SURGICAL MANAGEMENT OF ASTIGMATISM [1 ed.] 9389188857, 9789389188851

Citation preview

SURGICAL MANAGEMENT OF ASTIGMATISM

SURGICAL MANAGEMENT OF ASTIGMATISM Editors Dimitri T Azar   MD MBA Senior Director, Ophthalmic Innovations Clinical Lead, Ophthalmology Programs Alphabet Verily Life Sciences San Francisco, CA, USA Distinguished University Professor BA Field Chair of Ophthalmological Research and Former Medical School Executive Dean University of Illinois College of Medicine, Chicago, IL, USA

Jorge L Alió  MD PhD FEBOphth Professor and Chairman of Ophthalmology VISSUM Instituto Oftalmológico, Alicante, Spain Division of Ophthalmology Universidad Miguel Hernández, Alicante, Spain

M Soledad Cortina  MD Associate Professor of Ophthalmology, Director Comprehensive Ophthalmology Faculty Practice (COFP) and General Eye Clinic University of Illinois College of Medicine Department of Ophthalmology and Visual Sciences University of Illinois at Chicago, Chicago, IL, USA

Joelle A Hallak  MS PhD Assistant Professor and Executive Director Ophthalmic Clinical Trials and Translational Center Director, Ophthalmic Data Science Laboratory Department of Ophthalmology and Visual Sciences University of Illinois at Chicago, Chicago, IL, USA

JAYPEE BROTHERS MEDICAL PUBLISHERS The Health Sciences Publisher New Delhi | London

Jaypee Brothers Medical Publishers (P) Ltd Headquarters Jaypee Brothers Medical Publishers (P) Ltd 4838/24, Ansari Road, Daryaganj New Delhi 110 002, India Phone: +91­11­43574357 Fax: +91­11­43574314 E­mail: [email protected] Overseas Office JP Medical Ltd 83 Victoria Street, London SW1H 0HW (UK) Phone: +44 20 3170 8910 Fax: +44 (0)20 3008 6180 E­mail: [email protected] Website: www.jaypeebrothers.com Website: www.jaypeedigital.com © 2020, Jaypee Brothers Medical Publishers The views and opinions expressed in this book are solely those of the original contributor(s)/author(s) and do not necessarily represent those of editor(s) of the book. All rights reserved. No part of this publication may be reproduced, stored or transmitted in any form or by any means, electronic, mechanical, photo­ copying, recording or otherwise, without the prior permission in writing of the publishers. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The publisher is not associated with any product or vendor mentioned in this book. Medical knowledge and practice change constantly. This book is designed to provide accurate, authoritative information about the subject matter in question. However, readers are advised to check the most current information available on procedures included and check information from the manufacturer of each product to be administered, to verify the recommended dose, formula, method and duration of administration, adverse effects and contraindications. It is the responsibility of the practitioner to take all appropriate safety precautions. Neither the publisher nor the author(s)/editor(s) assume any liability for any injury and/or damage to persons or property arising from or related to use of material in this book. This book is sold on the understanding that the publisher is not engaged in providing professional medical services. If such advice or services are required, the services of a competent medical professional should be sought. Every effort has been made where necessary to contact holders of copyright to obtain permission to reproduce copyright material. If any have been inadvertently overlooked, the publisher will be pleased to make the necessary arrangements at the first opportunity. The CD/DVD-ROM (if any) provided in the sealed envelope with this book is complimentary and free of cost. Not meant for sale.

Inquiries for bulk sales may be solicited at: [email protected] Surgical Management of Astigmatism First Edition: 2020 ISBN: 978­93­89188­85­1

Dedicated to Our families. Without their constant support, this book would not have been possible.

Contributors Afua Annor MD Ophthalmologist University of California Davis Eye Center Sacramento, CA, USA

Alanna S Nattis  DO FAAO Ophthalmologist Department of Cornea and Refractive Surgery SightMD, NY, USA

Alexander Pleet MD Glaucoma Fellow Department of Ophthalmology Feinberg School of Medicine Northwestern University Chicago, IL, USA

Alfredo Vega-Estrada  MD PhD Ophthalmologist Department of Cornea and Refractive Surgery Vissum/Instituto Oftalmológico de Alicante, Spain

Almutez M Gharaibeh MD Associate Professor of Ophthalmology Faculty of Medicine The University of Jordan Amman, Jordan

Andrea Arteaga  MD Ophthalmology Resident Department of Ophthalmology and Visual Sciences University of Illinois Chicago, IL, USA

Andrzej Grzybowski  MD PhD MBA Professor Department of Ophthalmology Chair of Ophthalmology University of Warmia and Mazury Olsztyn, Poland Head, Institute for Research in Ophthalmology Poznan, Poland

Arthur Cummings  FRCS MD

Daniel G Fernández-Pacheco  PhD

Medical Director Wellington Eye Clinic Consultant Ophthalmologist Head, Department of Ophthalmology Beacon Hospital, Dublin, Ireland

Faculty Department of Graphical Expression Technical University of Cartagena Cartagena, Spain

Asier Villanueva MSc

Senior Director, Ophthalmic Innovations Clinical Lead, Ophthalmology Programs Alphabet Verily Life Sciences San Francisco, CA, USA Distinguished University Professor BA Field Chair of Ophthalmological Research and Former Medical School Executive Dean University of Illinois College of Medicine Chicago, IL, USA

Researcher Department of Optical and Visual Sciences University of Alicante, Alicante, Spain

Brent A Kramer MD Ophthalmology Resident Department of Ophthalmology University of North Carolina at Chapel Hill Chapel Hill, NC, USA

Bryan S Lee  MD JD

Dimitri T Azar  MD MBA

Eric D Donnenfeld MD

Ophthalmologist Department of Cornea and Refractive Surgery Altos Eye Physicians Los Altos, CA, USA

Ophthalmologist Long Island Cornea, LASIK and Cataract Specialist Department of Ophthalmology Ophthalmic Consultants of Long Island Garden City, NY, USA

Damien Gatinel  MD PhD

Eric D Rosenberg  DO MSE

Chief of Service Department of Anterior Segment and Refractive Surgery Rothschild Foundation Paris, France

David F Chang  MD Clinical Professor University of California San Francisco San Francisco, CA, USA

David R Hardten  MD FACS Ophthalmologist and Founding Partner Minnesota Eye Consultants Minnesota, USA

Debora E Garcia-Zalisnak  MD Cornea, Refractive Surgery and External Disease Specialist Clear Vision Ophthalmology Tucson, Arizona, USA

Ophthalmologist Valhalla, NY, USA

F Cavas Martínez  PhD Faculty Department of Graphical Expression Technical University of Cartagena Cartagena, Spain

FJ Fernández Cañavate  PhD Faculty Department of Graphical Expression Technical University of Cartagena Cartagena, Spain

George Stamatelatos  BScOptom Clinical Director ASSORT Senior Optometrist New Vision Clinics Melbourne, Australia

viii Surgical Management of Astigmatism Gunther Grabner MD

Jorge L Alió del Barrio  MD PhD

Noel Alpins  AM FRANZCO FRCOphth FACS

Professor Department of Ophthalmology Paracelsus Medical University Salzburg, Austria

Surgeon, Cornea, Cataract, and Refractive Surgery Division of Ophthalmology Universidad Miguel Hernández Alicante, Spain

Clinical Professor and Medical Director Newvision Clinics CEO of ASSORT Department of Ophthalmology University of Melbourne Melbourne, Australia

Joao Crispim  MD Ophthalmologist Department of Ophthalmology and Visual Sciences Federal University of São Paulo São Paulo, Brazil

Joelle A Hallak  MS PhD

Jose de la Cruz  MD Assistant Professor Department of Ophthalmology and Visual Sciences University of Illinois at Chicago Chicago, IL, USA

Assistant Professor and Executive Director Ophthalmic Clinical Trials and Translational Center Director Ophthalmic Data Science Laboratory Department of Ophthalmology and Visual Sciences University of Illinois at Chicago Chicago, IL, USA

Kai Kang  MD

John SM Chang  MD

Leon Strauss  MD PhD

Director, Guy Hugh Chan Refractive Surgery Centre Hong Kong Sanatorium and Hospital Happy Valley, Hong Kong

John P Berdahl  MD Ophthalmologist Surgeon Vance Thompson Vision Sioux Falls, SD, USA

Jonathan Rubenstein  MD Professor Department of Ophthalmology Rush University Medical Center Chicago, IL, USA

Jorge L Alió  MD PhD FEBOphth Professor and Chairman of Ophthalmology VISSUM Instituto Oftalmológico Alicante, Spain Division of Ophthalmology Universidad Miguel Hernández Alicante, Spain

Piotr Kanclerz  MD PhD Ophthalmologist Department of Ophthalmology Medical University of Gdansk Gdansk, Poland

Assistant Professor Department of Ophthalmology and Visual Sciences University of Illinois at Chicago Chicago, IL, USA

Praneetha Thulasi MD

Laurent Bataille MSc

Renan Ferreira Oliveira MD

Director Research and Development VISSUM Instituto Oftalmológico Alicante, Spain Instructor Department of Ophthalmology The Wilmer Eye Institute The Johns Hopkins University School of Medicine Baltimore, MD, USA

M Soledad Cortina MD Associate Professor of Ophthalmology Director Comprehensive Ophthalmology Faculty Practice (COFP) and General Eye Clinic University of Illinois College of Medicine Department of Ophthalmology and Visual Sciences University of Illinois at Chicago Chicago, IL, USA

Mauro C Tiveron Jr MD ICO Fellow Department of Cornea, Cataract and Refractive Surgery Massachusetts Eye and Ear Infirmary Harvard Medical School Boston, MA, USA

Assistant Professor Department of Ophthalmology Emory Eye Center Atlanta, GA, USA

Ophthalmologist, Cataract and Refractive Surgeon Sadalla Amin Ghanem Eye Hospital Joinville, SC, Brazil

Ricardo M Nosé MD Research Fellow Department of Cornea, Cataract and Refractive Surgery Massachusetts Eye and Ear Infirmary Harvard Medical School Boston, MA, USA

Roberto Fernández-Buenaga  MD PhD Consultant Ophthalmologist Department of Cornea, Cataract and Refractive Surgery Vissum Corporación Madrid, Spain

Roberto Pineda II MD Associate Professor Department of Ophthalmology Director Department of Refractive Surgery Massachusetts Eye and Ear Infirmary Harvard Medical School Boston, MA, USA

Contributors

Samantha Williamson MD

Suphi Taneri MD

Cornea and External Disease Specialist Department of Ophthalmology Kaiser Permanente Mid-Atlantic Medical Group Baltimore, MD, USA

Director Center for Refractive Surgery Owner, Eye Department St Franziskus Hospital, Munster Lecturer, Ruhr University Bochum, Germany

Shilpa Gulati MD Retina Fellow Tufts New England Eye Center Ophthalmic Consultants of Boston Boston, MA, USA

Susie Drake MD Cornea and External Disease Specialist Center for Eye Care Watertown Regional Medical Center Watertown, WI, USA

Tomoaki Nakamura MD Ophthalmologist Department of Refractive Surgery Nagoya Eye Clinic Namiyose Atsuta-ward Nagoya, Japan

Verónica Vargas MD Refractive Surgery Fellow Department of Investigation, Development and Innovation Vissum Alicante, Alicante, Spain

ix

Preface Astigmatism is one of the most frequent problems encountered in the surgical management of cataract, cornea, and refractive conditions. In essence, the correction of astigmatism is part of the success of any anterior segment surgical procedure. Regardless of its types, astigmatism can be difficult to manage and may complicate surgical outcomes. In contrast to myopia, there is no single focal point in astigmatism, which poses a challenge to the modern trends in refractive and intraocular lens surgery. The modern trend is to obtain a unique quality of retinal images, not distorted by astigmatism. This is why in this book we present various solutions to the different types of astigmatism that the practical surgeon will find in today’s world of refractive and cataract-lens surgical practices. The different chapters of this book are based on scientific evidence and practice. Well-known authors, with extensive practice, offer what they consider to be pearls of their knowledge to optimize and improve surgical outcomes for the correction of astigmatism with cornea or intraocular procedures. We would like to thank all the authors and contributors of the book for sharing their knowledge and expertise through comprehensive chapters. They have worked tirelessly on their chapters. Without their consistent efforts and dedication, the book would lack the essence that we want to transmit to the readers, which is how to manage astigmatism from the diagnostic stages, instrumental examinations, surgical plannings, surgical applications, and, last but not least, analysis of outcomes. Dimitri T Azar Jorge L Alió M Soledad Cortina Joelle A Hallak

Acknowledgments We would like to acknowledge and thank our students and mentors as well as our patients for trusting us in their care, and for helping us in advancing the management of astigmatism. Lastly, we would like to thank Shri Jitendar P Vij (Group Chairman), Mr Ankit Vij (Managing Director), Mr MS Mani (Group President), Ms Chetna Malhotra Vohra (Associate Director—Content Strategy), Ms Pooja Bhandari (Production Head) and Ms Prerna Bajaj (Development Editor) of M/s Jaypee Brothers Medical Publishers (P) Ltd, New Delhi, India, for giving a go-ahead at the very beginning and helping us in every way possible to bring out this book.

Contents 1. Etiology, Pathogenesis, and Epidemiology of Astigmatism and History of its Surgical Management

1

Andrzej Grzybowski, Piotr Kanclerz

2. Optics of Regular Astigmatism

12

Leon Strauss

3. Optical Clinical Assessment of Astigmatism, Surgical Vector Planning, and Analysis of Astigmatism for Refractive Surgery

22

Noel Alpins, George Stamatelatos

4. Keratometry and Placido‑based Topography: Techniques, Advantages, and Limitations

25

Damien Gatinel

5. Scanning-slit, Scheimpflug, and Optical Coherence Tomography in Regular and Irregular Astigmatism

36

Ricardo M Nosé, Mauro C Tiveron Jr, Roberto Pineda II

6. Morphogeometric Analysis of the Cornea and its Relation to Corneal Astigmatism

53

F Cavas Martínez, Asier Villanueva, Laurent Bataille, FJ Fernández Cañavate, DG Fernández-Pacheco

7. Corneal and Total Aberrometry: Orthogonal/Oblique and Secondary Higher‑order Aberrations in Astigmatism

60

Andrea Arteaga, Jose de la Cruz

8. Aberrated Cornea–Irregular Astigmatism in Real-world Optics: Topography, Aberrometry, and Applications in Refractive Surgery

66

Samantha Williamson, Joao Crispim, Dimitri Azar

9. Preoperative Evaluation of Astigmatism in Cataract Surgery

79

Debora E Garcia-Zalisnak, Joelle A Hallak, Soledad Cortina

10. Surgical Planning for the Correction of Astigmatism

85

Noel Alpins, George Stamatelatos

11. Posterior Corneal Astigmatism Measurement Techniques and Preoperative Planning for Cataract Surgery Kai Kang, Jose de la Cruz, Joelle A Hallak, Soledad Cortina, Dimitri Azar

91

xvi Surgical Management of Astigmatism 12. Toric Pseudophakic Intraocular Lenses: Materials/Characteristics of Toric Monofocal, Toric Bifocal, and Toric Trifocal Intraocular Lenses

99

Roberto Fernández-Buenaga, Jorge L Alió

13. Surgical Techniques: Axis Marking, Toric Intraocular Lens Positioning, and Incisional Approaches

117

Bryan S Lee, David F Chang

14. Intraoperative Alignment Techniques

128

Afua Annor, Jonathan Rubenstein

15. Induced and Residual Astigmatism after Cataract Surgery

136

Alexander Pleet, Soledad Cortina, Dimitri Azar

16. Correcting Astigmatic Surprises Following Toric Intraocular Lens Implantation

154

David R Hardten, John P Berdahl, Brent A Kramer

17. Cataract and Refractive Lens Exchange Surgery in Keratoconus

164

Jorge L Alió del Barrio, Roberto Fernández-Buenaga, Jorge L Alió

18. Astigmatic Keratotomy and Limbal Relaxing Incisions: Principles, Indications, and Nomograms

173

Eric D Rosenberg, Alanna S Nattis, Eric D Donnenfeld

19. Femtosecond Arcuate Keratotomy: Principles, Indications, and Techniques

180

Gunther Grabner

20. Laser Profiles and Depths for Myopic, Hyperopic, and Mixed Astigmatism

186

Shilpa Gulati, Joelle A Hallak, Dimitri Azar

21. Photoastigmatic Refractive Keratectomy and Laser-assisted in Situ Keratomileusis: Surgical Management of Astigmatism

195

Susie Drake, Soledad Cortina, Dimitri Azar, Joelle A Hallak

22. Small incision Lenticule Extraction versus Laser in Situ Keratomileusis in  Myopic Astigmatism

213

Suphi Taneri

23. Outcomes of Photorefractive Keratectomy, Laser in Situ Keratomileusis, and Smallincision Lenticule Extraction for Myopic, Mixed, and Hyperopic Astigmatism

221

Jorge L Alió del Barrio, Verónica Vargas, Roberto Fernández-Buenaga, Jorge L Alió

24. Custom Treatment of Irregular Astigmatism Arthur Cummings

231

Contents

25. Management of Postkeratoplasty Astigmatism

240

Praneetha Thulasi, Joelle A Hallak, Dimitri Azar, Soledad Cortina

26. Management of Astigmatism after Cataract Surgery: Excimer Laser, Astigmatic Keratotomy, Piggyback IOL and IOL Exchange

249

Renan Ferreira Oliveira, Roberto Fernández-Buenaga, Jorge L Alió del Barrio, Jorge L Alió

27. Surgical Management of Corneal Ectatic Disorders Using Intracorneal Ring Segments, Corneal Collagen Cross-linking, and Thermokeratoplasty Procedures 262 Alfredo Vega-Estrada, Renan Ferreira Oliveira, Jorge L Alio

28. Topography-guided Surgical Correction of Astigmatism: Indications, Techniques, Limitations, and Complications

271

John SM Chang

29. Toric Implantable Collamer Lenses for the Correction of Astigmatism

283

Tomoaki Nakamura

30. Iris Clip Toric Phakic Intraocular Lenses for Keratoconus and Irregular Astigmatism

296

Almutez M Gharaibeh, Jorge L Alió

Index 309

xvii

CHAPTER 1 Etiology, Pathogenesis, and Epidemiology of Astigmatism and History of its Surgical Management Andrzej Grzybowski, Piotr Kanclerz

CORE MESSAGES ll

ll

ll

ll

ll

This chapter covers the etiology, pathogenesis, and epidemiology of astigmatism, as well as the history of surgical treatment. The lack of standardized methods for populationbased studies on astigmatism make comparisons challenging. Visual impairment caused by astigmatism ranges from 2.2 to 34%. The 19th century contributed greatly to our under­ standing and management of astigmatism, with the first attempts to alter the refraction in astigmatic patients by changing the shape of the cornea made in the final two decades of the 19th century. The occurrence of acquired astigmatism is an issue in several ophthalmic procedures.

EPIDEMIOLOGY According to the World Health Organization, in 2004, a total of 153 million people (range of uncertainty: 123–184 million) were estimated to have visual impairment due to uncorrected refractive errors, of whom 8 million were blind.1 Other sources report that uncorrected refractive errors and cataract are the principal causes of visual impairment globally (43 and 33%, respectively).2-4 Naidoo et al. put focus on the increase in vision impairment due to uncorrected refractive errors: 15% in years 1990–2010.5 Visual impairment caused by astigmatism ranges from 2.2 to 34% depending on the region and the urban/rural

setting.1 This gives an estimate of 2.7 up to 62.5 million people with vision loss as a result of this disorder.2-5 In some studies, the prevalence of astigmatism in people aged over 50 years reaches 77%.6,7 Assessment of the prevalence of astigmatism encounters a number of technical problems. Compa­ risons between studies might be problematic, as no standardization has been introduced. Refractive error measurements in population-based studies are usually noncycloplegic. Commonly, stationary autorefractors are used, however, home evaluation with handheld autorefractors was employed in single studies. In children, diagnostics of the refractive error are determined by objective refraction under cycloplegia and subjective refraction. In infants and preschool children, most measurements are obtained by retinoscopy. Astigmatism is usually defined as a refractive error of ≥1.00 cylindrical diopter, less commonly as ≥0.75 cylindrical diopter.8 The axis of the cylinder is classified as with-the-rule if the minus cylinder axis is 0 ± 15°,8,9 against-the-rule if the axis is 90 ± 15°, and oblique astigmatism when the axis does not comply the preceding criteria. In some studies, the range was 45° (0 ± 22.5° for with-the-rule, 90 ± 22.5° for against-the-rule).10 Moreover, a range of 40° or 60° is used in some papers.11 Several studies lacked information regarding the preceding criteria.12,13 Furthermore, there is a deficiency of cross-sectional reviews. Several studies relate to a specific age group or are related to very limited geographical areas.

2

Surgical Management of Astigmatism

Major Cross-sectional Studies The European Eye Epidemiology Consortium conducted a meta-analysis of data from 33 adult cohort and cross-sectional studies. Refractive data were collected between 1990 and 2013, mostly from Northern and Western Europe. Fifteen population-based studies met the eligibility criteria. In patients aged over 15 years, the prevalence of astigmatism was 27.3% with an agestandardized prevalence of 23.9%.12 Astigmatism rates were fairly constant in the analyzed studies (15–25%), but were higher after the age of 65. As data on ethnicity were available for only 50% of participants with minimal ethnic diversity, analysis of ethnicity was not carried out. Across all age groups the prevalence of astigmatism was higher in men than in women, and this difference was particularly pronounced in middle aged individuals. As most of the studies were conducted in Northern and Western Europe, no conclusions regarding predominance in particular regions could be drawn.11 The prevalence of astigmatism in the United States was evaluated in the National Health and Nutrition Examination Survey on 14,213 adult participants, which were examined in the years 1999–2004.13 The reported rates varied by age; for age groups of 20–39, 40–59, and ≥60 years, prevalence estimates were 23.1, 27.6, and 50.1%, respectively. The prevalence of astigmatism varied slightly by race–ethnicity category. In those aged 60 years and older, astigmatism was more prevalent among males (54.9%) than among females (46.1%). The Blue Mountains Eye Study was a populationbased assessment of visual impairment of the representative older Australian community sample. The examinations were performed in years 1992–1994.8 The prevalence of astigmatism (equal or higher than 0.75 cylindrical diopter) was 37%, and high cylinder values (higher than 1.5 D) were reported in 13% of eyes. The gender-adjusted mean cylinder increased with age: for ages 49–59, 60–69, 70–79, and 80–97 years, it was −0.6 D, −0.7 D, −1.0 D, and −1.2 D, respectively. There was no statistically significant difference in the mean cylindrical error between men and women. Furthermore, a trend of increased cylindrical error with the spherical error (either myopic or hyperopic) was found. There was no difference in the axis of astigmatism by gender or age group.

Most of the aforementioned population-based studies on refractive errors correspond to European or Europeanderived populations. Some large population-based studies in Asian populations have been conducted in Singapore, Nepal, Hong Kong, China, and Malaysia.14 The Singapore Epidemiology of Eye Disease Study reported the prevalence of astigmatism as 58.8% in a multiethnic cohort aged 40 years and over.15 The prevalence of astigmatism increased with increasing age, while no sex difference was found. The prevalence of astigmatism in the Chinese Singaporeans was significantly higher in all age groups compared with that in Singaporean Indians. Furthermore, the authors observed that the prevalence of astigmatism increased significantly during the 12 years of the study. The major increase in the astigmatism rate was due to the increase in the myopic astigmatism rate, which accounted for 72% of the observed increase in the rate of astigmatism, while hyperopic astigmatism accounted only for 16%. Therefore, the authors postulated that the increase in myopic astigmatism was due to the rise in environmental factors such as a more competitive education environment. The Baltimore Eye Study was a population-based survey of ocular disorders among patients aged 40 years and older living in East Baltimore, Maryland, United States. It was conducted in the years 1985–1988. The rates of astigmatism were higher for White people than for Black people at all ages. In this study, astigmatism increased with age in men and was more prevalent in men than in women.16 These results were similar to the findings from the European Eye Epidemiology Consortium. Most of the studies found that the rates of astigmatism are higher in men or revealed no differences between genders. However, some reports from China and Bangladesh noted a higher prevalence of astigmatism in women.17-19 A correlation between astigmatism and cataract formation has been found, however, the mechanism by which cataract induces astigmatism is not always clear. It is argued that mainly cortical cataract can lead to astigmatic shift and it might be caused by asymmetrical refractive index changes within the parts of the cortex of the lens.20 Moreover, an association between nuclear cataract and astigmatism was described.19 Other epidemiological risk factors for astigmatism include lower educational background and living in an urban population.11,17,19

Etiology, Pathogenesis, and Epidemiology of Astigmatism and History of its Surgical Management

Relationship between Age and Astigmatism Nearly half of 0- to 6-month-old infants have significant astigmatism of one or more diopters. The prevalence found in the first year of life is greatly reduced by the age of 4, reaching