection of the schwannoma from Fig. 25.1 via a lateral orbitotomy approach

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40, 48]. Extension medially into the sphenoid or maxillary sinus or anteriorly and inferomedially into the skull base is a more challenging problem for open skull base approaches without significant vascular and neurological risk. In these situations, considering subtotal resection with adjuvant radiosurgery, a purely endoscopic endonasal approach, or combining endonasal with anterior transmaxillary or open approaches may be necessary for the best surgical outcome.

Endoscopic Endonasal Approach The endoscopic endonasal approach (EEA) has been revolutionary particularly in ventral skull base surgery, and cavernous sinus pathology has benefited greatly from advances in this field. EEA has been utilized in cavernous sinus pathology since its inception, initially for pituitary adenomas that extended into the cavernous sinus but later for unique cavernous sinus pathologies [49]. The anterior and medial cavernous sinus are particularly accessible through an expanded EEA, much more so than an open skull base approach provides (Figs.  25.2, 25.4, and 25.5). The outcomes from medial cavernous surgery demonstrate generally better rates of gross total

Sella I C A Schwannoma

Fig. 25.4 Intraoperative, endoscopic endonasal view showing resection of a medial cavernous meningioma involving the anterior/inferior compartment of the cavernous sinus and likely originating from the plexus around the internal carotid artery (ICA)

resection, decreased postoperative neurological deficits, and decreased rates of vascular injury across multiple large sample studies [50–52]. While this has not been looked at specifically with regard to schwannoma resection, contained schwannomas of the abducens nerve and internal carotid sympathetic plexus, as well as other schwannomas with significant extension medially and anteriorly, would be best approached through an EEA. The medial cavernous sinus can be accessed through a traditional endoscopic endonasal (transsphenoidal) approach while the lateral cavernous sinus, as well as Meckel’s cave, is better approached through the endoscopic endonasal lateral pterygoid approach [21]. This makes the lateral pterygoid approach the preferred approach for the majority of cavernous sinus schwannomas. Similar to the open approaches, access through the lateral pterygoid approach allows the surgeon to stay outside of the cavernous sinus by working within the dual dural layer of the lateral cavernous wall. Furthermore, as these tumors can extend into surrounding compartments, this approach allows for access posteriorly to Meckel’s cave as well as more temporal structures as needed. Care must be taken to consider V1 involvement and deficit in the setting of a transpterygoid approach. Sacrifice of the Vidian nerve with resultant decreased lacrimation in the setting of an insensate cornea could have significant potential for corneal keratopathy, which can even result in loss of the eye. Our own experience reflects the importance of the lateral pterygoid approach in these cases. In an attempt to better understand outcomes using this approach, we recently performed a retrospective analysis on trigeminal schwannomas resected using a lateral pterygoid EEA.  Of 16 patients resected at our institution, mostly V3 or V2 tumors, 13 patients underwent single-stage EEA while three others required multistage approach (EEA in two, open cranial approach in one). In this series, nine patients achieved gross total resection with seven receiving subtotal resection. Of the patients with subtotal resection, four underwent stereotactic radiosurgery for persistent disease. Postoperatively, the most common

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Fig. 25.5  Postoperative T2-weighted and T1-weighted, postcontrast axial MR images showing resection of the schwannoma from Fig. 25.2

complaint was persistent trigeminal hypoesthesia (n = 12) followed by masseter atrophy (n = 3) and facial paresthesias (n = 2). Postoperative trigeminal nerve dysfunction was found to correlate with preoperative pain syndromes, trigeminal dysfunction, and analgesic use. Four surgical complications were noted consisting of ICA injury (n  =  1) without sacrifice or sequelae, transient abducens palsy (n = 2), and transient vagus palsy (n = 1, following retromastoid craniotomy as part of combined approach).

Radiosurgery While surgical gross total resection can be completed with good outcomes and minimal complications, for those cavernous schwannomas or patients not amenable to gross total resection or surgery at all, radiosurgery can generally be safely delivered with comparable outcomes and disease control. The frequency of nonvestibular schwannomas is limited and therefore standard treatment protocols are difficult to create; as a result, the treatment plans were largely adapted from the well-established vestibular schwan-

noma literature. D’astous et  al. retrospectively reviewed 88 patients at their institution with nonvestibular intracranial schwannomas treated with stereotactic radiosurgery. Local control was achieved in 94% of tumors with a 7% complication rate and complete resolution of symptoms or stable symptoms in 82% of patients [23]. Other studies have found similar positive results using only stereotactic radiosurgery or Gamma Knife (Fig. 25.6) [24, 39, 53, 54].

Conclusion Cavernous sinus schwannomas are a rare entity, each of which must be carefully considered for origin, location within the cavernous sinus, and resectability based on potential morbidity. Small tumors with minimal symptoms can usually be observed or treated with radiosurgery, but larger tumors, especially with ophthalmoplegia, should be considered for resection. The majority of these rare tumors occur in the lateral cavernous wall and would therefore be best accessed via an open skull base approach. Rarely, medially originating schwannomas in the cavernous sinus could also

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a

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b

d

c

Fig. 25.6 Gamma Knife radiosurgery planning MR images. (a–c) The treatment plan for a cavernous schwannoma (possibly from V1) with extension into the posterior

fossa. (d) A 14-month follow-up MRI shows a dramatic decrease in the size of the tumor

be accessed via an endoscopic endonasal approach. Regardless of approach, any treatment should be designed and executed with the goal of maximal preservation of nonpathological but involved nerve fibers.

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S. Elhamdani et al. 31. Sun H, Sharma K, Kalakoti P, Thakur JD, Patra DP, Konar S, et al. Factors associated with abducens nerve recovery in patients undergoing surgical resection of sixth nerve schwannoma: a systematic review and case illustration. World Neurosurg. 2017;104:883–99. 32. Muhammad S, Niemelä M. Management of oculomotor nerve schwannoma: systematic review of literature and illustrative case. Surg Neurol Int. 2019;10:40. 33. Mariniello G, Horvat A, Dolenc VV. En bloc resection of an intracavernous oculomotor nerve schwannoma and grafting of the oculomotor nerve with sural nerve. Case report and review of the literature. J Neurosurg. 1999;91(6):1045–9. 34. Türe U, Seker A, Kurtkaya O, Pamir MN.  Internal carotid plexus schwannoma of the cavernous sinus: case report. Neurosurgery. 2003;52(2):435–8; discussion 438–439. 35. Takase H, Araki K, Seki S, Takase K, Murata H, Kawahara N. Unique diagnostic features and surgical strategy for intracranial carotid sympathetic plexus schwannoma: case report and literature review. World Neurosurg. 2017;98:876.e1–8. 36. Goudihalli SR, Goto T, Bohoun C, Nagahama A, Tanoue Y, Morisako H, et  al. Sympathetic plexus schwannoma of carotid canal: 2 cases with surgical technique and review of literature. World Neurosurg. 2018;118:63–8. 37. Elmalem VI, Younge BR, Biousse V, Leavitt JA, Moster ML, Warner J, et  al. Clinical course and prognosis of trochlear nerve schwannomas. Ophthalmology. 2009;116(10):2011–6. 38. Torun N, Laviv Y, Jazi KK, Mahadevan A, Bhadelia RA, Matthew A, et al. Schwannoma of the trochlear nerve-an illustrated case series and a systematic review of management. Neurosurg Rev. 2018;41(3):699–711. 39. Peciu-Florianu I, Tuleasca C, Comps J-N, Schiappacasse L, Zeverino M, Daniel RT, et  al. Radiosurgery in trochlear and abducens nerve schwannomas: case series and systematic review. Acta Neurochir. 2017;159(12):2409–18. 40. Al-Mefty O, Smith RR.  Surgery of tumors invading the cavernous sinus. Surg Neurol. 1988;30(5):370–81. 41. Dolenc VV.  Frontotemporal epidural approach to trigeminal neurinomas. Acta Neurochir. 1994;130(1–4):55–65. 42. Couldwell WT, Sabit I, Weiss MH, Giannotta SL, Rice D. Transmaxillary approach to the anterior cavernous sinus: a microanatomic study. Neurosurgery. 1997;40(6):1307–11. 43. Altay T, Patel BCK, Couldwell WT.  Lateral orbital wall approach to the cavernous sinus. J Neurosurg. 2012;116(4):755–63. 44. Cohen MA, Couldwell WT.  Resection of cavernous sinus meningioma via lateral orbitotomy approach: 2-dimensional operative video. Oper Neurosurg (Hagerstown Md.). 2020;18(5):E164. 45. Raheja A, Couldwell WT.  Cavernous sinus meningioma. Handb Clin Neurol. 2020;170:69–85. 46. Chabot JD, Gardner PA, Stefko ST, Zwagerman NT, Fernandez-Miranda JC. Lateral orbitotomy approach

25 Schwannoma for lesions involving the middle fossa: a retrospective review of thirteen patients. Neurosurgery. 2017;80(2):309–22. 47. Abou-Al-Shaar H, Cohen MA, Bi WL, Gozal YM, Alzhrani G, Karsy M, et al. Surgical management of multifocal trigeminal schwannomas. Oper Neurosurg (Hagerstown Md). 2020;19(6):659–66. 48. Al-Mefty O, Fox JL, Smith RR.  Petrosal approach for petroclival meningiomas. Neurosurgery. 1988;22(3):510–7. 49. Kassam AB, Gardner P, Snyderman C, Mintz A, Carrau R.  Expanded endonasal approach: fully endoscopic, completely transnasal approach to the middle third of the clivus, petrous bone, middle cranial fossa, and infratemporal fossa. Neurosurg Focus. 2005;19(1):E6. 50. Koutourousiou M, Vaz Guimaraes Filho F, Fernandez-­ Miranda JC, Wang EW, Stefko ST, Snyderman CH, et  al. Endoscopic endonasal surgery for tumors of the cavernous sinus: a series of 234 patients. World Neurosurg. 2017;103:713–32.

389 51. Truong HQ, Lieber S, Najera E, Alves-Belo JT, Gardner PA, Fernandez-Miranda JC. The medial wall of the cavernous sinus. Part 1: surgical anatomy, ligaments, and surgical technique for its mobilization and/ or resection. J Neurosurg. 2018;131(1):122–30. 52. Cohen-Cohen S, Gardner PA, Alves-Belo JT, Truong HQ, Snyderman CH, Wang EW, et  al. The medial wall of the cavernous sinus. Part 2: selective medial wall resection in 50 pituitary adenoma patients. J Neurosurg. 2018;131(1):131–40. 53. Langlois A-M, Iorio-Morin C, Faramand A, Niranjan A, Lunsford LD, Mohammed N, et al. Outcomes after stereotactic radiosurgery for schwannomas of the oculomotor, trochlear, and abducens nerves. J Neurosurg. 2021;22:1–7. 54. Lunsford LD, Niranjan A, Martin JJ, Sirin S, Kassam A, Kondziolka D, et  al. Radiosurgery for miscellaneous skull base tumors. Prog Neurol Surg. 2007;20:192–205.

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Arianna Fava, Paolo di Russo, Thibault Passeri, Lorenzo Giammattei, Rosaria Abbritti, Fumihiro Matano, and Sébastien Froelich

Introduction Chordomas and chondrosarcomas are rare, infiltrative, and slow-growing tumors with quite similar clinical presentation, anatomical location, and radiological features but different histology and prognosis. Although they most often present a benign histopathology, their tendency to invade bone and soft tissue and to spread in close proximity to the brainstem, spinal cord, cavernous sinus (CS), cranial nerves, and vascular structures makes them aggressive tumors that are challenging to treat [1–5]. An optimum primary surgical treatment is crucial as it represents the only chance of cure for most patients. For this reason, chordoma should be referred to experienced skull base center and the treatment strategy

A. Fava (*) Laboratory of Experimental and Skull Base Neurosurgery, Department of Neurosurgery, Lariboisière Hospital, University of Paris, Paris, France P. di Russo · T. Passeri · L. Giammattei · R. Abbritti F. Matano · S. Froelich Department of Neurosurgery, Lariboisière Hospital, Assistance Publique – Hôpitaux de Paris, University of Paris, Paris, France e-mail: [email protected]; [email protected]; [email protected]

should be discussed among a multidisciplinary team, including dedicated neurosurgeons, with an expertise in both microscopic and endoscopic techniques, otolaryngologists, and radiation therapists [6–9]. Whenever preservation of neurological function and quality of life is possible, maximum cytoreductive surgery should be the first-line treatment [2–4, 6, 7, 9–14]. Their tendency to grow lateral from their clival or petroclival origin frequently results in invasion of the CS, which increases the complexity of the surgery and surgical risks, and reduces the chance to achieve oncological resection [1–5, 7, 8, 11, 15– 20]. Depending on their origin, tumor type, and relationship with the intracavernous neurovascular structures, complete resection is not always achievable, resulting in less favorable outcome [8]. Postoperative high-dose radiation therapy remains the standard treatment even if there is a tendency toward more integrated strategies dependent on tumor type, chromosome abnormalities, mutational landscape, and molecular characteristics [2, 3, 5, 7–9, 11, 16–18, 20, 21].

Definition and Histopathology Chordomas arise from remnants of the notochord, which represents the primary axis of the embryo and runs from the sacrum all along the spine across the vertebral bodies up to the lower clivus where it crosses the bone to reach the phar-

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 A. S. Youssef (ed.), Contemporary Skull Base Surgery, https://doi.org/10.1007/978-3-030-99321-4_26

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ynx. It then runs into the pharyngeal soft tissues in front of the clivus to its upper limit where it crosses again the bone and ends its course in the dorsum sellae [6, 10, 22, 23]. Chordomas are ­primarily midline tumors arising from the sacral region in 50% of adult patients followed by skull base and craniocervical junction (CCJ) (30%) and mobile spine (20%) [6]. The mean age at diagnosis is about 55 years, with skull base chordomas generally affecting a younger population [24]. Macroscopically, chordomas are gray to bluish-white tumors with a glistening intersection area, which often show a pseudocapsule [6]. Microscopically, chordomas are characterized by vacuolated (“bubble-bearing”) physaliphorous cells and commonly demonstrate immunoreactivity for S100 protein, vimentin, and epithelial-­ like markers such as EMA and cytokeratin [25]. Brachyury is typically overexpressed in chordomas and is used to differentiate chordoma from chondrosarcoma and other tumors. Chondrosarcomas are sarcomatous neoplasms that originate from primitive mesenchymal cells or from the embryonic rests of chondrocranium with various degrees of cartilaginous differentiation [25]. Cartilaginous rests may be the progenitors of chondrosarcomas at the petrous apex and posteromedial temporal bone, and between the internal auditory canal and jugular foramen. They usually arise at the level of sphenopetroclival synchondrosis and tend to be more lateral than midline chordomas [1]. When associated with Maffucci’s syndrome (multiple enchondromas with cutaneous and visceral hemangiomas) or Ollier’s disease (enchondral bone cysts), chondrosarcomas arise more frequently in the midline [26]. The mean age of tumor presentation is the fourth or fifth decade of life [9]. They are formed by epithelioid-like or spindle cells immersed in a myxoid or cartilaginous matrix similar to that seen in chordomas. As for chordomas, chondrosarcomas are positive to mesenchymal markers, including vimentin and S100 protein (focally), but they can be differentiated from chordomas because of the lack of immunoreactivity for epithelial markers, including EMA and cytokeratin [25]. Their behavior tends to be less aggressive than chordomas, with a better long-term outcome [1, 3].

Both chordomas and chondrosarcoma are primarily extradural, and transgression of the dura mater is a rare feature, and when it occurs, they often remain subdural without infiltrating the pia mater [1, 10]. For more details on pathology, see Chap. 40.

Clinical Presentation Due to their characteristic location and slow pattern of growth, chordomas and chondrosarcomas frequently remain asymptomatic until reaching a large size. Considering the clivus and sphenopetroclival region as epicenters of those tumors, the posterior aspect of the CS is usually infiltrated first and diplopia caused by abducens nerve paresis is the most common presenting sign, followed by headache [1, 4, 5, 10, 16]. The CS, defined as an anatomical “jewel box” by Parkinson, is a complex extracranial venous space that contains the internal carotid artery (ICA) with sympathetic fibers, cranial nerves (CN III, IV, V1, V2, VI), and fibrous tissue surrounded by venous plexus. Tumor lobulations tend to surround the vessels causing arterial encasement, while palsy of the cranial nerves of CS seems related to stretch [11]. However, CS syndrome, characterized by ophthalmoplegia, chemosis, proptosis, and Horner’s syndrome, is rare in chordoma and chondrosarcomas. Chondrosarcomas more often present with III nerve palsy compared to chordomas, presumably due to their lateral origin [27]. Trigeminal nerve dysfunction can occur with facial numbness or facial pain. Considering the proximity to the optic nerve, pituitary gland and stalk, visual field impairment and pituitary dysfunction can also occur [5, 8, 9, 11, 16–18, 20, 21].

Imaging Chordoma and chondrosarcoma have a similar radiological appearance and are sometimes difficult to differentiate before histological examination. Al-Mefty et  al. [1] showed that chondrosarcomas begin laterally and grow toward the midline, whereas chordomas originate from the midline and spread laterally.

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Bone destruction of the clivus on CT scan with sporadic calcifications is typical of chordomas, whereas a soft-tissue mass growing from the sphenopetroclival synchondrosis from lateral to medial is characteristic of chondrosarcomas [15, 19]. Moreover, calcification with a “ring and arc” pattern is seen in the majority of chondrosarcomas, typical of chondroid lesions [19]. In an MRI study, both lesions are usually hypo- to isointense on T1-weighted images, with various degrees of enhancement, and are hyperintense on T2-weighted images with hypointense areas representing fragments of bone in chordomas, or chondroid mineralization in chondrosarcomas. Sometimes they show small intratumoral collections of proteinaceous fluid or hemorrhage, resulting in hyperintense foci of high intensity on T1-weighted imaging [15, 19]. A recent study using diffusion-weighted imaging suggested that ADC values can help to differentiate chondrosarcoma from chordoma because of significantly higher values [27]. Considering the surgical planning, dural disruption is a key factor for the choice of the approach and closure technique. T2-weighted imaging may identify any dural defect but high-­ resolution SSFP-based sequences (steady-state-­ free precession) better discriminate tumor from nonenhancing dura and cranial nerves [28]. FIESTA-C sequences are also useful to study the cranial nerves’ trajectory, especially the VI nerve, in their cisternal segment before entering the skull base and the tumor. CT angiography is recommended to study the trajectory of important vessels especially for the ICA, and an angiography with a balloon test occlusion (BTO) may be considered in case of carotid artery encasement [17].

Surgical Considerations The Chordoma Global Consensus and literature about chordomas states that, whenever preservation of neurological function and quality of life is possible, gross total resection should be the goal of surgery, considering the better overall and progression-­free survival [1–10, 13, 17].

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The optimum treatment strategy for each patient should be discussed in a multidisciplinary team setting, and the surgical plan should be defined taking into consideration the following key factors: (1) anatomical characteristics of the skull base such as the degree of pneumatization, (2) relationship of the tumor with critical structures, (3) intradural extension, (4) tumor features such as origin, consistency, calcifications, and extension, (5) patient characteristics and clinical status, and (6) previous biopsy, surgeries, and radiation therapies. The CS, also known as the parasellar compartment, is bordered by the dura mater in its superior, lateral, and posterior wall. Its medial wall separating the content of the CS from the pituitary gland is incomplete [29–31]. The weakness of the medial wall of the CS also explains how tumors arising from the clivus easily extend into the CS. In 1992, El-Kalliny classified CS tumors into three types: type I – intracavernous, arising within the cavernous sinus (rare); type II – interdural, tumors of the lateral wall of the CS, which arise and remain between the two layers of dura mater; and type III – invasive tumors, which grow from structures outside the CS and invade it through its walls or along neurovascular structures that traverse the CS [32]. Typically, chordomas and chondrosarcomas demonstrate a type III pattern, invading often the CS from its medial and posterior walls. Classifying the pathology into these types helps the surgeon to predict the possibilities and the amount of tumor resection, choose between transcranial or endoscopic approach, and predict neurological and oncological outcomes [32]. Nowadays, various MRI sequences have proven to be able to generate high spatial resolution images that allow visualization of cranial nerves, dura mater, and vessels, and their involvement by adjacent pathology, thus becoming paramount to define the surgical strategy [28]. Knosp classification [33] (KS grade) defines the CS invasion by pituitary adenomas into four grades considering the medial-to-lateral spreading of the tumor toward the lines connecting the intra- and supracavernous ICA in the coronal plane. Similarly, Cottier et  al. [34] described a

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method to predict the percentage of cavernous ICA encased by the tumor. Interestingly, Patrona et al. [8], presenting their surgical results of nonadenomatous, nonmeningeal pathology involving the CS, such as chordomas and chondrosarcomas, have shown that KS grade and degree of carotid encasement could be helpful in predicting the extent of resection within the CS. Without a doubt, ICA encasement remains one of the limitations in the treatment of CS chordomas and chondrosarcomas. Considering their tendency to encase the ICA, intraoperative micro-­ Doppler is mandatory. In case of total ICA encasement, and even more in case of ICA stenosis, BTO should be performed and ICA occlusion should be considered preoperatively. On the other hand, chordomas and chondrosarcomas showing high and homogenous intensity on T2 are usually soft and easy to dissect and suction around the ICA. Considering the invasive nature and large size reached by some tumors before becoming symptomatic, skull base surgeons are frequently obliged to plan multiple surgeries in order to achieve a maximal safe resection. In 1997, Al Mefty and Borba [7] proposed a classification of skull base chordomas based on tumor spreading: type I – tumors restricted to one compartment of the skull base defined as a solitary anatomical area (e.g., sphenoid sinus, CS, lower clivus, or occipital condyle); type II – extended to two or more contiguous areas of the skull base and for which radical removal can be achieved using a single skull base approach; and type III – extend to several contiguous compartments of the skull base and require two or more skull base procedures to achieve a radical surgical removal. In their series, they found 64% of patients with type II chordoma, 12% type III, 24% with recurrent tumor, and none involving only one anatomical area. Surgery has to be planned based on patient characteristics as well as previous treatments. In particular, chordomas show a high rate of local recurrence even after radiation therapy with poor long-term outcome [4]. Instead, chondrosarcomas present a lower rate of recurrence and a better outcome [1–4]. Recurrent chordomas

represent a complex challenge because the tumor is more adherent to neurovascular structures, the anatomy is more distorted, the vessels are often more fragile after radiotherapy, craniovertebral fixation is often present, and vascularized soft tissues such as nasoseptal flap (NSF) or galea are no longer available for reconstruction, thus increasing the risk of CSF leak and meningitis. Therefore, the benefit of a surgical resection has to be carefully evaluated and balanced with the surgical risk. Moreover, the surgical challenge is not only to perform an extensive resection but also to preserve function and give an adequate quality of life to patients. Accordingly, these pathologies have to be referred to specialized centers in order to obtain a radical resection as much as possible at first presentation.

Surgical Approaches Multiple transcranial and transfacial approaches to the middle fossa and CS have been described [2–5, 7, 11, 13, 17, 32, 35]. Since the early efforts by Parkinson who ventured cavernous sinus surgery for carotid–cavernous fistulas, CS surgery has significantly evolved over the years [36]. The frontotemporal or orbitozygomatic epidural [37] approaches became widely used to access the CS using the so-called “Dolenc,” “Hakuba,” and “Kawase” techniques [38–40]. Optimal surgical results are achieved for tumors of the lateral wall of CS with these transcranial approaches through a lateral-to-medial trajectory. In case of invasive tumors originating from the midline and spreading inside the CS, the transcranial routes are less favorable because of higher risk of traversing cranial nerves and the resulting deficits [18]. Thus, microscopic transsphenoidal [41, 42] and extended transsphenoidal approaches [43] were described and used for extradural tumors such as chordomas and chondrosarcomas limited to the upper clivus, with or without invasion of the cavernous sinus [7]. In the last decades, the development of endoscopic endonasal techniques has completely changed the surgical strategies for those tumors, and nowadays, the expanded endonasal approach

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(EEA) is often the approach of choice for chordomas and chondrosarcomas [8, 16, 44–49]. The direct line of sight through the nose and a medial-­ to-­lateral approach allows to access directly the lesion without manipulation of the cranial nerves and vessels that are most often lateral to the tumor. Using angled endoscopes, the entire clivus, CS, petrous apex, and craniocervical junction can be

reached, enabling tumor dissection from ICA, vertebral and basilar arteries, and cranial nerves with an acceptable lower risk [8, 14, 16, 46, 50]. Several cadaveric studies describing various endoscopic endonasal techniques to access the CS have been published [50–54] (Fig. 26.1). Nevertheless, when tumors extend more laterally beyond the CS, petrous apex, and jugular

a

b

c

d

Fig. 26.1 Cadaveric dissection illustrating the endoscopic endonasal anatomical view of CS using a 30° endoscope. (a) EEA showing the sellar floor and clivus (partially drilled) medially, the medial wall of CS, and the cavernous segment of ICA laterally. (b) Medial-to-lateral dissection exposing cavernous, paraclival, and lacerum segments of ICA. (c) Endoscopic view of the lateral wall

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of the CS obtained by mobilizing the anterior genu of the ICA medially. (d) Lateral view showing the abducens nerve trajectory after removing the petrous apex and opening the dura mater. ON optic nerve, VN Vidian nerve, Opth.A. ophthalmic artery, Symp.N. sympathetic plexus, BA basilar artery

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foramen, EEA becomes more challenging because of the long distance and the necessity to work between cranial nerves and the carotid artery. In those cases, transcranial approaches or a combination of both should be preferred (Chap. 22). EEA becomes a perfect approach for purely extradural chordomas and chondrosarcomas, but when tumors show a large dural invasion and a significant intradural component, the risk of CSF leak increases significantly and even more in case of recurrent tumor. Several reconstruction techniques have been described [14] such as the nasoseptal flap (NSF) [55], multilayer vascularized technique [56], gasket seal technique [57], and temporoparietal fascia flap [58] (Chap. 10). Recently, the 3F (fat, flap, flash) technique [59] has demonstrated that not only an accurate closure is essential to reduce CSF leak but also an early control of the intracranial pressure with fast patient mobilization. Nevertheless, effective dural reconstruction remains the main issue for EEA and postoperative CSF leak represents a significant serious risk for the patient; however, in tertiary care centers, the CSF leak rate decreases to around 11% following EEA for skull base chordomas [14].

Adjuvant Treatment The Chordoma Global Consensus [6] group states that after surgical resection, adjuvant radiotherapy is recommended. A baseline patient evaluation including neurological examination, visual field assessment, audiometry exam, and pituitary gland function should be performed before radiation. Considering the chordomas’ radioresistance to conventional radiotherapy, high-dose photon, proton, and carbon ion therapies are used [6]. The physical properties of proton particles allow a more precise targeting and minimize the exposure of surrounding structures to radiation. Routinely, fractionated treatment is planned, while carefully adjusting the target volumes in proximity to functional areas, such as the CS and brainstem, in order to avoid unacceptable damage [6]. For chordomas, particle therapies and adequate dose uniformity within the target volume are associated with better local control.

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In 2011, a report of 71 patients with chordomas from the North American Gamma Knife Consortium [60] demonstrated that GKS is a potent treatment option for small-sized chordomas, especially in younger patients, as part of a multipronged attack. However, a recent study including 12 patients with residual clival chordomas after surgery showed that GKS should not be considered an effective treatment in terms of local control of residual tumor [61]. Overall, with a better understanding of the oncogenesis of chordomas and a better knowledge of the prognostic factors, it is expected that more adaptive treatment strategies based on the tumor growth profile, radiological characteristics, cytogenetics, and molecular characteristics of each tumor will emerge in the future. Conversely, recommendations for skull base chondrosarcomas are not as clear regarding the extent of resection and additional radiation therapy. A trend has emerged toward a less aggressive treatment strategy with follow-up imaging only after complete resection in grade II chondrosarcomas when compared to chordomas. Low-grade chondrosarcomas should be treated with radical surgical resection alone with the aim of minimizing the morbidity of surgery and their consequences on the quality of life in patients who otherwise potentially have long-term survival [3, 9, 62]. For WHO grade II chondrosarcomas, if radical resection has been achieved, it has been also recommended to keep adjuvant radiotherapy for the management of recurrence [63]. In case of close proximity to critical structures, such as the CS, a subtotal resection should be considered in order to avoid postoperative neurological deficits considering the good prognosis and the possibilities of an additional surgery and/ or adjuvant treatment at the time of recurrence [62, 64–66]. Contrarily, for high-grade chondrosarcomas, such as dedifferentiated and mesenchymal variants, adjuvant therapy is mandatory [12]. As for chordomas, proton therapy is ­considered the treatment of choice for chondrosarcomas due to their radioresistance. For chondrosarcomas, GKS seems to be a reasonable option for patients with residual or newly diagnosed small skull base chondrosarcomas [64– 66]. It also may improve cranial nerve function,

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especially for patients who present with diplopia related to abducens nerve palsy [66]. For both chordoma and chondrosarcoma, in elderly patients, or for tumor in which surgery is too risky, high-dose radiation therapy alone may

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be considered as an alternative primary treatment [62]. A management algorithm for the management of chordomas and chondrosarcomas involving the cavernous sinus is shown in Fig. 26.2.

Skull Base Lesion Involving CS

EEA Biopsy

Chordoma/Chondrosarcoma

EEA Resection

STR

GTR

2nd Stage Open Approach

Chondrosarcoma Low Grade

Surveillance

Chondrosarcoma High Grade

Chordoma

Radiotherapy

Fig. 26.2  Management algorithm for chordomas and chondrosarcomas involving the cavernous sinus. CS cavernous sinus, EEA endoscopic endonasal approach, GTR gross total resection, STR subtotal resection

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Illustrative Cases Case 1: Clival Chordoma Involving CS A 26-year-old woman presented with intermittent left abducens nerve palsy. CT scan showed an osteolytic mass eroding the left petroclival region (Fig.  26.3a, b), and MRI demonstrated a large clival lesion extended extradurally from the lower clivus to the sphenoid sinus with a complete invasion of left CS displacing the ICA and intradurally from the prepontine to the interpeduncular cistern, with a severe brainstem compression. The lesion appeared hyperintense on T2-weighted images with hypointense areas and septa (Fig.  26.3c–f). An endoscopic endonasal biopsy was performed demonstrating a classic chordoma. a

d

b

Accordingly, a staged surgery was planned to remove the extra- and intradural components in two different surgical procedures. The rationale for this two-stage surgery was to first resect the extradural part of the tumor with an endonasal approach and then resect the intradural part of the tumor through a transcranial approach using microscopic bimanual dissection in order to reduce the risk of cranial nerves’ deficit and CSF leak. Firstly, a one-nostril minimally invasive endonasal approach was performed to remove all the extradural components using the chopstick technique [67], with the help of neuromonitoring, neuronavigation, and micro-Doppler. The resection stopped at the level of the dural defect through which the tumor was extending intradurally. Tachosil® and autologous fat filling the c

e

Fig. 26.3  Case 1. (a, b) Preoperative axial and sagittal CT scan showing an osteolytic lesion eroding the left petroclival junction and clivus. (c–e) Preoperative sagittal and axial T2-weighted MRI images demonstrating a large hyperintense tumor with both extra- and intradural com-

f

ponents involving the left CS. (f) Preoperative sagittal FIESTA sequence highlighting the left cavernous sinus complete invasion with anterior displacement of left ICA and intradural extension toward the right-side cerebellopontine angle

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Fig. 26.4  Case 1. (a–c) Postoperative MRI after an endoscopic endonasal approach showing the complete resection of the extradural and intracavernous components. (d–f) Postoperative MRI after a right-side combined transpetro-

sal approach demonstrating the complete removal of the intradural portion of chordoma with brainstem decompression

tumor bed and sphenoid sinus secured with fibrin glue were used for closure. Postoperative MRI showed a total removal of the extradural and intracavernous part of the tumor (Fig. 26.4a–c). Pathology confirmed a classic chordoma. After 2  months, a right-side combined transpetrosal approach was performed in order to gain access to the anterolateral aspect of brainstem and interpeduncular cistern from an inferior to superior, posterior to anterior and lateral to medial trajectory. The final MRI showed GTR (Fig. 26.4d–f). After surgery, the patient complained of diplopia in the downward gaze related to a partial right fourth nerve deficit that resolved after 2 months. Patients then underwent proton beam therapy.

 ase 2: Petroclival Chondrosarcoma C Involving CS A 41-year-old woman presenting with diplopia was referred to our center after CT and MRI studies showed an extradural osteolytic lesion centered on the left petroclival region and extending into the entire clivus and CS displacing the ICA anteriorly. The lesion appeared hyperintense on T2-weighted images with hypointense areas and heterogenous contrast enhancement (Fig. 26.5a– d). An endoscopic endonasal biopsy was performed, demonstrating a chondrosarcoma. Considering the extradural nature of the tumor and the CS invasion, a one-nostril EEA was performed. The sphenoid sinus and the clivus were

A. Fava et al.

400 Fig. 26.5  Case 2. (a, b) Preoperative axial CT scan showing an osteolytic lesion centered on left petroclival region. (c, d) Preoperative sagittal and axial T2-weighted MRI images demonstrating a left-side petroclival extradural lesion involving the left CS. (e, f) Postoperative axial MRI images showing a gross total resection

a

b

c

d

e

f

exposed and the left lacerum segment of the ICA was identified following the Vidian nerve. The soft consistency of the tumor allowed the surgeon to remove it safely. The intracavernous portion of the tumor was resected from a medial-to-lateral

trajectory. Finally, autologous fat covered with an NSF was used for closure. The postoperative MRI demonstrated GTR (Fig. 26.5e, f), and the diagnosis of low-grade chondrosarcoma was confirmed. Accordingly, no adjuvant radiation

26  Chordomas and Chondrosarcomas Involving the Cavernous Sinus

therapy was performed. In the postoperative course, the patient experienced a left partial abducens nerve palsy that recovered within 10 days.

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from the medical and patient community. Lancet Oncol. 2015;16(2):e71–83. https://doi.org/10.1016/ S1470-­2045(14)71190-­8. 7. Al-Mefty O, Borba LAB.  Skull base chordomas: a management challenge. J Neurosurg. 1997;86(2):182–9. 8. Patrona A, Patel KS, Bander ED, Mehta A, Tsiouris AJ, Anand VK, et  al. Endoscopic endonasal surConclusion gery for nonadenomatous, nonmeningeal pathology involving the cavernous sinus. J Neurosurg. 2017;126(3):880–8. Chordomas and chondrosarcomas are rare, infil 9. Wanebo JE, Bristol RE, Porter RR, Coons SW, trative, and slow-growing tumors that arise in the Spetzler RF. Management of cranial base chondrosarskull base from clival and petroclival region and comas. Neurosurgery. 2006;58(2):249–54. can involve the cavernous sinus. Gross total resec- 10. George B, Bresson D, Herman P, Froelich S.  Chordomas: a review. Neurosurg Clin N Am. tion, preserving function and quality of life, repre2015;26(3):437–52. sents the gold standard for chordomas and 11. Goel A, Muzumdar DP, Nitta J.  Surgery on lesions chondrosarcomas. This can be successfully involving cavernous sinus. J Clin Neurosci. achieved through expanded endoscopic endonasal 2001;4:71–7. 12. Kremenevski N, Schlaffer SM, Coras R, Kinfe approaches for the extradural portion and possible TM, Graillon T, Buchfelder M.  Skull base chordoopen skull base approaches for the intradural pormas and chondrosarcomas. Neuroendocrinology. tion as a second stage. While radical surgery is 2020;110(9–10):836–47. considered curative for low-grade chondrosar- 13. Lanzino G, Dumont AS, Lopes MB, Laws ER.  Skull base chordomas: overview of disease, coma, high-grade chondrosarcoma and chordoma ­management options, and outcome. Neurosurg Focus. will still need postoperative radiotherapy. As radi2001;10(3):1–9. cal primary surgery is the only chance of cure, it 14. Wang EW, Zanation AM, Gardner PA, Schwartz is of tremendous importance to refer those patients TH, Eloy JA, Adappa ND, et  al. ICAR: endoscopic skull-base surgery. Int Forum Allergy Rhinol. to specialized skull base centers with expertise in 2019;9(S3):S145–365. both microscopic and endoscopic techniques, 15. Abdel Razek AAK, Castillo M.  Imaging lesions supported by a multidisciplinary team. of the cavernous sinus. Am J Neuroradiol. 2009;30(3):444–52. 16. Koutourousiou M, Guimaraes Filho FV, Fernandez-­ Miranda JC, Wang EW, Stefko ST, Snyderman CH, References Gardner PA.  Endoscopic endonasal surgery for tumors of the cavernous sinus: a series of 234 patients. 1. Almefty K, Pravdenkova S, Colli BO, Al-Mefty O, World Neurosurg. 2017;103:713–32. https://doi. Gokden M.  Chordoma and chondrosarcoma: simiorg/10.1016/j.wneu.2017.04.096. lar, but quite different, skull base tumors. Cancer. 17. Lanzino G, Sekhar LN, Hirsch WL, Sen CN, Pomonis 2007;110(11):2467–77. S, Snyderman CH.  Chordomas and chondrosarco 2. Crockard HA, Steel T, Plowman N, Singh A, mas involving the cavernous sinus: review of surgical Crossman J, Revesz T, et  al. A multidisciplinary treatment and outcome in 31 patients. Surg Neurol. team approach to skull base chordomas. J Neurosurg. 1993;40(5):359–71. 2001;95(2):175–83. 18. Pamir MN, Kilic T, Özek MM, Özduman K, Türe 3. Crockard HA, Cheeseman A, Steel T, Al E. A multiU. Non-meningeal tumours of the cavernous sinus: a disciplinary team approach to skull base chondrosarsurgical analysis. J Clin Neurosci. 2006;13(6):626–35. coma. J Neurosurg. 2001;95(2):175–83. 19. Tang Y, Booth T, Steward M, Solbach T, Wilhelm 4. Gay E, Sekhar LN, Rubinstein E, Wright DC, Sen T. The imaging of conditions affecting the cavernous C, Janecka IP, Snyderman CH.  Chordomas and sinus. Clin Radiol. 2010;65(11):937–45. https://doi. chondrosarcomas of the cranial base: results and org/10.1016/j.crad.2010.06.009. follow-up of 60 patients clinical study. Neurosurgery. 20. Zada G, Lopes MB, Mukundan S, Laws ER, editors. 1997;36(5):887–97. Atlas of sellar and parasellar lesions: clinical, radio 5. Arnold H, H.  DH.  Skull base chordoma with cavlogic, and pathologic correlations. Springer; 2016. ernous sinus involvement. Partial or radical tumour-­ 21. Al-Mefty O.  Chordoma. Acta Neurochir. removal? Acta Neurochir. 1986;51:48–51. 2017;159(10):1869–71. 6. Stacchiotti S, Sommer J.  Building a global con- 22. Salisbury JR, Deverell MH, Cookson MJ, Whimster sensus approach to chordoma: a position paper WF.  Three-dimensional reconstruction of human

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A. Fava et al. giomas: surgical method and results in 10 patients. Neurosurgery. 1991;28(6):866–9. 39. Hakuba A, Tanaka K, Suzuki T, Nishimura S. A combined orbitozygomatic infratemporal epidural and subdural approach for lesions involving the entire cavernous sinus. J Neurosurg. 1989;71(5 Pt 1):699–704. 40. Dolenc V. Direct microsurgical repair of intracavernous vascular lesions. J Neurosurg. 1983;58(6):824–31. 41. Laws ERJ. Transsphenoidal surgery for tumors of the clivus. Otolaryngol Neck Surg. 1984;92(1):100–1. 42. Fatemi N, Dusick JR, De Paiva Neto MA, Kelly DF.  The endonasal microscopic approach for pituitary adenomas and other parasellar tumors: a 10-year experience. Neurosurgery. 2008;63(4 Suppl):244–56. 43. Lalwani AK, Kaplan MJ, Gutin PH.  The transsphenoethmoid approach to the sphenoid sinus and clivus. Neurosurgery. 1992;31(6):1008–14. discussion 1014 44. Carrabba G, Dehdashti AR, Gentili F.  Surgery for clival lesions: open resection versus the expanded endoscopic endonasal approach. Neurosurg Focus. 2008;25(6):1–8. 45. Chibbaro S, Cornelius JF, Froelich S, Tigan L, Kehrli P, Debry C, et al. Endoscopic endonasal approach in the management of skull base chordomas  – clinical experience on a large series, technique, outcome, and pitfalls. Neurosurg Rev. 2014;37(2):217–25. 46. Fraser JF, Nyquist GG, Moore N, Anand VK, Schwartz TH.  Endoscopic endonasal transclival resection of chordomas: operative technique, clinical outcome, and review of the literature – clinical article. J Neurosurg. 2010;112(5):1061–9. 47. Zoli M, Milanese L, Bonfatti R, Faustini-Fustini M, Marucci G, Tallini G, et  al. Clival chordomas: considerations after 16 years of endoscopic endonasal surgery. J Neurosurg. 2018;128(2):329–38. 48. Hasegawa H, Shin M, Kondo K, Hanakita S, Mukasa A, Kin T, et al. Role of endoscopic transnasal surgery for skull base chondrosarcoma: A retrospective analysis of 19 cases at a single institution. J Neurosurg. 2018;128(5):1438–47. 49. Frank G, Sciarretta V, Calbucci F, Farneti G, Mazzatenta D, Pasquini E. The endoscopic transnasal transsphenoidal approach for the treatment of cranial base chordomas and chondrosarcomas. Neurosurgery. 2006;59(1 SUPPL. 1):10–2. 50. Fernandez-Miranda JC, Zwagerman NT, Abhinav K, Lieber S, Wang EW, Snyderman CH, et al. Cavernous sinus compartments from the endoscopic endonasal approach: anatomical considerations and surgical relevance to adenoma surgery. J Neurosurg. 2018;129(2):430–41. 51. Doglietto F, Lauretti L, Frank G, Pasquini E, Fernandez E, Tschabitscher M, et  al. Microscopic and endoscopic extracranial approaches to the cavernous sinus: anatomic study. Neurosurgery. 2009;64(5 Suppl 2):412–3. 52. Cavallo LM, Cappabianca P, Galzio R, Iaconetta G, de Divitiis E, Tschabitscher M. Endoscopic transnasal approach to the cavernous sinus versus transcra-

26  Chordomas and Chondrosarcomas Involving the Cavernous Sinus nial route: anatomic study. Neurosurgery. 2005;56(2 Suppl):379–89. 53. Alfieri A, Jho HD.  Endoscopic endonasal cavern ous sinus surgery: an anatomic study. Neurosurgery. 2001;48(4):827. 54. Truong HQ, Lieber S, Najera E, Alves-Belo JT, Gardner PA, Fernandez-Miranda JC. The medial wall of the cavernous sinus. Part 1: surgical anatomy, ligaments, and surgical technique for its mobilization and/ or resection. J Neurosurg. 2019;131(1):122–30. 55. Kassam AB, Thomas A, Carrau RL, Snyderman CH, Vescan A, Prevedello D, et al. Endoscopic reconstruction of the cranial base using a pedicled nasoseptal flap. Neurosurgery. 2008;63(1 Suppl 1):ONS44–52; discussion ONS52–3 56. Simal-Julián JA, Miranda-Lloret P, Mena LP, Sanromán-Álvarez P, García-Piñero A, Sanchis-­ Martín R, Botella-Asunción C, Kassam A.  Impact of multilayer vascularized reconstruction after skull base endoscopic endonasal approaches. J Neurol Surg B Skull Base. 2020;81(2):128–35. 57. Garcia-Navarro V, Anand VK, Schwartz TH. Gasket seal closure for extended endonasal endoscopic skull base surgery: efficacy in a large case series. World Neurosurg. 2013;80(5):563–8. 58. Thomas R, Girishan S, Chacko AG. Endoscopic transmaxillary transposition of temporalis flap for recurrent cerebrospinal fluid leak closure. J Neurol Surg B Skull Base. 2016;77(6):445–8. 59. Cavallo LM, Solari D, Somma T, Cappabianca P. The 3F (fat, flap, and flash) technique for Skull Base reconstruction after endoscopic endonasal suprasellar approach. World Neurosurg. 2019;126:439–46. 60. Kano H, Iqbal FO, Sheehan J, Mathieu D, Seymour ZA, Niranjan A, et  al. Stereotactic radiosurgery for

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Part V Middle Cranial Fossa: Meckel’s Cave

Open Surgical Approaches to Meckel’s Cave

27

Akal Sethi and A. Samy Youssef

Introduction Lesions involving Meckel’s cave can be challenging, even to the experienced surgeon, given the proximity of critical neurovascular structures such as posterior cavernous sinus, internal carotid artery (ICA), and cranial nerves III– VIII. Different pathologies can arise from or secondarily involve Meckel’s cave with different patterns of nerve involvement. Understanding the nuances of each pathology and its pattern of involvement of Meckel’s cave is necessary in order to select the most direct approach with the ultimate goal of preserving neurovascular structures [1]. Maximum preservation of the trigeminal nerve fascicles is key to successful surgery in and around Meckel’s cave. Trigeminal schwan-

Supplementary Information The online version contains supplementary material available at [https://doi. org/10.1007/978-­3-­030-­99321-­4_27]. A. Sethi Department of Neurological Surgery, University of Colorado, Aurora, CO, USA e-mail: [email protected] A. S. Youssef (*) Department of Neurological Surgery, University of Colorado School of Medicine, Aurora, CO, USA Department of Otolaryngology, University of Colorado School of Medicine, Aurora, CO, USA e-mail: [email protected]

nomas (Chap. 29) and upper petroclival meningiomas (Chap. 37) are the most frequent tumors encountered in Meckel’s cave. The subtemporal middle fossa approach provides the most direct and short access to Meckel’s cave with wide working space. The dural architecture is unique for this region, and a thorough understanding of this intricate anatomy is key to performing middle fossa approaches to lesions originating in or around Meckel’s cave.

Anatomy Meckel’s cave resides in the middle cranial fossa, which is an area posterior to the sphenoid ridge and chiasmatic sulcus, and anterior to the border created by the petrous ridge, dorsum sella, and posterior clinoid processes (Chap. 3). The cavernous sinus sits just lateral to the sella turcica, and its complex dural anatomy adds to the complexity of operating in this area. Meckel’s cave itself sits just posterolateral to the posterior cavernous sinus. The dura mater consists of two layers: an outer periosteal layer and inner meningeal layer (dura propria). These two layers tightly fuse except where they are separated to provide space for the dural venous sinuses, venous plexi, and cranial nerves that pass through the parasellar region. The dura propria and arachnoid typically follow cranial nerves for varying distances as they leave the cranial cavity [2, 3]. The dura propria that ­follows each cranial nerve becomes the epineu-

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 A. S. Youssef (ed.), Contemporary Skull Base Surgery, https://doi.org/10.1007/978-3-030-99321-4_27

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rium, whereas the pia-arachnoid continues as the perineurium that invests each nerve fascicle [3–7]. The trigeminal nerve passes from the posterior fossa over the trigeminal impression of the petrous apex between the periosteal and meningeal (dura propria) layers of middle fossa dura, carrying with it arachnoid and dura propria from the posterior fossa. Meckel’s cave is a cleft-like dural pocket that originates from the dura propria of the posterior fossa, between the two layers of the middle fossa dura [3, 8–15]. The contents of Meckel’s cave are the sensory and motor roots of the trigeminal nerve, Gasserian ganglion, and arachnoid layer. The subarachnoid space within Meckel’s cave is behind the Gasserian ganglion and is the actual space that constitutes the trigeminal cistern. At the anterior convex margin of the ganglion, the dura propria of Meckel’s cave becomes the epineural sheath of each division of the trigeminal nerve; the pia-arachnoid becomes the perineurium that invests each fascicle of the trigeminal nerve divisions (Fig. 27.1). This meningeal architecture is identical to that of the spiFig. 27.1 Illustration of the coronal sections along the line between the porus trigeminus and foramen ovale showing the dural layers. (With permission from Operative Neurosurgery) [1]

nal ganglion and nerve [6], with the dura propria and pia-arachnoid becoming epineurium and perineurium, respectively. The Gasserian ganglion and trigeminal root have two layers of dura propria on their dorsolateral aspects. The inner layer, the dura propria, constitutes the dorsolateral wall of Meckel’s cave. The outer layer is the dura propria of the middle fossa. The cleavage plane between these two layers of dura propria continues distally as the cleavage plane between the epineural sheaths of the trigeminal nerve divisions and the dura propria of the middle fossa. This cleavage plane serves as the anatomic basis for the interdural exposure of the contents of Meckel’s cave [16–19]. The transition from the petrous segment (C2) [6] through the lacerum segment (C3) to the cavernous segment (C4) of the internal carotid artery (ICA) occurs ventromedial to the GG (Fig. 27.2). The anterior portion of the ventromedial wall of Meckel’s cave can be divided into two parts: an upper one-third and lower two-thirds. The upper one-third, covering a portion of the Gasserian Superior petrosal sinus

trigeminal cistern

porus trigeminus

Meckel’s cave 3

tentorium trigeminal nerve root

2

inferior petrosal sinus

GG 1 V3

C2

clivus

sphenoid bone trigeminal impression

foramen ovale petrolingual ligament

arachnoid from posterior fossa periosteal layer of middle fossa dura propria of middle fossa epineural sheath

27  Open Surgical Approaches to Meckel’s Cave

a

b

Fig. 27.2 (a) Dorsolateral wall of Meckel’s cave (dotted line) behind the anterior margin of the Gasserian ganglion (dashed line) exposed via subtemporal interdural approach. (b) Removal of the dura propria of the tentorium, lateral wall of cavernous sinus, and dorsolateral wall of Meckel’s cave. The trigeminal exit from the brainstem is shown. (With permission from Operative Neurosurgery) [1]

ganglion, gives rise to the ophthalmic division (V1). The lower two-thirds are separated from the ICA lacerum segment (C3) and the medial portion of the horizontal part of the ICA petrous segment by the petrolingual ligament and the periosteum that covers the roof of the carotid canal (Fig. 27.3). After the abducens nerve passes underneath the petrosphenoidal ligament (Gruber’s ligament), it then courses medial to the upper portion of the medial wall of Meckel’s cave toward the posterior and lateral aspects of the vertical segment of the intracavernous ICA, making an anterior and downward angulation at the petrous apex tip [20]. The porus trigeminus is an oval-shaped opening of Meckel’s cave posteriorly that communicates with the posterior fossa. Two layers of dura propria (from the middle and posterior fossae) form the roof of the porus trigeminus. The superior petrosal sinus (SPS) lies between these two layers. The two-layered roof of the porus trigemi-

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nus continues posteriorly as the tentorium cerebelli. If the cleavage plane between the two layers of dura propria covering the Gasserian ganglion and trigeminal roots on their dorsolateral aspect is followed toward the porus trigeminus, the SPS will be opened (Fig. 27.3). The cavernous sinus venous plexus gives rise to the SPS through a space defined inferiorly by the superior border of Meckel’s cave and superiorly by the trochlear nerve (i.e., posterior portion of Parkinson’s triangle) [21, 22]. The middle meningeal artery enters the cranial cavity through the foramen spinosum, which is just posterolateral to the foramen ovale. This artery courses in the periosteal layer of middle fossa dura [2]. Consequently, dividing this artery near the foramen spinosum can lead to the cleavage plane between the periosteal and meningeal dural layers. This helps preserve the greater superficial petrosal nerve in the plane between the two layers of the dura. Understanding this intricate anatomy is key to performing middle fossa approaches to lesions originating in or around Meckel’s cave.

Open Approach to Meckel’s Cave Open lateral surgical approaches offer the most direct access to Meckel’s cave. For lesions that are centered over Meckel’s cave and expanding to the middle fossa, the middle fossa approach is appropriate in that it grants an interdural access to the lesion with better nerve preservation, which is usually on the medial side. In true petroclival meningiomas, the nerve maybe pushed laterally by the tumor and maybe encountered immediately upon splitting the SPS and tentorium. For lesions that are mainly in the posterior fossa with a small extension to Meckel’s cave through porus trigeminus, a posterior approach allows easy access to the trigeminal nerve as it exits the brain stem and involves a standard retrosigmoid craniotomy with possible drilling of the petrous apex (Chaps. 28 and 34). In this chapter, we will focus on the middle fossa approach to Meckel’s cave.

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arachnoid from posterior fossa periosteal layer of middle fossa dura propria of middle fossa epineural sheath

a

b

c

cavemous sinus venous plexus

C4 CS

superior orbital fissure

III

pituitary gland

IV outflow to superior petrosal sinus

TC PLL

VI Gasserion gandlion petrolingual ligament

MR SS

C3

V3

C4

sphenoid sinus C4h

GG

PG

Meckel’s cave trigeminal cistern motor root

C3

V3

VI clivus

C4v

C2 trigeminal cistern

V1 Gasserion ganglion Meckel’s cave trigeminal nerve root

petrous apex

formen lacerum

d

e

Fig. 27.3 Relationship of Meckel’s cave and the ICA. The oblique coronal section along the angle of the petrous ICA. (a) Histological section passing through the foramen lacerum showing the relationship of the ventromedial wall and surrounding anatomy. (b) Upper one-­ third of the ventromedial wall of Meckel’s cave forms the lateral wall of the posteroinferior portion of the cavernous sinus. The lower two-thirds of the wall lie on the petrolingual ligament and periosteum of the roof of the carotid canal. (c) Axial section through the course of V1. Posterior cavernous sinus directly contacts the superior part of the ventromedial wall of Meckel’s cave. (d) The Gasserian

ganglion (GG) and root were incised between V1, V2, and V3; the middle part of the GG was reflected anteriorly to show the ventromedial wall (asterisk) of Meckel’s cave, petrolingual ligament (PLL), and C3 segment of the ICA.  The upper one-third of this wall forms the lateral wall of the posteroinferior portion of the cavernous sinus. (e) Upper part of Meckel’s cave and V1 were retracted inferolaterally to show the C4 segment of the ICA.  The abducens nerve passes underneath the petrosphenoidal ligament and then courses medial to the upper portion of the medial wall of Meckel’s cave. (With permission from Operative Neurosurgery) [1]

Subtemporal/Middle Fossa Interdural Approach

then affixed to the Mayfield head holder, and registration to neuronavigation can proceed in the standard fashion.

Positioning The patient is usually positioned in the supine position with the head turned so that the sagittal suture is parallel to the floor. A shoulder bump may be required in patients with a less flexible neck and in older or obese patients. The patient is

Craniotomy After positioning, a question mark incision is marked and swung just posterior to the pinna, and the temporalis muscle is incised and reflected

27  Open Surgical Approaches to Meckel’s Cave

anteroinferiorly. A small frontotemporal craniotomy is turned with burr holes just above the supramastoid crest 1 cm posterior to the external acoustic meatus. Zygomatic osteotomy is not necessary except in the case of a large tumor with significantly high subtemporal extension. In such case, zygomatic osteotomy with the masseter muscle left attached will allow the temporalis muscle to be retracted lower, which minimizes retraction on the temporal lobe. The lateral sphenoid wing is drilled away down to the superior orbital fissure to ease angles of exposure.

 iddle Fossa Dissection M and Meningeal Dura Elevation (Video 27.1) Dural elevation proceeds similarly to an expanded middle fossa exposure (Chap. 31). The dura is dissected away in a posterior to anterior manner to protect the greater superficial petrosal nerve (GSPN). The arcuate eminence is identified posteriorly, and the middle meningeal artery (MMA) is cauterized and cut at the foramen spinosum to allow easier retraction of the temporal lobe in an extradural fashion. After cutting of the MMA, the two layers of dura are better identified and the dura propria and the periosteal dura can be split by careful blunt dissection. Peeling toward the foramen ovale and rotundum, the dura propria and the epineural sheath of V3 and V2 can be separated where the periosteal dura is incised over the anterior margin of the foramina, which allows further dural elevation in the plane between dura propria and the epineurium of V2 and V3. The dura is continually peeled superiorly and medially working toward the superior orbital fissure exposing V1 in its epineural sheath, which lies in continuity with the inner layer of the lateral wall of the cavernous sinus. In this manner, the dura propria is dissected away from the epineural sheaths of all three branches of the trigeminal nerve at their respective cranial exit points allowing for placement of retractors and the next phase of drilling (Fig. 27.4). The petrous ridge is

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identified and retractor blades are placed between the dura and the petrous ridge. In the case of a dumbbell schwannoma, the interdural dissection will expose and permit resection of the middle fossa tumor in an extradural fashion. The posterior fossa component can be exposed after anterior petrosectomy and intradural resection is performed after dural and tentorial incision. It is common that the tumor has already created a window to the posterior fossa by remodeling and expanding the petrous apex. In such cases, tumor resection can be extended to the posterior fossa component without anterior petrosectomy.

Anterior Petrosectomy Once fixed retractors are placed and the dura peeled sufficiently for exposure, drilling of the middle fossa floor can take place. The petrous apex (Kawase quadrangle) is identified and drilling is performed as described in Chap. 31.

Dural/Tentorial Incision The intradural portion of the case begins with a linear incision in the middle fossa dura above and parallel to the SPS from the petrous apex to the arcuate eminence. The posterior fossa dura is incised just below the superior petrosal sinus toward the lateral portion of the porus trigeminus. Hemoclips are used to ligate the SPS while preserving drainage from the petrosal vein. The ligated sinus is sharply severed while taking care not to cut through the trigeminal nerve root, which is directly below. In the case of petroclival meningiomas that originate from the tentorium, the tentorium is significantly thick and should be cut in a piecemeal fashion until the trigeminal nerve root fascicles are identified and preserved. The cut made through the SPS is extended medially through the tentorium to the incisura keeping in mind the location of the trochlear nerve. The two leaflets of the tentorium are retracted with sutures (Fig. 27.4).

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Fig. 27.4  Step-by-step procedure for subtemporal interdural approach. (a) Elevation of middle fossa dura in posterior to anterior direction to protect GSPN. (b) Relationship between petrous ICA, GSPM, IAC, trochlear nerve, and arcuate eminence. (c) Kawase’s quadrilateral after retraction of middle fossa dura and dural layer splitting. (d) Anterior petrosectomy showing ICA, inferior petrosal sinus, and posterior fossa dura.

(e) Identification of trochlear nerve after middle and posterior fossa dural incisions; tentorial incision between the superior petrosal sinus. (f) After transection of the superior petrosal sinus, sutures retract the tentorial leaflets. (g) Incision extends along the dorsolateral surface of Meckel’s cave. (h) Roof of the porus trigeminus reflected medially. (With permission from Operative Neurosurgery) [1]

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Fig. 27.5  MRI T1with gadolinium axial, sagittal, and coronal views, showing a middle fossa trigeminal schwannoma with small Meckel’s cave component

Porus Trigeminus Opening The dural cleft surrounding the trigeminal nerve root as it crosses the porus trigeminus should be incised along the lateral margin below the transected superior petrosal sinus. This incision is extended along the dorsolateral wall of Meckel’s cave in order to reflect the roof of the porus trigeminus medially in addition to the tentorial edge, thus sparing traction injury on the trochlear nerve.

Tumor Resection The tumor now is completely exposed as the middle and posterior fossa compartments are combined into one compartment after anterior petrosectomy and dural and tentorial incisions. Tumor resection is performed in a piecemeal fashion while maximally preserving the trigeminal nerve fascicles.

Closure Primary dural closure is possible for the temporal convexity dura. Exposed petrous air cells can be sealed with bone wax. Posterior fossa dura is reconstructed with muscle graft to obliterate the petrous defect, and collagen-based dural substitute is laid over. This can be further sprayed with a dural sealant or fibrin glue. The bone flap can

be secured in the standard fashion, and bone cement can be used to fill in the gaps with the calvarium.

Case Example (Video 27.1) A 40-year-old lady was diagnosed with trigeminal schwannoma that measured 3.6 × 2.8 cm in the middle fossa with a small component in Meckel’s cave (Fig. 27.5). She has been experiencing hypesthesia in the ipsilateral V2 distribution. She underwent surgical resection through a middle fossa approach. Gross total resection was achieved with trigeminal nerve fascicle preservation technique. She had an uneventful postoperative course with temporary postoperative paresthesia along V2 that eventually improved in 6  months. At the 3-year follow-up, she had no evidence of recurrence.

Conclusions Meckel’s cave lesions can be directly accessed through a subtemporal middle fossa interdural approach with maximum preservation of trigeminal nerve fascicles and function. The subtemporal approach is versatile and can be used for diverse pathology and tumor sizes. In trigeminal schwannomas, the approach remains extradural for the middle fossa component with maximum functional preservation.

414 Disclosure Funding: This study did not receive any funding relative to its elaboration. Conflict of interest: ASY is a consultant for Stryker Corp and has received royalty from Mizuho America. Ethical approval and informed consent (to participate and for publication): Informed consent and ethical approval were not deemed necessary by the local ethics in view of the design of the study. This study did not receive financial support. Availability of data and material (data transparency): This manuscript has not been previously published in whole or in part or submitted elsewhere for review.

References 1. Youssef S, et al. The subtemporal interdural approach to dumbbell-shaped trigeminal schwannomas: cadaveric prosection. Operative Neurosurg. 2006;59:270–6. 2. Clemente CD. Gray’s anatomy. 30th ed. Philadelphia: Lea & Febiger; 1984. p. 1121–33. 3. Taptas JN.  The so-called cavernous sinus: a review of the controversy and its implications for neurosurgeons. J Neurosurg. 1995;82:719–25. 4. Asbury AK.  Peripheral nerves. In: Haymaker W, Adams RD, editors. Histology and histopathology of the nervous system. Springfield: Charles C. Thomas; 1982. p. 1566–70. 5. Parent A.  Carpenter’s human neuroanatomy. 9th ed. Baltimore: Williams & Wilkins; 1996. p. 264–8. 6. Romanes GJ.  The central nervous system. In: Romanes GJ, editor. Cunningham’s textbook of anatomy. 12th ed. Oxford: Oxford University Press; 1981. p. 729–33. 7. Shanthaveerappa TR, Bourne GH.  Perineural epithelium: a new concept of its role in the integrity of the peripheral nervous system. Science. 1966;154:1464–7. 8. Burr HS, Robinson GB. An anatomical study of the gasserian ganglion, with particular reference to the

A. Sethi and A. S. Youssef nature and extent of Meckel’s cave. Anat Record. 1925;29:269–82. 9. Frazier CH, Whitehead E. The morphology of the gasserian ganglion. Brain. 1925;48:458–75. 10. Goel A.  Infratemporal fossa interdural approach for trigeminal neurinomas. Acta Neurochir. 1995;136:99–102. 11. Goel A, Muzumdar D, Raman C.  Trigeminal neuroma: analysis of surgical experience with 73 cases. Neurosurgery. 2003;52:783–90. 12. Hakanson S.  Transoval trigeminal cisternography. Surg Neurol. 1978;10:137–44. 13. Henderson WR.  The anatomy of the gasserian ganglion and the distribution of pain in relation to injections and operations for trigeminal neuralgia. Ann R Coll Surg Engl. 1965;37:346–73. 14. Kaufman B, Bellon EM. The trigeminal nerve cistern. Radiology. 1973;108:597–602. 15. Kehrli P, Maillot C, Wolff MJ. Anatomy and embryology of the trigeminal nerve and ints branches in the parasellar area. Neurol Res. 1997;19:57–65. 16. Al-Mefty O, Ayoubi S, Gaber E. Trigeminal schwannomas: removal of dumbbell-shaped tumors through the expanded Meckel cave and outcomes of cranial nerve function. J Neurosurg. 2002;96:453–63. 17. El-Kalliny M, van Loveren HR, Keller JT, Tew JM Jr. Tumors of the lateral wall of the cavernous sinus. J Neurosurg. 1992;77:508–14. 18. Kawase T, van Loveren HR, Keller JT, Ter JM Jr. Meningeal architecture of the cavernous sinus. Clinical and surgical implications. Neurosurgery. 1996;39:527–36. 19. Yoshida K, Kawase T. Trigeminal neurinomas extending into multiple fossae: surgical methods and review of the literature. J Neurosurg. 1999;91:202–11. 20. Umansky F, Elidan J, Valarezo A. Dorello’s canal: a microanatomical study. J Neurosurg. 1991;74:837–44. 21. Volenc VV.  Anatomy and surgery of the cavernous sinus. Vienna: Springer-Verlag; 1989. p. 28–35. 22. Parkinson D.  A surgical approach to the cavernous portion of the carotid artery. Anatomical study and case report. J Neurosurg. 1965;23:474–83.

Endoscopic Endonasal Approach to Meckel’s Cave

28

Carl H. Snyderman and Paul A. Gardner

Introduction In the coronal plane, endonasal approaches are defined by the cranial fossae and the relationship to the internal carotid artery (ICA). Endoscopic endonasal approaches (EEAs) to the middle cranial fossa include approaches both medial and lateral to the parasellar and paraclival segments of the ICA [1]. Meckel’s cave is situated lateral to the paraclival segment and superior to the horizontal petrous segment. It is bounded by the lateral cavernous sinus and the abducens nerve superiorly, and anterior access is defined by the Vidian nerve and foramen rotundum (Fig. 28.1) [2]. An endonasal approach to Meckel’s cave is most often used for biopsy of indeterminate lesions and resection of benign tumors such as meningiomas and trigeminal schwannomas (Fig.  28.2). It is also used to resect sinonasal malignancies with perineural extension to Meckel’s cave such as adenoid cystic carcinoma

C. H. Snyderman (*) Departments of Otolaryngology and Neurological Surgery, University of Pittsburgh School of Medicine, Pittsburgh, PA, USA e-mail: [email protected] P. A. Gardner Departments of Neurological Surgery and Otolaryngology, University of Pittsburgh School of Medicine, Pittsburgh, PA, USA e-mail: [email protected]

Fig. 28.1  The quadrangular space (Meckel’s cave) is bounded by the internal carotid artery (ICA) medially and inferiorly, the maxillary nerve (V2) laterally, and the lateral cavernous sinus with abducens nerve (CNVI) superiorly. The Vidian nerve and artery are important landmarks for the petrous segment of the ICA with a transpterygoid approach. Meckel’s cave is situated between the pterygoid foramen and foramen rotundum. BA: basilar artery; VA: vertebral artery. (Reprinted with permission from Kassam et al. [1])

and squamous cell carcinoma (Fig.  28.3). The endonasal approach to Meckel’s cave is often combined with other surgical modules to provide

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Operative Technique

Fig. 28.2  MRI (coronal view) of trigeminal schwannoma arising from the maxillary nerve (V2)

Fig. 28.3  MRI (axial view) of right sinonasal adenoid cystic carcinoma with perineural extension along branches of trigeminal nerve. Note the proximity to petrous segment of internal carotid artery (ICA)

access to adjacent areas such as the lateral cavernous sinus, medial petrous apex, and floor of the middle cranial fossa. The transpterygoid approach (Chap. 35) is a prerequisite for access to Meckel’s cave.

1. For right-handed surgeons (standing on patient’s right side), the patient is in a supine position with the head fixated in a Mayfield clamp. The head is rotated slightly toward the surgeons with the vertex of the head angled away and slightly extended to provide an inline ergonomic approach (Chap. 5). Greater rotation may be necessary for an approach to the right middle cranial fossa. 2. Registration of the navigation system is performed using images from a computed tomography angiogram (CTA). This provides superior localization of the ICA and bony landmarks (foramen rotundum, foramen ovale, Vidian canal, etc.). It may be fused with magnetic resonance imaging (MRI) depending on the pathology. 3. In addition to somatosensory-evoked potentials (SSEPs), neuromonitoring may include electromyography of the third division of the trigeminal nerve and the abducens nerve (Chap. 7). 4. Antibiotic prophylaxis with a third-­ generation cephalosporin is administered in nonallergic patients. 5. Pledgets soaked in oxymetazoline (0.05%) solution are placed intranasally for decongestion of nasal mucosa. 6. The midface is prepped with betadine (protecting the eyes) and draped in the usual manner with the endotracheal tube secured on the left side. If desired, the nasal vestibule can be prepped but antiseptic rinses of the nasal cavity are not performed due to deleterious effects on mucociliary function and olfaction. 7. For most cases, a binarial approach is planned. For a limited exposure and biopsy, a unilateral approach may be considered. 8. The inferior aspect of the middle turbinate is resected on the side of the transpterygoid approach. The posterior stump is carefully cauterized to prevent bleeding from a branch of the sphenopalatine artery.

28  Endoscopic Endonasal Approach to Meckel’s Cave

9. If a dural defect is anticipated, a contralateral nasoseptal flap is elevated so that the vascular pedicle is not compromised by the transpterygoid approach (Chap. 10). 10. The cartilage and bone of the posterior half of the nasal septum are removed to the rostrum of the sphenoid to provide binarial access and for the construction of a reverse septal flap [3]. Releasing incisions are made and the mucosa of the posterior septum opposite the flap is transposed to cover the cartilage of the donor site. This promotes rapid healing and may prevent postoperative saddle nose deformity in a small percentage of patients. 11. SPIWay nasal sleeves are inserted bilaterally to protect the nasal mucosa of the nasal septum and turbinates from trauma caused by the passage of instruments and burn injury from powered instrumentation [4]. 12. The bone of the sphenoid rostrum is removed, and the sphenoid sinus is maximally opened bilaterally with Kerrison rongeurs. Septations within the sphenoid sinus are partially removed with rongeurs or drilling. Be cautious with lateral septations since they lead to the ICA [5]. Mucosa is stripped to expose the underlying bone, and key bony landmarks are identified with the aid of navigation. 13. On the side of the lesion, a maxillary antrostomy is performed with maximal enlargement of the opening posteriorly. The mucoperiosteum of the lateral nasal wall is elevated inferior to the crista ethmoidalis, and the sphenopalatine artery is identified at the foramen. 14. The bone of the sphenopalatine foramen and posterior wall of the maxilla is removed with Kerrison rongeurs to expose the contents of the pterygopalatine space (Fig. 28.4). 15. The sphenopalatine artery and its distal branches are cauterized with bipolar electrocautery, and the mucosal tissues are resected to expose the medial pterygoid “wedge”  – the junction of the floor of the sphenoid sinus with the pterygoid body and medial pterygoid plate.

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Fig. 28.4  Removal of the posterior wall of the maxillary sinus exposes the contents of the pterygopalatine space. Branches of the internal maxillary artery (IMA) are sacrificed to expose branches of the maxillary nerve, including the infraorbital nerve (ION) and descending palatine nerve (PN)

16. In order to gain access to the pterygoid canal transmitting the Vidian nerve, it is necessary to cauterize and transect the palatosphenoidal branch of the sphenopalatine artery as well. This vessel passes through a bony canal that traverses the bone inferior to the sphenoidotomy and is easily mistaken for the Vidian nerve. Following transection of the vessel, the pterygoid canal is found immediately lateral. 17. The contents of the pterygopalatine space can then be elevated in a subperiosteal plane from the bone of the pterygoid base in a medial to lateral direction. The Vidian nerve (and sometimes artery) is situated directly posterior to the stump of the sphenopalatine artery. With a well-pneumatized sinus, the course of the pterygoid canal may be visible on the floor of the sphenoid sinus. It curves laterally to meet the horizontal petrous ICA where it connects with the greater superficial petrosal nerve and deep petrosal nerve. Further dissection along the pterygoid base superolateral to the pterygoid canal exposes the maxillary nerve at foramen rotundum (Fig. 28.5).

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Fig. 28.5  The Vidian nerve (VN) exits the pterygoid canal (smaller circle). Foramen rotundum (larger circle) with V2 is located superolateral to the lateral recess of the sphenoid sinus. Major branches of the maxillary nerve include the infraorbital nerve (ION) and descending palatine nerve (PN)

18. Meckel’s cave is situated lateral to the paraclival ICA and superior to the horizontal petrous ICA. The maxillary nerve and Vidian nerve converge on Meckel’s cave. With a well-pneumatized sinus, the course of the maxillary nerve may be evident on the lateral wall of the sphenoid sinus. The distance between the Vidian and maxillary nerves is increased with greater pneumatization of the lateral recess [6]. Access to Meckel’s cave is limited without sacrificing the Vidian nerve and artery. After transection of the Vidian nerve, the bone of the base of pterygoid is drilled circumferential to the pterygoid canal using a 4-mm coarse diamond bit. Removal of the bone lateral to the pterygoid canal helps define the depth of the petrous ICA; drilling medial to the canal should be done cautiously to avoid the paraclival ICA and foramen lacerum. 19. The bone overlying the maxillary nerve (thickest at foramen rotundum) is then thinned with the drill and carefully elevated to expose the underlying dura. The bone between the Vidian nerve and maxillary nerve is drilled to expose the dura of Meckel’s cave. If desired, the bone over the lateral cavernous sinus and ICA can be similarly

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removed and will provide greater mobility for dissection. The bone directly inferolateral to foramen rotundum is drilled to gain access to the mandibular nerve in foramen ovale. A nerve stimulator probe is useful for identifying the motor component (V3) of the trigeminal nerve. 20. The dura is incised parallel to the course of the maxillary nerve to avoid transection of nerve fibers. Opening of the dura extends posteriorly to Meckel’s cave, inferiorly to foramen ovale, and superiorly to the parasellar ICA. 21. Tumor dissection can even continue through the porus trigeminus into the posterior fossa, though visualization is limited unless the porus is greatly expanded. Much of the dissection is performed with a stimulating dissector to localize the abducens nerve that marks the upper boundary of the endonasal approach to Meckel’s cave and to avoid damage to mandibular (V3) fibers. Larger tumors should be debulked prior to capsule dissection to avoid unnecessary damage to the critical structures associated with this approach (CN VI, ICA, uninvolved trigeminal fibers). Limiting dissection where foramen rotundum enters Meckel’s cave will reliably avoid a CSF leak and may provide an adequate margin for aggressive sinonasal malignancies. 22. Localization of the ICA is critical for safe resection in Meckel’s cave. This can be done with an extended Doppler probe, neuronavigation, indocyanine green endoscopic angiography, or all of the above. Single-shaft clip appliers and aneurysm clips should be available in the room whenever dissecting adjacent and lateral to the ICA. 23. Following removal of the lesion, reconstruction of the defect with vascularized tissue is preferred even in the absence of a CSF leak. A vascularized flap provides coverage of the dural defect as well as the carotid artery. If there is a dural defect, a multilayer reconstruction is performed with an inlay collagen graft covered externally by a nasoseptal flap. For larger defects, a fascia lata graft or fat

28  Endoscopic Endonasal Approach to Meckel’s Cave

graft may be interposed. Fat grafts are especially useful for large defects within the posterior fossa and result in very low rates of postoperative CSF leak [7]. 24. The reconstruction is supported with Merocel tampons, and the nasal septum is protected with silastic splints. Nasal packing and splints are removed 5–7 days following surgery. Oral antibiotics (second-generation cephalosporin) are administered for the duration of nasal packing. 25. Given the low rates of postoperative CSF leak, lumbar spinal drainage is not routinely employed unless the reconstruction is suboptimal or the patient has increased risk factors.

Case Example A 45-year-old man was diagnosed with a right sinonasal tumor (Fig. 28.3). Biopsy confirmed an adenoid cystic carcinoma. MRI revealed perineural extension of tumor along the branches of the maxillary nerve. A right endoscopic transpterygoid and transpalatal approach was used to resect the tumor with the contents of the pterygopalatine space (Fig. 28.6). Deep margins included the

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Fig. 28.7  Following tumor resection, the stumps of the maxillary nerve (V2) and Vidian nerve (VN) are sampled for frozen section analysis. Note the relationship of the trigeminal nerve and VN to the internal carotid artery (ICA). SS sphenoid sinus

Vidian nerve in the pterygoid canal and the maxillary nerve at foramen rotundum (Fig. 28.7). The exposed dura was covered with a contralateral nasoseptal flap.

Conclusions An endoscopic endonasal approach to Meckel’s cave is a suprapetrous approach to the middle cranial fossa. It requires a transpterygoid approach to identify key landmarks (Vidian nerve at the pterygoid canal and V2 at foramen rotundum) and provide full access to the lateral recess of the sphenoid sinus. It is most often used for benign tumors such as meningiomas and trigeminal schwannomas as well as sinonasal malignancies with perineural extension to Meckel’s cave.

References

Fig. 28.6  Dissection within the pterygopalatine space. The infraorbital nerve is transected proximally and distally (asterisk). Bipolar electrocautery is used to cauterize branches of the internal maxillary artery. SS sphenoid sinus

1. Kassam AB, Prevedello DM, Carrau RL, Snyderman CH, Gardner P, Osawa S, Seker A, Rhoton AL Jr. The front door to Meckel’s cave: an anteromedial corridor via expanded endoscopic endonasal approach – technical considerations and clinical series. Neurosurgery. 2009;64(3 Suppl):71–82. discussion 82-83 2. Pinheiro-Neto CD, Fernandez-Miranda JC, Rivera Serrano CM, Paluzzi A, Snyderman CH, Gardner PA,

420 Sennes LU. Endoscopic anatomy of the palatovaginal canal (palatosphenoidal canal): a landmark for dissection of the vidian nerve during endonasal transpterygoid approaches. Laryngoscope. 2012;122(1):6–12. 3. Caicedo-Granados E, Carrau R, Snyderman CH, Prevedello D, Fernandez-Miranda J, Gardner P, Kassam A.  Reverse rotation flap for reconstruction of donor site after vascular pedicled nasoseptal flap in skull base surgery. Laryngoscope. 2010;120(8):1550–2. 4. Velasquez N, Ahmed OH, Lavigne P, Goldschmidt E, Gardner PA, Snyderman CH, Wang EW.  Utility of nasal access guides in endoscopic endonasal skull base surgery: assessment of use during cadaveric dissection and workflow analysis in surgery. J Neurol Surg B Skull Base. 2020;82(5):540–6.

C. H. Snyderman and P. A. Gardner 5. Fernandez-Miranda JC, Prevedello DM, Madhok R, Morera V, Barges-Coll J, Reineman K, Snyderman CH, Gardner P, Carrau R, Kassam AB. Sphenoid septations and their relationship with internal carotid arteries: anatomical and radiological study. Laryngoscope. 2009;119(10):1893–6. 6. Vaezi A, Cardenas E, Pinheiro-Neto C, Paluzzi A, Branstetter BF 4th, Gardner PA, Snyderman CH, Fernandez-Miranda JC.  Classification of sphenoid sinus pneumatization: relevance for endoscopic skull base surgery. Laryngoscope. 2015;125(3):577–81. 7. Shin SS, Gardner PA, Stefko ST, Madhok R, Fernandez-Miranda JC, Snyderman CH.  Endoscopic endonasal approach for nonvestibular schwannomas. Neurosurgery. 2011;69(5):1046–57.

Trigeminal Schwannoma

29

Wei Huff, Benjamin K. Hendricks, and Aaron A. Cohen-Gadol

Abbreviations CN cranial nerve CP cerebellopontine CSF cerebrospinal fluid CT computed tomography CTA computed tomography angiography MR magnetic resonance SRS stereotactic radiosurgery TS trigeminal schwannoma

Introduction Trigeminal schwannomas (TSs) are rare benign tumors representing the most common nonvestibular schwannoma type and 0.07–0.36% of all intracranial tumors [1]. They can occur sporadically, but case reports have suggested that patients with neurofibromatosis type 2 have an increased propensity to form these tumors in addition to the more commonly found vestibular schwannomas. W. Huff · A. A. Cohen-Gadol (*) Department of Neurological Surgery, Indiana University School of Medicine, Indianapolis, IN, USA e-mail: [email protected]; [email protected] B. K. Hendricks Department of Neurosurgery, Barrow Neurological Institute, Phoenix, AZ, USA e-mail: [email protected]

Malignant TSs are reported very infrequently and most often occur in association with neurofibromatosis type 1 [2]. Schwannomas emerge from the peripheral nerve sheath, distal to the oligodendroglia– Schwann cell junction [3]. After the vestibular nerve, the trigeminal nerve (cerebellopontine [CP] angle) and the Gasserian ganglion (cavernous sinus and Meckel’s cave) are the most common sites for intracranial schwannomas (Fig.  29.1). Purely extradural TSs are exceptionally rare as one of the three postganglionic divisions of the trigeminal nerve; the ophthalmic branch is the site of origin more frequently than is either the maxillary or mandibular branch [4]. Similar to vestibular schwannomas, gross-total resection of a TS results in a good prognosis regarding long-term tumorfree survival; however, if only partial resection is achieved, recurrence is common [1].

Classification Several classification systems for TSs have been proposed. The first classification system was proposed in 1955 by Jefferson [5], who categorized TSs into three different types: type A, tumors originating from the Gasserian ganglion in the middle cranial fossa; type B, tumors originating from the roots of the trigeminal nerve in the posterior fossa; and type C, the so-called hour-glass tumors that occupy both the middle

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Fig. 29.1  Schwannomas present as well-circumscribed, heterogeneously enhancing lesions that are isointense or hypointense on T1-weighted MR images and hyperintense on T2-weighted MR images. They do not harbor the dural tail associated with meningiomas. Enlargement of

the internal auditory meatus signifies a vestibular schwannoma. (a and b) A classic TS with extension into Meckel’s cave. (c and d) A giant tumor with intraorbital and extracranial or infratemporal extensions. (e and f) An isolated TS within the CP angle

Table 29.1  Jefferson’s classification of TSs [5]

nal nerve; type M, middle fossa tumors originating from the Gasserian ganglion or the peripheral branch at the lateral wall of the cavernous sinus; type E, tumors arising from the extracranial peripheral branches of the trigeminal nerve; and types MP, ME, and MPE, combinations of P, M, and E tumors. The operative approach to treating TSs and the surgical difficulties depend highly on the location of the tumor along the trigeminal nerve.

Type Root (A)

Description Nerve root derived with posterior fossa involvement Ganglion (B) Gasserian ganglion derived with middle fossa involvement Dumbbell (C) Combination posterior fossa–middle fossa involvement Division (D) Peripheral segment involvement  V1 Orbital  V2 Pterygopalatine fossa  V3 Infratemporal fossa

and posterior fossae. Some authors have added a fourth classification, type D, to separately classify tumors with extracranial extension [6–8]. These TS types are summarized in Table  29.1. Yoshida and Kawase [9] expanded the classification into six types: type P, posterior fossa tumors originating from the root of the trigemi-

Clinical Presentation Patients with a TS are nearly uniformly symptomatic with trigeminal nerve dysfunction at presentation [9, 10]. Symptoms typically include hypoesthesia within a variable distribution depending on the number of trigeminal seg-

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ments affected. Keratitis can be an important feature heralding a loss of function within the trigeminal nerve due to the diminished corneal reflex. Motor fibers of the nerve innervate the muscles of mastication, including the tensor, digastric, and mylohyoid muscles, with expected deficits or atrophy in these muscles that are sometimes clinically apparent. Trigeminal neuropathy follows an indolent course, corresponding to the slow growth of the tumor [9, 10]. Facial pain has also been reported in the setting of a TS. The pain is different from that experienced with trigeminal neuralgia [10, 11]. Although tic-like in onset, the long duration of pain and lack of a defined stimulus are characteristic of TS-induced facial pain. These patients’ pain is also refractory to carbamazepine and other neuropathic pain medications used for trigeminal neuralgia. Despite this fact, the literature suggests that a small percentage of patients with a TS experience pain consistent with the diagnosis of trigeminal neuralgia. The key for symptomatic relief in these patients is to examine the entire nerve root for possible vascular compression after total or subtotal tumor resection [10]. All three divisions of the nerve are affected to different degrees at presentation. Other presenting symptoms can be attributed to the mass effect on the surrounding CNs. Deficits of facial motor, vestibular, and auditory functions are possible with larger tumors. Diplopia is a result of compression on the oculomotor or abducens nerve or compression on the globe resulting in exophthalmos-related diplopia.

Evaluation Patients with a TS should undergo a detailed neurologic examination, with special focus on trigeminal sensory and motor functions. A preoperative audiogram establishes the baseline functional status of CN VIII. The complexity of the skull base anatomy relevant to the resection of these tumors requires a thorough imaging evaluation. Magnetic resonance (MR) imaging characterizes the tumor’s size and degree of extension and localizes the adjacent CNs and cerebrovascular structures

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(Fig.  29.1). Vestibular, facial, and oculomotor schwannomas within the CP angle can be difficult to differentiate from TSs. The radiographic evaluation includes computed tomography (CT) for defining the bony anatomy of the skull base and any resultant bony erosion. The proximity of the tumor to the carotid canal, cochlea, and internal auditory canal should be assessed to qualify the patient’s operative risk. CT angiography (CTA) defines the caliber of the involved arterial anatomy and elucidates the displacement of major vessels.

General Considerations for Management Because of the benign, slow-growing nature of these lesions, multiple management strategies for TSs can be considered [12]. The most conservative approach is observation with serial imaging. This option includes a 6- to 12-month interval between the initial and first follow-up scans and is most applicable for patients with a small tumor or elderly patients in whom the risk of surgery outweighs the risk of continued tumor growth. Microsurgical or endoscopic transnasal resection is the most definitive method for managing these lesions. If gross-total resection is achieved, the risk of recurrence is low; however, subtotal resection resulting from an effort to preserve function can be supplemented with radiosurgery. Stereotactic radiosurgery (SRS) alone is an option for patients with a small tumor for which evidence of growth is found on serial imaging. In the unique situation of bilateral TSs, generally isolated to patients with neurofibromatosis type 2, the surgeon should operate on the most symptomatic side only. The contralateral nonsymptomatic tumor can be handled via observation or SRS.

Preoperative Considerations and Surgical Approach The microsurgical or endoscopic approach utilized for TSs depends highly on the location of the tumor. Jefferson’s classification simplifies the

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424 Table 29.2  Optimal surgical approaches to TSs Type Root (A) Ganglion (B) Dumbbell (C)

Approach Retrosigmoid Extradural subtemporal Extradural frontotemporal Extradural subtemporal Extradural subtemporal or anterior transpetrosal Extradural modified orbitozygomatic Combined petrosal

geon’s efforts to preserve cranial nerve functions. These monitoring tools include somatosensory evoked potentials and brainstem auditory evoked responses.

Operative Anatomy

The trigeminal nerve root originates from the lateral aspect of the rostral pons, travels superiorly, Modified orbitozygomatic laterally, and anteriorly toward the petrous apex, Pterional/endoscopic and enters Meckel’s cave through the trigeminal Extradural subtemporal/endoscopic impression, just inferior to the superior petrosal sinus. This first segment, which extends from the operative planning for these tumors. Table  29.2 brainstem to Meckel’s cave, is referred to as the lists the optimal surgical approaches for each cisternal or CP angle segment. type of tumor. Those tumors within the ganglion The root is myelinated by oligodendrocytes or peripheral category can be efficiently from its origin at the brainstem to the level of the approached extradurally to minimize the risk to central myelin–peripheral myelin transition zone, adjacent CNs. where the Schwann cells appear. Schwannomas Because of the soft consistency of TSs, gross-­ arise from the peripheral myelin zone. After total removal of large, multicompartment tumors entering the trigeminal impression, the nerve with a predominantly middle fossa component is courses within two leaflets of the dura, also achievable via a single extradural approach known as Meckel’s cave. through the middle fossa (Chap. 27). This prinTSs involve the Gasserian ganglion (middle ciple also holds true for tumors that harbor a pre- fossa), the nerve root (CP angle), and the three dominantly posterior fossa extension with a divisions of the nerve (middle fossa). As expected, middle fossa component that can be extracted via some tumors extend into more than one of these a modified retrosigmoid corridor and opening compartments and are typically dumbbell-­ over the expanded Meckel’s cave. shaped. Rare tumors invade the extracranial However, the reach from the posterior fossa space via the extracranial nerve branches in the corridor into the middle fossa is very limited. orbit and infratemporal fossa. These tumors reach Therefore, tumors with equally dominant mid- the orbit through the superior orbital fissure and dle and posterior fossa extensions should be the infratemporal fossa through the foramen approached first with middle fossa surgery in ovale or foramen rotundum. an attempt to remove the entire tumor; a The most common tumors are those found in second-­ stage retrosigmoid craniotomy might the middle cranial fossa (50%), followed by posbe needed if the posterior fossa component terior fossa (30%) and dumbbell (20%) tumors. cannot be resected safely during the initial Extension into the cavernous sinus is common. operation. These schwannomas do not infiltrate but rather A lumbar drain, or alternatively an external displace their surrounding structures; therefore, ventricular catheter, should be placed preopera- entry into the lateral cavernous sinus for their tively to facilitate brain relaxation. The use of removal is appropriate in most cases (Figs. 29.2, neurophysiologic monitoring augments the sur- 29.3, and 29.4). Division (D)  V1  V2  V3

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a

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Fig. 29.2  Anatomy for extradural dissection of the cavernous sinus via orbitozygomatic craniotomy through the extradural clinoidectomy approach. (a) The middle meningeal artery was sacrificed lateral to the posterior edge of V3. The outer wall of the cavernous sinus is peeled off from its inner layer after partial transection of the meningo-orbital dural band at the lateral edge of the superior orbital fissure. These maneuvers expose the nerves invested in the inner layer of the lateral wall as the meningeal (outer) layer is peeled away. (b) Elevation of the middle fossa dura continues posteriorly and medially while the greater petrosal nerve is found. (c and d) Magnified views after the extradural clinoidectomy. The inner dural layer of the lateral sinus wall is resected to expose the relevant structures that are usually displaced medially by the tumor. This maneuver is replaced by a linear incision over the tumor capsule between and parallel to the V1 and V2 divisions during surgery. The tumor expands the porus of Meckel’s cave and creates a route toward the posterior fossa so that the surgeon can deliver the component of the tumor within the CP angle. The inci-

sion over the tumor can be extended into the posterior fossa dura over the bulk of the tumor capsule while coagulating and transecting the superior petrosal sinus. (e) Note the approximate location of the middle fossa triangles, the anteromedial triangle (between V1 and V2), the anterolateral triangle (between V2 and V3), the posterolateral triangle, also called Glasscock’s triangle (between V3 and the greater petrosal nerve), and the posteromedial triangle, also called Kawase’s triangle (lateral to the trigeminal nerve and posterior to the greater petrosal nerve). (f) The petrous carotid is exposed under the greater petrosal nerve. Sup. Orb. Fiss., superior orbital fissure; Fr., frontal; Mid. Men. A, middle meningeal artery; Ant. Clin., anterior clinoid process; Gr. Pet. N., greater petrosal nerve; Clin. Seg., clinoid segment; Triang., triangle; Car., carotid; Carotidoculom. Memb., carotid-oculomotor membrane; Horiz. Seg., horizontal segment; Post. Vert. Seg., posterior vertical segment; Post. Lat., posterior lateral; Post. Med., posterior medial; Tens. Tymp. M., tensor tympani muscle; Gen. Gang., geniculate ganglion. (Images courtesy of A. L. Rhoton, Jr)

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a

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c

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Fig. 29.3  Sequential Dissection of the medial middle fossa. (a) The native appearance of the middle fossa dura is demonstrated. (b) The middle fossa dura and bone over the petrous internal carotid artery are removed to demonstrate the relationship of the petrous internal carotid artery and Meckel’s cave. (c) Additional bone remove permits an exposure of the internal auditory canal and its relationship to the greater petrosal nerve. (d) Further petrous bone removal elucidates the location of additional structures within the petrous bone, even though they are not commonly exposed during surgery. These structures should be protected during microdissection. Note the need for disconnection of the superior petrosal sinus for reaching the CP angle via the middle fossa exposure around Meckel’s

cave. Tent. Edge, tentorial edge; Car. A., carotid artery; Ant. Clin., anterior clinoid process; Arc. Emin., arcuate eminence; Horiz. Seg., horizontal segment; Gr. Pet. N., greater petrosal nerve; Mid. Men. A., middle meningeal artery; Sup. Semicirc. Canal, superior semicircular canal; Tymp. Seg., tympanic segment; Laby. Seg., labyrinthine segment; Gen. Gang., geniculate ganglion; Mid. Men. A., middle meningeal artery; Tymp. Cavity, tympanic cavity; Bas. A., basilar artery; A.I.C.A., anterior inferior cerebellar artery; Tens. Tymp. M., tensor tympani muscle; Eust. Tube, eustachian tube; Ext. Ac. Meatus, external acoustic meatus; Inf. Pet. Sinus, inferior petrosal sinus; Sphen. Sinus, sphenoid sinus; Inf. Temp. Fossa, inferior temporal fossa. (Images courtesy of A. L. Rhoton, Jr)

Microsurgical Approaches for the Resection of TSs

This “untethering” of the sigmoid sinus enables its lateral mobilization using retraction sutures after dural opening and expands the lateral operative trajectory toward the CP angle while reducing retraction on the cerebellar hemisphere. In patients without obstructive hydrocephalus, a lumbar puncture is used before draping the incision to drain approximately 35–40 cc of cerebrospinal fluid (CSF) and achieve posterior fossa decompression. This maneuver reduces the intracranial tension and significantly facilitates the

Retrosigmoid Approach The extended retrosigmoid approach applies to root-type tumors within the CP angle (Fig. 29.5). The extended retromastoid approach is a ­modification of the standard retromastoid craniotomy, which includes partial removal of the bone over the transverse and sigmoid sinuses.

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a

b

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f

Fig. 29.4  Anatomy of the trigeminal nerve originating from the brainstem. (a) The origin of the trigeminal nerve is a reasonable landmark for defining the border between the pons and the middle cerebellar peduncle. (b) The tentorium and the occipital lobe have been detached to expose the route of the trigeminal nerve through the trigeminal impression over the petrous apex. (c) The two motor rootlets of the trigeminal nerve, the superior and second inferior motor rootlets, are evident. The dura of the anterior-posterior fossa has been peeled away to uncover the basilar plexus, cavernous sinus, and inferior petrosal sinus. (d) Meckel’s cave is located at the trigeminal impression between the meningeal layer (dura propria) and the periosteal layer of the dura. (e and f) The superior petrosal sinus travels over Meckel’s cave to join the cavernous sinus. Opening the roof of Meckel’s cave requires

clipping or coagulation of this sinus before its division to avoid venous bleeding. Lat. pon. Sul., lateral pontine sulcus; MCP, middle cerebellar peduncle; Floccul., flocculus; Inf. olive, inferior olive; Sup. Pet. Sinus, superior petrosal sinus; Int. Ac. Canal, interior acoustic canal; AICA, anterior inferior cerebellar artery; Sup. motor roots, superior motor roots; Inf., inferior; Seg., segment; Trig. Gang., trigeminal ganglion; Bas., basilar; Cav., cavernous; Inf. Pet. Sinus, inferior petrosal sinus; Int. Ac. Meatus, internal acoustic meatus; Glossophar. Meatus, glossopharyngeal meatus; Tent. edge, tentorial edge; Arachnoid Memb; arachnoid membrane; ACP, anterior clinoid process; SOF, superior orbital fissure; For. Rotund., foramen rotundum; For. Oval., foramen ovale. (Images courtesy of A. L. Rhoton, Jr)

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a

b

c

d

e

f

Fig. 29.5  A primarily CP angle TS (a and b) is exposed via a right-sided retrosigmoid craniotomy (c). Note the upper fascicles of the nerve engulfed by the tumor (yellow arrow). The tumor is mobilized away from the brainstem

(d), and the attenuated nerve is carefully released (e). Ultimately, most of the trigeminal nerve (yellow V) was preserved (f)

safety of opening the dura and “going around” the cerebellum to reach the CP angle cisterns during the early intradural dissection process. The suprameatal tubercle is just above the internal acoustic meatus and posterior to the tri-

geminal nerve. After removal of the dura from the surface of the suprameatal tubercle and drilling the tubercle, additional exposure along the trigeminal nerve can be achieved to expand the retrosigmoid approach to remove tumors that

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extend slightly into Meckel’s cave. Given the close proximity of the internal carotid artery, stealth CTA is helpful for guiding the osteotomy. In addition, an endoscope can be used to assist with removing tumor from Meckel’s cave and checking the extend of resection.

Pterional/Frontotemporal Approach The extended pterional or frontotemporal craniotomy is applicable for small TSs. As a supratentorial skull base approach workhorse, extended pterional craniotomy offers simplicity, flexibility, efficiency, and familiarity to neurosurgeons treating lesions along the anterior and middle skull base. In comparison to the regular frontotemporal craniotomy, the extended approach includes (1) osteotomy along the lateral sphenoid wing to the level of the superior orbital fissure, (2) drilling along the roof of the orbit to flatten its surface, and (3) roungering temporal squama toward the floor of the middle fossa. These modifications provide unobstructed operative working angles toward most of the anterior and middle skull base with minimal brain retraction. The patient is placed in the supine position with knees flexed and the head of the table elevated approximately 15–20 degrees. The head is immobilized in a skull clamp, turned 20–45 degrees away from the side of the approach, and moderately hyperextended to allow the frontal lobes to fall away from the floor of the anterior cranial fossa. For small TSs over the cavernous sinus, positioning requires little head deflection and greater head rotation so that the orbital ridge is left in the superior plane.

 rbitozygomatic or Zygomatic O Infratemporal Approach The zygomatic or orbitozygomatic osteotomy is used optimally for schwannomas that involve different segmental distributions of the trigeminal nerve within the cavernous sinus. In cases in which the tumor extends into the orbit, the superior and lateral walls of the orbit should be

Fig. 29.6  After completion of the orbitozygomatic craniotomy, reflection of the temporal dura or the lateral wall of the cavernous sinus (the Kawase method) exposes the tumor covered by the inner cavernous wall dura. The meningo-orbital dural band at the lateral edge of the superior orbital fissure is partially transected, and an extradural clinoidectomy is pursued. Elevation of the middle fossa dura continues posteriorly and medially while the greater petrosal nerve is found and sacrificed. A linear incision in the dura of the cavernous sinus, between and parallel to V1 and V2, unveils the tumor capsule. The tumor often expands the porus of Meckel’s cave and creates a route toward the posterior fossa so that the surgeon can deliver the component of the tumor within the CP angle into the middle fossa resection cavity. The dural incision over the tumor can be extended into the posterior fossa over the bulk of the tumor capsule within Meckel’s cave by coagulating and transecting the superior petrosal sinus. (Used with permission from The Neurosurgical Atlas by Aaron Cohen-Gadol, MD)

removed after the orbitozygomatic osteotomy. If the surgeon requires a wider operative field for approaching a V1 lesion, the optic canal can be unroofed and an extradural anterior clinoidectomy performed (Figs. 29.6, 29.7, and 29.8).

Subtemporal Approach The subtemporal approach provides a wide operative corridor to the floor of the middle fossa and upper petroclival territories. This approach is flexible and reaches middle fossa lesions that extend into the posterior fossa via traversing

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Fig. 29.7  An alternative method is the Dolenc approach, which uses the oculomotor nerve as the center of attention for the entry point into the cavernous sinus. After an orbitozygomatic craniotomy and extradural clinoidectomy, the bone over the lateral and anterior aspects of the foramen rotundum and foramen ovale are drilled to allow mobilization of these nerves within the foramina during tumor manipulation. The dura is incised along the Sylvian fissure, and this dural incision is extended medially to the level of the distal or outer carotid ring. Then, the distal ring is released, and the anterior cavernous sinus is entered. The oculomotor nerve is found as it enters the edge of the tentorium. The dura is opened over this nerve using an arachnoid knife. The trochlear nerve is posterolateral to the oculomotor nerve. The V1 is near the superior orbital fissure and is dissected from the overlying dura; this dissection along the nerve completes the exposure of the lateral wall of the cavernous sinus. The abducens nerve is the only nerve within the cavernous sinus and is immediately lateral to the internal carotid artery; it should be protected during the dissection of the medial tumor capsule. (Used with permission from The Neurosurgical Atlas by Aaron Cohen-Gadol, MD)

Meckel’s cave. If the caliber of Meckel’s cave is insufficient to gain access into the posterior fossa, the exposure can be expanded via an incision in the dura of Meckel’s cave that continues toward the superior petrosal sinus. Through extracapsular and intra-arachnoidal dissection, the posterior fossa portion of the tumor can be resected. This approach also enables entry into the cavernous sinus via its lateral wall for the resection of enclosed tumors, which are generally the larger TSs (Fig. 29.9). One caveat is the need to identify the petrous internal carotid artery to provide proximal control over the cavernous segment during the dissection. Intracavernous

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Fig. 29.8  The tumor is debulked and dissected from the adjacent neurovascular structures within the cavernous sinus. Often, the V1 is the source of the schwannoma, and part of this nerve has to be sacrificed; the patient is most likely suffering from preoperative V1 numbness, therefore allowing this maneuver. (Used with permission from The Neurosurgical Atlas by Aaron Cohen-Gadol, MD)

dissection requires minimal manipulation of the adjacent CNs to avoid postoperative deficits. The tumor is internally debulked and its capsule mobilized circumferentially after sufficient debulking. This technique requires microdissection of the capsule away from the adjacent trigeminal fascicles. Part of the trigeminal nerve that is incorporated into the tumor is resected.

Anterior Petrosectomy Approach Anterior petrosectomy is an extension of the subtemporal craniotomy and involves expanded bony resection at the petrous apex to expose the upper petroclival region and ventrolateral brainstem (Fig. 29.9). In combination with other cranial approaches, this operative pathway is optimized for large multicompartment lesions that involve both the posterior and middle fossae. Because of the limited working space offered via anterior petrosectomy and its inflexibility regarding intraoperative expansion, careful evaluation of preoperative MR and CT/CTA images for appropriate preoperative planning is crucial, and any need for the use of combined operative cor-

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a

b

c

d

e

Fig. 29.9  Subtemporal and anterior petrosectomy surgical approach. (a and b) Patient’s head position with the zygoma as the highest point on the operative field. The single pin of the skull clamp often has to be placed on the patient’s forehead to avoid the interference of the pins with the incision. The vertex of the patient’s head is tilted slightly toward the floor. This maneuver maximizes the effect of gravity retraction on the temporal lobe. (c and d) Exposure of Kawase’s triangle (indicated by hash marks in d). With the dura completely elevated to the level of the petrous ridge, the landmarks of Kawase’s quadrilateral are clearly visible: laterally, the greater superficial petrosal nerve (and often

the lesser superficial petrosal nerves); posteriorly, the arcuate eminence; anteriorly, the posterior edge of the Gasserian ganglion; and medially, the petrous ridge. (e) The dural opening is planned as a T-shaped incision. The first incision is made along the basal temporal dura parallel to the inferior edge of the craniotomy. The second cut is perpendicular to the first, crossing the superior petrosal sinus into the posterior fossa dura. The superior petrosal sinus is then ligated using a Weck clip, and the dural incisions are connected. Note the location of the trigeminal nerve within the dura. (Used with permission from The Neurosurgical Atlas by Aaron Cohen-Gadol, MD)

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ridors to achieve sufficient tumor exposure should be planned preoperatively. Intraoperative neurophysiologic monitoring of the facial nerve (electromyography) and brainstem auditory evoked responses is very helpful for localizing the facial nerve and warning the surgeon of maneuvers that might place the brainstem at risk. Incisions within the dura must be placed carefully and typically in a T-shaped fashion. The tentorium is opened posteriorly to the point at which CN V enters the dura to extend exposure from the middle to posterior fossa. The dural defect around the superior petrosal sinus can be repaired by using a pedicle of the temporal fascia after the removal of the tumor. Some degree of temporal lobe retraction is required during the surgery, and the use of a lumbar drain is highly advised, especially for lesions on the dominant side. In addition, the lumbar drain can be continued for a few days to reduce the risk of CSF leakage. In summary, appropriate patient selection, comprehensive preoperative planning, patience with drilling, and caution during dural opening are keys to this approach.

Postoperative Considerations During the early postoperative period, the patient will experience trigeminal neuropathy as a result of intraoperative manipulation of the nerve. This dysfunction will improve gradually during the postoperative period. Diminution or loss of the corneal reflex is of particular concern acutely because of the risk for keratitis if this deficit goes unnoticed and proper eye care is not implemented in a timely manner. Surgery of the cavernous sinus often requires manipulation of CNs III, IV, and VI. Therefore, postoperative diplopia from paresis of one or all of these CNs is not uncommon. These deficits are also generally transient and should improve within 3 months. Surgical complications depend on tumor size, location, and approach(es) chosen, but the most frequently reported postoperative complications are meningitis, fluid fistulas, masseter muscle atrophy, trigeminal pain, and facial paresthesia,

in addition to paresis of CNs III through VI [13]. As a result of the high surgical morbidity seen after TS resection, the field is progressively evolving toward using minimally invasive techniques.

 ndoscopic Endonasal Surgical E Approach More recently, endoscopic endonasal surgical approaches have been reported as suitable for lesions affecting branches of V2 and V3. This technique provides access to tumors in a paramedian location, in the parasellar region, and in Meckel’s cave and lesions with discrete extension into the posterior fossa, the middle fossa, pterygopalatine fossa, and infratemporal fossa [14–16]. Advocators of this approach argue that it requires less retraction on temporal structures or the cerebellum and provides good access to structures in the vicinity of the brainstem. However, risks associated with endoscopic approaches include a higher rate of CSF leak and vascular complications [17].

Stereotactic Radiosurgery A significant evolution in the availability of treatment methods occurred when radiosurgery emerged as an alternative treatment for unresectable lesions or in patients who are poor surgical candidates. Radiosurgery aims to control tumor growth without open surgical access and hence intends to minimize surgery-associated risks and neurological deficits. Small tumors are treated with SRS, and large tumors have been treated successfully with fractionated SRS, resulting in excellent 5- and 10-year tumor control [18]. A recent meta-analysis of 18 studies found that a total of 564 patients after SRS reported an average tumor control rate of 92.3% (range, 90.1– 94.5%) and tumor reduction rates of 62.7% (range, 54.3–71%). Tumor progression rates were 9.4% (range, 6.8–11.9%). The average rate of clinical improvement in trigeminal neuralgia was 63.5% (range, 52.9–74.1%), and that in ocu-

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lomotor nerves was 48.2% (range, 36–60.5%). The clinical worsening rate was 10.7% (range, 7.6–13.8%) [11]. Therefore, SRS for the treatment of TS is associated with high tumor control rates and favorable clinical outcomes, especially for trigeminal neuralgia and oculomotor nerve dysfunction. However, patients should be advised about the risk of tumor progression and potential clinical worsening. When noninvasive SRS treatment is chosen empirically, the indication is based on a clinical and radiological diagnosis alone, which carries the risk of misdiagnosis because the differential diagnoses for tumors in this location include other entities, such as meningiomas, epidermoid cysts, metastasis, chondrosarcomas, chordomas, chondromas, other schwannomas, and maxillary sinus tumors. One needs to consider that such treatment without histopathology can occasionally lead to inadequate initial treatment or delay in obtaining the correct diagnosis. Percutaneous biopsy is rarely employed, but it offers a diagnostic alternative for lesions in Meckel’s cave or lesions along the third division of the trigeminal nerve. The biopsy can be performed using a percutaneous technique similar to percutaneous approaches for trigeminal neuralgia and can be done before any adjuvant treatment (e.g., SRS) [19]. Sindou et al. [20] reported on a cohort of 50 patients who underwent percutaneous biopsy, and pathological diagnosis revealed a sensitivity of 0.83 and specificity of 1 by using this method. Among these 50 patients who underwent biopsy as a result of insufficient imaging features to determine the correct diagnosis, only 3 cases of schwannoma were identified. This finding exemplifies the importance of a minor procedure for avoiding unnecessary open surgery when another nonsurgical pathology is found.

Conclusion/Pearls and Pitfalls • TSs are rare, but they are the most common nonvestibular schwannoma. TSs can differ significantly in their size, shape, and location.

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•

•

•

•

The classification of TSs is helpful in managing treatment options and approaches. Familiarity with a wide variety of skull base approaches is necessary for optimal TS management. Lumbar drains are useful for minimizing brain retraction and facilitating tumor exposure in patients without hydrocephalus. Microsurgery is a definitive treatment for TS; however, radiosurgery is an effective method for local tumor control in patients at high surgical risk. Appropriate management of skull base dural repair is critical for reducing complications of a CSF leak.

References 1. Neves MWF, de Aguiar PHP, Belsuzarri TAB, de Araujo AMS, Paganelli SL, Maldaun MVC.  Microsurgical management of trigeminal schwannoma: cohort analysis and systematic review. J Neurol Surg B. 2019;80:264–9. 2. Jusué-Torres I, Martinez-Gutierrez JC, Elder BD, Olivi A.  Giant trigeminal schwannoma presenting with obstructive hydrocephalus. Cureus. 2015;7:e386. 3. Peker S, Bayrakli F, Kiliç T, Pamir MN. Gamma-knife radiosurgery in the treatment of trigeminal schwannomas. Acta Neurochir. 2007;149:1133–7. 4. Pollack IF, Sekhar LN, Jannetta PJ, Janecka IP.  Neurilemomas of the trigeminal nerve. J Neurosurg. 1989;70:737–45. 5. Jefferson G.  The trigeminal neurinomas with some remarks on malignant invasion of the gasserian ganglion. Clin Neurosurg. 1955;1:11–54. 6. Goel A, Muzumdar D, Raman C.  Trigeminal neuroma: analysis of surgical experience with 73 cases. Neurosurgery. 2003;52:783–90. discussion 790 7. Guthikonda B, Theodosopoulos PV, van Loveren H, Tew JM Jr, Pensak ML.  Evolution in the assessment and management of trigeminal schwannoma. Laryngoscope. 2008;118:195–203. 8. Samii M, Migliori MM, Tatagiba M, Babu R. Surgical treatment of trigeminal schwannomas. J Neurosurg. 1995;82:711–8. 9. Yoshida K, Kawase T. Trigeminal neurinomas extending into multiple fossae: surgical methods and review of the literature. J Neurosurg. 1999;91:202–11. 10. Neff BA, Carlson ML, O’Byrne MM, Van Gompel JJ, Driscoll CLW, Link MJ. Trigeminal neuralgia and neuropathy in large sporadic vestibular schwannomas. J Neurosurg. 2017;127:992–9.

434 11. Peciu-Florianu I, Régis J, Levivier M, Dedeciusova M, Reyns N, Tuleasca C.  Tumor control and trigeminal dysfunction improvement after stereotactic radiosurgery for trigeminal schwannomas: a systematic review and meta-analysis. Neurosurg Rev. 2020;44(5):2391–403. https://doi.org/10.1007/ s10143-­020-­01433-­w. 12. Makarenko S, Ye V, Akagami R. Natural history, multimodal management, and quality of life outcomes of trigeminal schwannomas. J Neurol Surg B Skull Base. 2018;79:586–92. 13. Samii M, Alimohamadi M, Gerganov V. Endoscope-­ assisted retrosigmoid intradural suprameatal approach for surgical treatment of trigeminalschwannomas. Neurosurgery. 2014;10(Suppl4):565–75. discussion 575 14. Raza SM, Donaldson AM, Mehta A, Tsiouris AJ, Anand VK, Schwartz TH.  Surgical management of trigeminal schwannomas: defining the role for endoscopic endonasal approaches. Neurosurg Focus. 2014;37:E17. 15. Park HH, Hong SD, Kim YH, Hong C-K, Woo KI, Yun I-S, Kong D-S.  Endoscopic transorbital and endonasal approach for trigeminal schwannomas: a

W. Huff et al. retrospective multicenter analysis (KOSEN-005). J Neurosurg. 2020;133:467–76. 16. Di Somma A, Langdon C, de Notaris M, Reyes L, Ortiz-Perez S, Alobid I, Enseñat JJ.  Combined and simultaneous endoscopic endonasal and transorbital surgery for a Meckel’s cave schwannoma: technical nuances of a mini-invasive, multiportal approach. Neurosurgery. 2020;134(6):1836–45. 17. Raza SM, Amine MA, Anand V, Schwartz TH.  Endoscopic endonasal resection of trigeminal schwannomas. Neurosurg Clin N Am. 2015;26:473–9. 18. Champ CE, Mishra MV, Shi W, Siglin J, Werner-­ Wasik M, Andrews DW, Evans JJ. Stereotactic radiotherapy for trigeminal schwannomas. Neurosurgery. 2012;71:270–7. discussion 277 19. Janjua RM, Wong KM, Parekh A, van Loveren HR.  Management of the great mimicker: meckel cave tumors. Neurosurgery. 2010;67(2 Suppl Operative):416–21. 20. Sindou M, Messerer M, Alvernia J, Saint-Pierre G. Percutaneous biopsy through the foramen ovale for parasellar lesions: surgical anatomy, method, and indications. Adv Tech Stand Neurosurg. 2012;38:57–73.

Part VI Intracanalicular Vestibular Schwannoma

Middle Fossa Approach for Hearing Preservation

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Nathan D. Cass and Samuel P. Gubbels

Introduction Vestibular schwannoma (VS) surgery was initially fraught with high mortality and morbidity, which is further detailed in Chap. 38. As surgical goals progressed from preservation of life to that of facial function, surgeons naturally turned their attempts toward preservation of (binaural) hearing, the most common collateral damage inflicted by these tumors [1]. Via a variety of mechanisms including binaural redundancy, head shadow effect, and binaural squelch, the benefits of binaural hearing include three-dimensional sound localization and the ability to detect important auditory signals in the presence of background noise [2]. Adults with unilateral hearing loss experience decreased overall quality of life [3]. Hearing preservation represents the final frontier, the “holy grail” of VS surgery: to leave the patient without neurologic deficit save for the unilateral vestibular loss intrinsic to the tumor itself. One major advance in the pursuit of hearing preservation surgery for VS was the development and

N. D. Cass Department of Otolaryngology—Head and Neck Surgery, University of Kentucky, Lexington, KY, USA S. P. Gubbels (*) Department of Otolaryngology, University of Colorado School of Medicine, Aurora, CO, USA e-mail: [email protected]

adaptation of the middle cranial fossa (MCF) approach to the skull base. The MCF approach was pioneered in the 1890s by neurosurgeons Victor Horsley, Frank Hartley, Fedor Krause, Charles Frazier, and Harvey Cushing, for the trigeminal nerve section [4–9]. In 1904, RH Parry performed an MCF approach for vestibular nerve section in a patient with vertigo and tinnitus [10]. Unfortunately, minimal symptomatic improvement was noted, and the facial nerve was severed at the labyrinthine segment during the operation, potentially reducing further appetite for accessing the internal auditory canal (IAC) from that approach. Between 1900 and 1950, the MCF approach was utilized for petrous apicectomy for chronic petrous apicitis, superior canal destruction (for Meniere’s disease), superior canal fenestration (for otosclerosis or chronic otitis media), and for facial nerve grafting in cases of traumatic facial paralysis [11]. In 1961, William House described his work on 14 cases of surgical exposure of the IAC and its contents through the MCF [12]. The approach was initially used in cases of far-advanced otosclerosis, in an attempt to decompress suspected otosclerotic plaques believed to be compressing the cochlear nerve within the IAC. House adapted the MCF approach for use as both a diagnostic and therapeutic measure in patients with vertigo and hearing loss in whom either Meniere’s disease or VS were suspected. After accessing and

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opening the IAC, the tumor, if present, was resected; if no tumor was found, a diagnosis of Meniere’s disease was made, and a vestibular nerve section performed for relief of vertigo. Coexisting imaging modalities (x-ray, myelography) and retrosigmoid (RS) approach for vestibular nerve section were both unable to accurately diagnose small, intracanalicular tumors, and the MCF approach was thus deemed superior for intraoperative diagnosis and decision-making in such cases. In 1963, House presented his first 50 cases of MCF approach performed for a variety of indications: removing otosclerotic plaques, vestibular nerve section, petrous apicectomy, facial nerve decompression, and VS removal [13]. Despite the latter constituting only 10 of the 50 cases, he had sparked renewed interest in the approach worldwide. He had succeeded, in the words of audience commentator and neurosurgeon Norman Dott, in “opening a door to a territory hitherto inaccessible.” Five years later, House published his results on the use of the MCF approach for removal of VS in an attempt to preserve existing hearing [14]. He preserved hearing in four of five (80%) patients with intracanalicular tumors, but only three of fourteen (21.4%) with even minimal extension into the CPA. He also commented on the perceived limitations of the approach: poor access to the posterior fossa for control of bleeding, and increased risk to the facial nerve given its anterosuperior position within the IAC in this superiorly-based approach. He concluded that entirely intracanalicular tumors were ideal for the MCF approach but preferred the TL approach for those tumors with any extension into the CPA.

Hearing Preservation The question of hearing preservation hinges on what constitutes “hearing” and “preservation.” A variety of definitions have been proposed. One of the more restrictive definitions is postoperative hearing within 10–15  dB pure tone average (PTA) and 15%-word recognition score (WRS) compared to preoperative values [15, 16]. In

1988, Gardner and Robertson developed a classification system (GR) for reporting hearing preservation in patients undergoing VS surgery, which involved thresholds in both PTA and WRS [17]. In 1995, the American Academy of Otolaryngology–Head and Neck Surgery (AAO-­ HNS) Committee on Hearing and Equilibrium developed guidelines for the same subject, one of which was a similar classification scale of hearing utilizing PTA and WRS (Table  30.1) [18]. Mario Sanna developed a more granular scale with a greater number of classifications [19]. A more simplified approach to hearing classification is used by the Iowa group, who argue that WRS alone is more indicative of the usefulness and audibility of the ear, regardless of pure tone audiometry results (Table  30.2) [20, 21]. Importantly, neither the GR, AAO, Sanna, or WRS classification scales take preoperative hearing into account, but rather categorize the absolute postoperative hearing levels into classes. As a result of the wide variability within these classes, in 2012 the AAO-HNS advocated for scattergram reporting of individualized results, which does take preoperative hearing level into account as it displays a change in sensitivity in both PTA and WRS [22]. While each patient may have different goals, it is likely that both overall levels of postoperative hearing (and implications for audibility) as well as change from preoperative hearing are important to the patient. This chapter will discuss outcomes in the most widely used 1995 AAO-HNS classification, with some mention made of WRS scoring. In the future, Table 30.1  AAO–HNS Hearing Classification Scale Hearing Classification A B C D

PTA (dB) 0–30 31–50 >50 Any Level

WRS (%) 70–100 50–69 50–69 3 years of follow-up, 84% of whom maintained AAO-A/B hearing; based on their 78 patients and Kaplan-Meier survival probability curves of time to loss of serviceable hearing, they estimated a 72% rate of preservation of AAO-­A/B hearing at 10  years [27]. Hunt et  al. performed a time-based systematic review of long-term hearing outcomes after MCF approach for VS resection: they found that time to reported final hearing outcome was not associated with decreased rates of serviceable hearing, supporting the hypothesis that, when preserved after surgery, hearing seems to be durable over time [37].

 actors Impacting Hearing F Preservation Tumor Size Some authors have found no significant effect of tumor size (usually measured in cm in greatest linear dimension) on hearing preservation [23, 29, 30]. However, other studies have found a clear correlation between tumor size and hearing preservation outcome. Meyer et  al. found that patients with preoperative AAO-A/B hearing maintained that level postoperatively in 66%

with tumor size 1 cm (n = 8) [28]. Kosty et al. (n = 41) similarly reported a preservation of AAO-A/B hearing in 54% of those with tumors