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94570764 diagnostic-procedures-in-ophthalmology-full-colour(1)

  1. 1. Diagnostic Procedures in OPHTHALMOLOGY
  2. 2. Diagnostic Procedures in OPHTHALMOLOGY SECOND EDITION ® JAYPEE BROTHERS MEDICAL PUBLISHERS (P) LTD New Delhi • Ahmedabad • Bengaluru • Chennai • Hyderabad Kochi • Kolkata • Lucknow • Mumbai • Nagpur • St Louis (USA) HV Nema Former Professor and Head Department of Ophthalmology Institute of Medical Sciences Banaras Hindu University Varanasi, Uttar Pradesh, India Nitin Nema MS Dip NB Assistant Professor Department of Ophthalmology Sri Aurobindo Institute of Medical Sciences Indore, Madhya Pradesh, India
  3. 3. Published by Jitendar P Vij Jaypee Brothers Medical Publishers (P) Ltd Corporate Office 4838/24 Ansari Road, Daryaganj, New Delhi - 110 002, India, +91-11-43574357 (30 lines) Registered Office B-3 EMCA House, 23/23B Ansari Road, Daryaganj, New Delhi 110 002, India Phones: +91-11-23272143, +91-11-23272703, +91-11-23282021, +91-11-23245672, Rel: +91-11-32558559 Fax: +91-11-23276490, +91-11-23245683 e-mail: jaypee@jaypeebrothers.com, Website: www.jaypeebrothers.com Branches • 2/B, Akruti Society, Jodhpur Gam Road Satellite Ahmedabad 380 015 Phones: +91-79-26926233, Rel: +91-79-32988717 Fax: +91-79-26927094 e-mail: ahmedabad@jaypeebrothers.com • 202 Batavia Chambers, 8 Kumara Krupa Road, Kumara Park East Bengaluru 560 001 Phones: +91-80-22285971, +91-80-22382956, +91-80-22372664 Rel: +91-80-32714073, Fax: +91-80-22281761 e-mail: bangalore@jaypeebrothers.com • 282 IIIrd Floor, Khaleel Shirazi Estate, Fountain Plaza, Pantheon Road Chennai 600 008 Phones: +91-44-28193265, +91-44-28194897, Rel: +91-44-32972089 Fax: +91-44-28193231 e-mail: chennai@jaypeebrothers.com • 4-2-1067/1-3, 1st Floor, Balaji Building, Ramkote Cross Road Hyderabad 500 095 Phones: +91-40-66610020, +91-40-24758498, Rel:+91-40-32940929 Fax:+91-40-24758499 e-mail: hyderabad@jaypeebrothers.com • No. 41/3098, B & B1, Kuruvi Building, St. Vincent Road Kochi 682 018, Kerala Phones: +91-484-4036109, +91-484-2395739, +91-484-2395740 e-mail: kochi@jaypeebrothers.com • 1-A Indian Mirror Street, Wellington Square Kolkata 700 013 Phones: +91-33-22651926, +91-33-22276404, +91-33-22276415 Rel: +91-33-32901926, Fax: +91-33-22656075, e-mail: kolkata@jaypeebrothers.com • Lekhraj Market III, B-2, Sector-4, Faizabad Road, Indira Nagar Lucknow 226 016 Phones: +91-522-3040553, +91-522-3040554 e-mail: lucknow@jaypeebrothers.com • 106 Amit Industrial Estate, 61 Dr SS Rao Road, Near MGM Hospital, Parel Mumbai 400012 Phones: +91-22-24124863, +91-22-24104532, Rel: +91-22-32926896 Fax: +91-22-24160828 e-mail: mumbai@jaypeebrothers.com • “KAMALPUSHPA” 38, Reshimbag, Opp. Mohota Science College, Umred Road Nagpur 440 009 (MS) Phone: Rel: +91-712-3245220, Fax: +91-712-2704275 e-mail: nagpur@jaypeebrothers.com USA Office 1745, Pheasant Run Drive, Maryland Heights (Missouri), MO 63043, USA, Ph: 001-636-6279734 e-mail: jaypee@jaypeebrothers.com, anjulav@jaypeebrothers.com DiagnosticProceduresinOphthalmology © 2009, HV Nema, Nitin Nema All rights reserved. No part of this publication should be reproduced, stored in a retrieval system, or transmitted in any form or by any means: electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the editors and the publisher. This book has been published in good faith that the material provided by contributors is original. Every effort is made to ensure accuracy of material, but the publisher, printer and editors will not be held responsible for any inadvertent error(s). In case of any dispute, all legal matters to be settled under Delhi jurisdiction only. First Edition: 2002 Second Edition: 2009 ISBN 978-81-8448-595-0 Typeset at JPBMP typesetting unit Printed at Replika Press
  4. 4. Contributors Jorge L Alió MD, PhD Director, Vissum Institute of Ophthalmology of Alicante Alicante, Spain Sonal Ambatkar DNB Glaucoma Service Aravind Eye Hospital Tirunelveli, Tamil Nadu, India Francisco Arnalich MD Vissum Institute of Ophthalmology of Alicante Alicante, Spain Sreedharan Athmanathan MD, DNB Virologist LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India Mandeep S Bajaj MD Professor Dr RP Centre for Ophthalmic Sciences AIIMS, New Delhi, India Tinku Bali MS Consultant Department of Ophthalmology Sir Ganga Ram Hospital, New Delhi, India Rituraj Baruah MS Senior Registrar Lady Hardinge Medical College New Delhi, India Jyotirmay Biswas MS, FAMS Head, Ocular, Pathology and Uveitis Sankara Nethralaya, Chennai Tamil Nadu, India Ambar Chakravarty MS, FRCP Honorary Professor and Head Department of Neurology Vivekananda Institute of Medical Sciences Kolkata, West Bengal, India Surbhit Chaudhary MS Ex-Fellow Sankara Nethralaya Chennai, Tamil Nadu, India Taraprasad Das MS Director LV Prasad Eye Institute Bhubaneswar, Orissa, India Munish Dhawan MD Dr RP Centre for Ophthalmic Sciences AIIMS, New Delhi, India Lingam Gopal MS, FRCS Chairman Medical Research Foundation Sankara Nethralaya, Chennai Tamil Nadu, India AK Grover MD, FRCS Chairman Department of Ophthalmology Sir Ganga Ram Hospital New Delhi, India Roshmi Gupta MD Consultant, Narayana Nethralaya Bengaluru, Karnataka, India Sanjiv Gupta MD Dr RP Centre for Ophthalmic Sciences AIIMS, New Delhi, India Stephen C Hilton OD West Virginia University Morgantown, USA Santosh G Honavar MD, FACS Director Department of Ophthalmic Plastic Surgery and Ocular Oncology, LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India
  5. 5. Anjali Hussain MS Consultant LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India Subhadra Jalali MS Head Smt Kanuri Santhamma Retina-Vitreous Centre LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India Sadao Kanagami FOPS Professor Kitasato University School of Medicine Teikyo, Japan Sangmitra Kanungo MD, FRCS Consultant LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India Shahnawaz Kazi MS Fellow Sankara Nethralaya Chennai, Tamil Nadu, India R Kim DO Head Retina-Vitreous Service Aravind Eye Hospital and Postgraduate Institute of Ophthalmology Madurai, Tamil Nadu, India Parmod Kumar OD Glaucoma Imaging Centre New Delhi, India S Manoj MS Consultant Retina-Vitreous Service Aravind Eye Hospital and Postgraduate Institute of Ophthalmology, Madurai, Tamil Nadu, India S Meenakshi MS Consultant Pediatric Ophthalmology Sankara Nethralaya Chennai, Tamil Nadu, India Amit Nagpal MS Consultant Sankara Nethralaya, Chennai Tamil Nadu, India A Narayanaswamy Consultant Sankara Nethralaya Chennai, Tamil Nadu, India Rajiv Nath MS Professor Department of Ophthalmology KG Medical University Lucknow, Uttar Pradesh, India Tomohiro Otani MD Professor Department of Ophthalmology Gunma University School of Medicine Maebashi, Japan Nikhil Pal MD Senior Resident Dr RP Centre for Ophthalmic Sciences AIIMS, New Delhi, India Rajul Parikh MS Consultant, Sankara Nethralaya Chennai, Tamil Nadu, India David Piñero OD Vissum Institute of Ophthalmology of Alicante Alicante, Spain K Kalyani Prasad MS Consultant Krishna Institute of Medical Sciences Hyderabad, Andhra Pradesh, India Leela V Raju MD Monongalia Eye Clinic Morgantown, USA VK Raju MD, FRCS, FACS Clinical Professor Department of Ophthalmology West Virginia University Morgantown, USA LS Mohan Ram D Opt, BS LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India viii Diagnostic Procedures in Ophthalmology
  6. 6. R Ramakrishnan MS Professor and CMO Aravind Eye Hospital Tirunelveli, Tamil Nadu, India Manotosh Ray MD, FRCS Associate Consultant National University Hospital Singapore Pukhraj Rishi MD Consultant Sankara Nethralaya Chennai, Tamil Nadu, India Monica Saha MBBS Department of Ophthalmology KG Medical University Lucknow, Uttar Pradesh, India Chandra Sekhar MD Director LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India Harinder Singh Sethi MD, DNB, FRCS Senior Research Associate Dr RP Centre for Ophthalmic Sciences AIIMS, New Delhi, India Pradeep Sharma Professor Dr RP Centre for Medical Sciences AIIMS, New Delhi, India Rajani Sharma MD (Ped) Senior Resident Department of Pediatrics AIIMS, New Delhi, India Savitri Sharma MD Head Jhaveri Microbiological Centre LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India Tarun Sharma MD, FRCS Director Retina Service, Sankara Nethralaya Chennai, Tamil Nadu, India Yog Raj Sharma MD Professor Dr RP Centre for Ophthalmic Sciences AIIMS, New Delhi, India Deependra Vikram Singh MD Senior Resident Dr RP Centre for Ophthalmic Sciences, AIIMS New Delhi, India Devindra Sood MD Consultant, Glaucoma Imaging Centre New Delhi, India MS Sridhar MD Consultant LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India S Sudharshan MS Fellow Sankara Nethralaya Chennai, Tamil Nadu, India Kallakuri Sumasri B Optm Retina-Vitreous Centre LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India T Surendran MS, M Phil Vice Chairman and Director Pediatric Ophthalmology Sankara Nethralaya Chennai, Tamil Nadu, India Garima Tyagi B Opt Retina-Vitreous Centre LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India Vasumathy Vedantham MS, DNB, FRCS Consultant, Retina-Vitreous Service Aravind Eye Hospital and Postgraduate Institute of Ophthalmology Madurai, Tamil Nadu, India L Vijaya MS Head Glaucoma, Sankara Nethralaya Chennai, Tamil Nadu, India ixContributors
  7. 7. Preface to the Second Edition The goal of this second edition of Diagnostic Procedures in Ophthalmology remains the same as that of the first—to provide the practicing ophthalmologists with a concise and comprehensive text on common diagnostic procedures which help in the correct and speedy diagnosis of eye diseases. Like other disciplines of medicine, the knowledge of ophthalmology continues to expand and anumberofnewerandsophisticatedinvestigativeprocedureshavebeenintroducedrecently.Extensive and detailed information on recent diagnostic approaches is available in resource textbooks or online to ophthalmologists. To search these is time consuming, tiring and at times not practical in a busy clinical practice set-up. Therefore, this ready reckoner has been conceptualized. The book covers most of the basic and well-established diagnostic procedures in ophthalmology. It starts with visual acuity and describes color vision and color blindness, slit-lamp examination, tonometry, gonioscopy, evaluation of optic nerve head in glaucoma, perimetry, ophthalmoscopy and ophthalmic photography. Most of these procedures are considered basic and carried out routinely but to obtain an evidence-based diagnosis, a correct procedure for the examination must be followed. Corneal topography is very useful in detection of corneal pathologies such as early keratoconus, pellucid marginal corneal degeneration, corneal dystrophies, etc. It guides the ophthalmic surgeon to plan appropriate refractive surgery. Recent development in the application of wavefront technology can reduce different types of optical aberrations and may provide supervision and improve results of the LASIK surgery. A new chapter on Confocal Microscopy is included. Confocal microscopy, a noninvasive procedure, allows in vivo observation of normal and pathogenic corneal microstructure at a cellular level. It can identify subclinical corneal abnormalities. Procedures like Fundus Fluorescein Angiography and Indocyanine Green Angiography are invaluable diagnostic tools. They are not only useful in the diagnosis, documentation and follow- upbutalsoinmonitoringthemanagementoftheposteriorsegmenteyediseases.Withthedevelopment of high quality fundus camera and digital imaging, utility of both techniques has significantly increased. Ultrasonography, as a diagnostic procedure, has immense importance in the modern ophthalmology. Both A-scan and B-scan ultrasonography are dynamic procedures wherein diagnosis is made during examination in correlation with clinical features. Three-dimensional ultrasound tomography allows improved visualization and detection of small ophthalmic lesions. Ultrasound biomicroscopy is a method of high frequency ultrasound imaging used for evaluating the structural abnormality and pathology of the anterior segment of the eye both qualitatively and quantitatively. It is very helpful in understanding the pathomechanism of various types of glaucoma.
  8. 8. OpticalCoherenceTomographyisanoninvasive,cross-sectionalimagingtechniquewhichprovides objective and quantitative measurements that are reproducible and show very good correlation with clinical picture of retinal pathology especially macula. Presently, OCT is often used in assessment of optic nerve head damage in glaucoma. One must remember that imaging technique alone may not contribute to a correct diagnosis. It is complementary to clinical examination. Therefore, results of imaging should always be interpreted in conjunction with clinical findings and results of other relevant tests. Electrophysiological tests are often ordered to assess the functional integrity of the visual pathway and in evaluating the cause of visual impairment in children. Multifocal ERG and multifocal VEP are newer techniques still under evaluation. It is claimed that multifocal ERG can distinguish between the lesions of the outer retina and the ganglion cells or optic nerve. Results of electrophysiological tests should never be analyzed in isolation but always be correlated with clinical findings to establish a definitive diagnosis. Etiological diagnosis of infectious keratitis and uveitis has been more vexing and often fraught with pitfalls. Collection of samples from eye, their microbiological work-up and interpretation of laboratory results have been described in chapters on keratitis and uveitis. Role of optical coherence tomography in the diagnosis and management of complications of uveitis is also discussed. A number of new chapters such as: Retinopathy of Prematurity, Localization of Intraocular Foreign Body, Comitant Strabismus, Incomitant Strabismus, Dry Eye, Epiphora, Proptosis and Neurological Disorders of Pupil have been added in the second edition of the book. Retinopathy of prematurity is one of the important causes of childhood blindness. Risk factors, documentation, staging, classification, screening procedure and management of the disease are briefly described. Precise localization of intraocular foreign body is a tedious procedure but is critically important for its removal and management. Computerized tomography and magnetic resonance imaging have replaced old cumbersome radiological methods for localization of intraocular foreign bodies, metallic and wooden. Strabismus often has an adverse effect on psychological functioning, personality trait and working capabilities of an individual. Patients with strabismus suffer from low self-esteem and have problem in social interaction. Therefore, early correction of strabismus is necessary for improving the quality of the life of the patient. The chapter on comitant strabismus presents various methods for examination and measurement of deviations. Incomitant strabismus, though less common, is more troublesome. It usually results from cranial nerves (III,IV,VI) paralysis. Restrictive strabismus may be associated with interesting clinical ocular syndromes. Dry eye is one of the most common external ocular diseases seen by ophthalmologists. Prevalence of dry eye is on rise mainly due to an environmental pollution, change in lifestyle and increase in aging population. Should dry eye be considered a disorder of tear film and excessive tear evaporation or a localized immune-mediated inflammatory response of ocular surface? Besides the controversy, what is more important is an early diagnosis of dry eye and its proper management. xii Diagnostic Procedures in Ophthalmology
  9. 9. Epiphora is an annoying symptom. It may occur either in infants or adults. An understanding of anatomy and physiology of the lacrimal apparatus is necessary for the evaluation of epiphora. A number of invasive and noninvasive tests are available to investigate patients with epiphora and localize site of obstruction in the lacrimal passage. Proptosis has a varied etiology. It may occur due to ocular, orbital and systemic causes. Generally, proptosis requires interdisciplinary cooperation amongst ophthalmologists, neurologists, oncologists, ENT surgeons, internists and radiologists. Investigation of patients with proptosis should begin with simple standard noninvasive techniques and, if necessary, progress to more elaborate and invasive procedures. Ultrasonography, CT and MRI are of immense value in the diagnosis. Examination of pupil (size, shape and pupillary reactions) is essential in neurological disorders. Typical pupillary signs can help in localizing lesions in the nervous system. Characteristics of Adie tonic pupil and Argyll-Robertson pupil and a detailed evaluation of the third cranial nerve palsy are described in the last chapter. Most of the contributors who have vast experience in their respective fields have written chapters for this book. To make the reader familiar, they have not only described diagnostic procedures but also given characteristic findings of eye disorders with the help of illustrations. The book has expanded greatly as many new chapters with numerous illustrations are added. We hope the book should be of great help to the practicing ophthalmologists and clinical residents providing a practical resource to investigative procedures in ophthalmology. HV Nema Nitin Nema xiiiPreface to the Second Edition
  10. 10. Preface to the First Edition The word diagnosis comes from a Greek word meaning to distinguish or discern. Besides history and clinical examination of the patient, diagnostic tests are required to aid in making correct diagnosis of eye diseases. The role of diagnostic technology is not inferior to that of a clinician’s acumen. A correct diagnostic report helps in differentiating functional from organic and idiopathic from non-idiopathic diseases. The number of diagnostic tests available to an ophthalmologist has increased significantly in the last two decades. Both selective and non-selective tests are presently used for the clinical and research purposes. Non-selective approach to testing is costly and does not provide useful information. In order to be useful, diagnostic tests have to be properly performed, accurately read, and correctly interpreted. The ordering oculist should always compare the results of test with the clinical features of the eye disease. The main aim of the book—Diagnostic Procedures in Ophthalmology is to provide useful information on diagnostic tests, which an ophthalmologist intends to perform or order during his clinical practice. Some of the procedures described in the book, assessment of visual acuity, slit lamp examination, tonometry, gonioscopy, perimetry and ophthalmoscopy, are routine examinations. However, the technique of proper examination and interpretation of findings to arrive at a correct diagnosis must be known to the practising ophthalmologist or optometrist. Procedures like ophthalmic photography, evaluation of optic nerve head, fundus fluorescein angiography and indocyanine green angiography are invaluable because they not only help in the diagnosis and documentation but also help in monitoring the management of eye disease. Corneal topography gives useful data about corneal surface and curvature and contributes to the success of Lasik surgery to a great extent. The role of A-scan ultrasonography in the measurement of axial length of the eye and biometry cannot be over emphasised. B-scan ultrasonography is needed to explore the posterior segment of the eye when media are opaque or an orbital mass is suspected. Ultrasound biomicroscopy (UBM) and Optical coherence tomography (OCT) are relatively new non-invasive tools to screen the eye at the microscopic level. UBM helps in understanding the pathogenesis of various forms of glaucoma and their management. OCT obtains a tomograph of the retina showing its microstructure incredibly similar to a histological section. It helps in the diagnosis and management of the macular and retinal diseases. Electrophysiological tests allow objective evaluation of visual system. They are used in determination of visual acuity in infants and in the diagnosis of the macular and optic nerve disorders. What diagnostic tests should be ordered in the evaluation of the patients with infective keratitis or uveitis? Chapters on Diagnostic Procedures in Infective Keratitis and Diagnostic Procedures in Uveitis provide an answer. The experts who have credibility in their fields have contributed chapters to the book. Not only the procedures of diagnostic tests are described but to make the reader conversant, characteristic findings in the normal and the diseased eye are also highlighted with the help of illustrations. The book should be of great help to the practising ophthalmologists, resident ophthalmologists, optometrists and technicians as it provides instant access to the diagnostic procedures in ophthalmology. We are indebted to all contributors for their excellent contributions in short time in spite of their busy schedule. Mr JP Vij deserves our sincere thanks for nice publication of the book. HV Nema Nitin Nema
  11. 11. Acknowledgements The publication of the second edition of Diagnostic Procedures in Ophthalmology is possible with the help and cooperation of many colleagues and friends. We wish to express our gratitude to allthecontributingauthorsfortheirtimeandpainstakingeffortsnotonlyforwritingthecomprehensive and well illustrative chapters but also updating and revising them to conform the format of the book. We are indebted to Prof JL Alió, Dr Vasumathy Vadantham and Dr Tarun Sharma for contributing chapters on a short notice because the initial contributors failed to submit their chapters. Our grateful thanks go to Dr Mahipal Sachdev for persuading Dr Manotosh Ray to write a chapter on Confocal Microscopy. Mrs Pratibha Nema deserves our deep appreciation; without her patience, tolerance and understanding, this book would not have become reality. Finally, Shri Jitendar P Vij (Chairman and Managing Director), Mr Tarun Duneja (Director- Publishing) and supporting staff of M/s Jaypee Brothers Medical Publishers (P) Ltd, New Delhi especially deserve our sincere thanks for their cooperation and keen interest in the publication of this book. HV Nema Nitin Nema
  12. 12. Contents 1. Visual Acuity ..................................................................................................................... 1 Stephen C Hilton, Leela V Raju, VK Raju 2. Color Vision and Color Blindness........................................................................... 12 Harinder Singh Sethi 3. Slit-lamp Examination ................................................................................................... 33 Harinder Singh Sethi, Munish Dhawan 4. Corneal Topography....................................................................................................... 46 Francisco Arnalich, David Piñero, Jorge L Alió 5. Confocal Microscopy...................................................................................................... 84 Manotosh Ray 6. Tonometry .......................................................................................................................... 95 R Ramakrishnan, Sonal Ambatkar 7. Gonioscopy ......................................................................................................................106 A Narayanaswamy, L Vijaya 8. Optic Disk Assessment in Glaucoma ...................................................................115 Rajul Parikh, Chandra Sekhar 9. Basic Perimetry..............................................................................................................128 Devindra Sood, Parmod Kumar 10. Ophthalmoscopy.............................................................................................................151 Pukhraj Rishi, Tarun Sharma 11. Ophthalmic Photography ............................................................................................165 Sadao Kanagami 12. Fluorescein Angiography ............................................................................................181 R Kim, S Manoj 13. Indocyanine Green Angiography ............................................................................200 Vasumathy Vedantham 14. A-scan Ultrasonography..............................................................................................216 Rajiv Nath, Tinku Bali, Monica Saha 15. B-scan Ultrasonography ..............................................................................................239 Taraprasad Das, Vasumathy Vedantham, Anjali Hussain Sangmitra Kanungo, LS Mohan Ram
  13. 13. 16. Ultrasound Biomicroscopy in Ophthalmology ....................................................259 Roshmi Gupta, K Kalyani Prasad, LS Mohan Ram, Santosh G Honavar 17. Optical Coherence Tomography ..............................................................................269 Tomohiro Otani 18. Electrophysiological Tests for Visual Function Assessment ........................279 Subhadra Jalali, LS Mohan Ram, Garima Tyagi, Kallakuri Sumasri 19. Diagnostic Procedures in Infectious Keratitis ...................................................316 Savitri Sharma, Sreedharan Athmanathan 20. Diagnostic Procedures in Uveitis ...........................................................................333 Jyotirmay Biswas, Surbhit Chaudhary, S Sudharshan, Shahnawaz Kazi 21. Retinopathy of Prematurity: Diagnostic Procedures and Management ....353 Yog Raj Sharma, Deependra Vikram Singh, Nikhil Pal, Rajani Sharma 22. Localization of Intraocular Foreign Body ............................................................362 Amit Nagpal, Lingam Gopal 23. Comitant Strabismus: Diagnostic Methods .........................................................369 Harinder Singh Sethi, Pradeep Sharma 24. Incomitant Strabismus.................................................................................................395 S Meenakshi, T Surendran 25. Diagnostic Procedures in Dry Eyes Syndrome..................................................405 MS Sridhar 26. Evaluation of Epiphora ...............................................................................................412 AK Grover, Rituraj Baruah 27. Diagnostic Techniques in Proptosis ......................................................................426 Mandeep S Bajaj, Sanjiv Gupta 28. Neurological Disorders of Pupil .............................................................................441 Ambar Chakravarty Index .................................................................................................................................................. 461 xviii Diagnostic Procedures in Ophthalmology
  14. 14. 1Visual Acuity Vision is the most important of all senses. Approximately 80% of the information from the outside world is incorporated through the visual pathway. Loss of vision has a profound effect on the quality of life. The process of vision includes: 1. Central resolution (visual acuity) 2. Minimal light sensitivity 3. Contrast sensitivity 4. Detection of motion 5. Color perception 6. Color contrast 7. Peripheral vision (spatial, temporal and motion detection). In the normal clinical settings, we measure only one of these functions – central resolution at high contrast (visual acuity).1 Definition and Terminology of Visual Acuity The most basic form of visual perception is detection of light. Visual acuity is more than just detectinglight.Itisthemeasurementoftheability to discriminate two stimuli separated in space at high contrast compared with the background. The minimal angle of resolution that allows a human optic system to identify two points as different stimuli is defined as the threshold of resolution. Visual acuity is the reciprocal of the threshold of resolution.2 Clinically, discrimina- ting letters in a chart determine this, but this task also requires recognition of the form and shape of the letters, which are processes that also involve higher centers of visual perception. Discrimination at a retinal level may, there- fore, be determined by less complex stimuli, such as contrast sensitivity gratings. Theoretically, the maximum resolving power of the human retina could be derived from an estimate of the angle of approximately 20 seconds of arc because this represents the smallest unit distance between two individually stimulated cones. Thus the resolving power of the eye could be much greater than what is measured by visual acuity charts.3 Cones have the highest discriminatory capacity, but rods can also achieve some resolution.Thegreaterthedistancefromthefovea the level of visual acuity falls off rapidly. At a 5° distance from the foveal center, visual acuity is only one quarter of foveal acuity.4 Luminance of test object, optical aberrations of the eye and the degree of adaptation of the observer also influence the visual acuity.5 STEPHEN C HILTON, LEELA V RAJU, VK RAJU Visual Acuity 1
  15. 15. 2 Diagnostic Procedures in Ophthalmology Visual thresholds can be broadly classified into three groups: 1. Light discrimination (minimum visible, minimum perceptible) 2. Spatial discrimination (minimum separable, minimum discriminable) 3. Temporal discrimination (perception of transient visual phenomena such as flickering stimuli). Many clinical tests can assess many visual functions simultaneously. In a healthy observer in best focus, the resolution limit, or as it is usually called, the minimum angle of resolution (MAR), is between 30 seconds of arc and one minute of arc. Clinically, we use Landolt C and Snellen E to assess visual acuity. The minimum discriminable hyper-acuity or vernier-acuity is another example of spatial discrimination. The eye is capable of subtle discrimination in spatial localization, and can detect misalignment of two line segments in a frontal plane if these segments are separated by as little as three to five seconds of arc, considerably less than the diameter of a single foveal cone. The mechanism subserving hyper-acuity is still being investigated. Charts and Scales to Record Visual Acuity The function of the eye may be evaluated by a number of tests. The cone function of the fovea centralis is assessed mainly by measurement of theformsense,theabilitytodistinguishtheshape of objects. This is designated as central visual acuity. It is measured for both near and far, with and without the best possible correction of any refractive error present. Because only cones are effective in color vision and because they are concentrated in the fovea, the measurement of the ability to recognize colors is also a measurement of foveal function. The function of the peripheral retina which contains mainly rods, may be assessed by peripheral visual field.1 Visual acuity is the first test performed after obtaining a careful history. Measurement of the central visual acuity is essentially an assessment of the function of the fovea centralis. An object must be presented so that each portion of it is separated by a definite interval. Customarily, this interval has become one minute of an arc, and the test object is one that subtends an angle of five minutes of an arc. A variety of test objects has been constructed on this principle, so that an angle of five minutes is at distances varying from a few inches to many feet5 (Figs 1.1 and 1.2). The most familiar examination chart is Snellen chart (Fig. 1.3). Conventionally, reading vision is examined at 40 cm (16 inches). The testing distance of a preferred near distance chart Fig. 1.1: Snellen letters subtend one minute of arc in each section, the entire letter subtends five minutes of arc Fig. 1.2: Each component of Snellen letters subtend one minute of visual angle the entire letter subtends five minutes of visual angle at stated distance
  16. 16. 3Visual Acuity should be observed accurately. The Snellen notation is simply an equivalent reduction for near, maintaining the same visual angle. Most of the Snellen-based distance acuity charts are also commercially available as ‘pocket’ charts to check the near acuity at a preferred distance for every patient or at a defined distance for clinical trial purposes including ETDRS (Fig. 1.4) and Snellen letter “E”. The Jaeger notation is a historic enigma and Jaeger never committed himself to the distance at which the print should be used. The numbers on the Jaeger chart simply refer to the numbers on the boxes in the print shop from which Jaeger selected his type sizes in 1854. They have no biologic or optical foundation. Clinically, Jaeger’s charts (Fig. 1.5) are widely used. Central visual acuity is designated by two numbers. The numerator indicates the distance between the test object and the patient; the denominator indicates the distance at which the test object subtends an angle of five minutes. In the United States these numbers are given in inches or feet, whereas in the Europe the designation is in meters. The test chart commonly used in the United States has its largest test object one that subtends an angle of five minutes at a distance of 200 feet (6 m). Then there are test objects of 100, 80, 70, 60, 50, 40, 30, 20 and 15 feet. If the individual is unable to recognize the largest test object, then he or she should be brought closer to it, and the distance at which he or she recognizes it should be recorded. Thus, if the individual recognizes the test object that subtends a five minute angle at 200 feet when he or she is at 12 feet, the visual acuity is recorded as 12/200. This is not a fraction but indicates two physical Fig. 1.3: Snellen chart Fig. 1.4: ETDRS chart
  17. 17. 4 Diagnostic Procedures in Ophthalmology Fig. 1.5A: Jaeger's type near vision chart
  18. 18. 5Visual Acuity Fig. 1.5B Fig. 1.5B: Near vision chart: Music type and numericals
  19. 19. 6 Diagnostic Procedures in Ophthalmology measurements, the test distance and the size of the test object. The most familiar test objects are letters or numbers. Such tests have the disadvantage of requiring some literacy on the part of the patient. Additionally, there is a variation in their ability to be recognized. “L” is considered the easiest letterinthealphabettoreadand“B”isconsidered themostdifficult.Toobviatethisdifficulty,broken rings (Fig. 1.6) have been devised in which the break in the ring subtends one minute angle, and the ring subtends a five minute angle. Similarly, the letter “E” may be arranged so that it faces in different directions (Fig. 1.6). These test objects are easier to see than letters, eliminate some of the difficulties inherent in reading, and can be used in the testing of illiterates and persons not familiar with the English alphabet. A variety of pictures (Fig. 1.6) have also been designed for testing the visual acuity of children. When a person is unable to read even a top letter,heorsheisaskedtomovetowardthechart or a chart can be brought closer. The maximum distancefromwhichheorsherecognizesthetop letterisnotedasthenominator.Whenvisualacuity is less than 1/60, the patient is asked to count fingers from close at hand (CF at 20 cm). When a patient cannot even count fingers, the patient is asked if he or she can see examiner’s hand movements (HM positive). When hand move- mentsarenotseenwehavetorecordwhetherthe perception of light (LP) is present or absent by asking the patient if he or she sees the light. Standard illumination should be used for the acuity chart (10 to 20 foot candles for wall charts). When a patient is examined with the Snellen chart in a dark room, the subject sees a high contrast and glare-free target. But in real circumstances, contrast and glare reduce visual acuity, and even more so in a pathological conditions. The contrast sensitivity function of a subject may be affected even when Snellen acuity is normal. The contrast sensitivity tests are more accurate in quantifying the loss of vision in cases of cataracts, corneal edema, neuro- ophthalmic diseases, and retinal disorders. A patient with a low contrast threshold has a high degree of sensitivity; therefore, a healthy young subjectmayhaveathresholdof1%,andacontrast sensitivity of 100% (inversely proportional). It is important to have adequate lighting when testing visual acuity so that it does not become a test of contrast sensitivity. Factors Affecting Visual Acuity Factors affecting visual acuity may be classified as physical, physiological and psychological. Fig. 1.6: Broken C, letter E and pictures of familiar objects for testing visual acuity in illiterates and children
  20. 20. 7Visual Acuity Uncorrected refractive error is a common cause of poor acuity. Physical factors include illumination and contrast. Increased illumination increases visual acuity from threshold to a point at which no further improvement can be elicited. In the clinical situation this is 5-20 foot candles. When contrast is reduced more illumination is required to resolve an object. Beyond a certain point, illumination can create glare. Therefore, visual acuity is recorded under photopic condition and one wants to evaluate best visual acuity at the fovea. Physiological conditions include pupil size, accommodation, light-dark adaptation and age.2 Pupil Size Thepupilsizehasgreatinfluenceonvisualacuity. Visualacuitydecreasesifpupilsaresmallerthan 2 mm due to diffraction. Pupil diameters larger than 3.5 mm increase aberration. Variation in pupilsizechangesacuitybyalteringillumination, increasing depth of focus, and modifying the diameter of the blur circle on the retina. Accommodation An accommodation creates miosis, which could account for small hyperopic prescriptions being rejected for distance viewing in younger individuals. Itisworthwhiletodiscusstheroleofapinhole in obtaining the best visual acuity in the clinical setting. The optimum pinhole is 2.5 mm in diameter. A pinhole in an occluder (Fig. 1.7) may be introduced in a trial frame with the opposite eye occluded. Single pinhole device is not adequate. The patient must be able to find a hole, therefore, multiple pinholes are preferred. If the patient is older or infirm, or has tremors, he is asked to read only a single letter from each line asweproceeddownthecharttorecordthevision. Many patients have been referred for neuro- -ophthalmologic consultation because of painless loss of vision in one eye only. The best visual acuity may be 20/60 in the affected eye but when properly tested with the pinhole, the acuity may improve to 20/20. This indicates that the macula and optic nerve are functioning normally. When the patient’s vision is improved withpinholeoneknowstheproblemisarefractive one and simply need the change in glasses. If the patient’s vision is less when looking through the pinhole; it indicates that the patient has either an organic lesion at macula, or a central scotoma, or functional amblyopia. A patient with 20/400 vision that improves with pinhole to 20/70 indicates that the improvement is refractive, but some pathology may also be present. Figs 1.7A and B: Occluder with multiple holes A B
  21. 21. 8 Diagnostic Procedures in Ophthalmology Visual Acuity Testing in Young Children Early determination of vision loss and refractive error is an essential component of assessing the infant’s ultimate visual development potential. The visual acuity of a newborn as measured by preferential looking is in the range of 30 minutes of arc (20/600); acuity rapidly improves to six minutesofarc(20/120)bythreemonths.Asteady but modest improvement to approximately three minutes of arc (20/60) occurs by 12 months of age.Oneminuteofarc(20/20)isusuallyobtained at the age of three to five years.6 The examination is generally performed on the parent’s lap. The room should never be totally darkened because this may provoke anxiety. Objective retinoscopy remains the best method of determining a child’s refraction. Other clinical methods involve estimation of fixation and following behavior. A test target should incorporate high contrast edges. For infants younger than six months the best target represents the examiner’s face. For the child of six months and older, an interesting toy can be used. After assessment of the binocular fixation pattern, the examiner should direct attention to differences between the two eyes when tested monocularly. Objection to occlusion of one eye may suggest abnormality with the less preferred eye.7 Three common methods are used for determining resolution acuity: 1. Behavioral technique (preferential looking Fig. 1.8) 2. Detecting optokinetic nystagmus (OKN Fig. 1.9) 3. Recording visual evoked potentials (VEP Fig. 1.10). It is desirable to measure the visual acuity of children sometime during their third year to detect strabismic or sensory amblyopia and to recognize the presence of severe refractive errors. Fig. 1.8: Preferential looking test chart Fig. 1.9: OKN drum In this group of preschool children, visual acuity testing is easier to perform with the use of the following charts: 1. Allen and Osterberg charts (Fig. 1.11) 2. Illiterate E chart 3. Landolt broken ring.
  22. 22. 9Visual Acuity Contrast is defined as the ratio of the difference in the luminance of these two adjacent areas to the lower or higher of these luminance values. The amount of contrast a person needs to see a target is called contrast threshold. The contrast sensitivity is assessed by using the contrast sensitivity chart. It has 5-8 different sizes of letters in six or more shades of gray. Some contrast sensitivity charts contain a series of alternating black and white bars; 100 line pairs per mm is equivalent to space of one minute between two black lines. The alternating bar pattern is described as spatial frequency. The contrast sensitivity is measured in units of cycles per degrees (CPD). A cycle is a black bar and white spaces. To convert Snellen units to units of cycles per degree, divide 180 by Snellen denominator. Contrast sensitivity measurements differ from acuity measurements; acuity is a measure of the spatial resolving ability of the visual system under conditions of very high contrast, whereas contrast sensitivity is a measure of the threshold contrast for seeing a target.8 Visual Acuity in Low Vision Patients Individual near acuity needs are different among different population groups. For low vision patients these differences are magnified. Two persons with the same severe visual impairment may exhibit marked differences in their ability to cope with the demands of daily living. Visual acuity loss, therefore, is the aspect that must be addressed in individual rehabilitation plans. Colenbrander9 subdivides several components of visual loss into impairment aspects (how the eye functions), visual ability (how the person functions in daily living), and social/economic aspects (how the person functions in society (Table 1.1). Fig. 1.10: VEP testing Fig. 1.11: Allen and Osterberg chart Contrast Sensitivity A general definition of spatial contrast is that it is a physical dimension referring to the light- dark transition at a border or an edge of an image that delineates the existence of a pattern or object.
  23. 23. 10 Diagnostic Procedures in Ophthalmology TABLE1.1:RANGESANDASPECTSOFVISIONLOSS ImpairmentaspectsVisualabilityaspects/functionalvisionSocialandeconomicaspects (howtheeyefunction)(howthepersonfunctions-dailylivingskills)(howthepersonfunctionsinsociety) RangesVisualNewsprintStatisticalestimateVASComments (ICD-9-CM)acuity(1M)ofreadingabilityVisualaids Normalvision20/12.563inNormalreadingspeedNone110Notethatnormaladult 20/1650inNormalreadingdistance105visionisbetterthan20/20 20/2040inReservecapacityfor100 20/2532insmallprint95 Mildvisionloss20/3225inNormalreadingspeed90Manyfunctionalcriteria 20/4020inReducedreadingdistance85(whetherforadriver’s 20/5016inNoreserveforsmall80licenseorforcataract 20/6312.5in75surgery)fallwithintherange Moderatevisionloss20/8010inNear-normalwithVision70IntheUnitedStates, 20/1008inappropriatereadingaidsenhancements65childreninthisrangequalifyfor 20/1256inLow-powermagnifiersaids60specialeducationalassistance 20/1605inandlarge-printbooks55 SeverevisionLoss20/2004inSlowerthannormalwith50IntheUnitedStates, 20/2503inreadingaids45personsinthisrange 20/3202.5inHigh-powermagnifiers40areconsideredlegally 20/4002in(restrictedfield)35blindandqualifyfor tax-breakdisabilitybenefits. Profoundvisionloss20/5001.6inMarginalwithaids30IntheEU,manybenefits 20/6301.2inUsesmagnifiersforspot25startatthislevel.The 20/8001inreading,butmayprefer20WHOincludesthisrange 20/1000talkingbooksforleisure15initsblindnesscategory. Near-blindness20/12501cmNovisualreadingVision10Inthisrange,residualvision 20/16001cmmustrelyontalkingsubstitution5tendstobecomeunreliable, 20/20001cmbooksorotheraids0thoughitnonvisualsourcesmay stillbeusedasanadjunctto visionsubstitutionskills. TotalBlindnessNLP (FromColenbranderA.Preservationofvisionorpreventionofblindness[editorial]?AmJOphthalmol2002;133:2.p.264.)
  24. 24. 11Visual Acuity Summary Both distance and near visual acuities are recordedforeacheyewithandwithoutspectacles. Distance visual acuity is recorded at a distance of 20 feet or in a room of at least 10 feet using mirrors and projected charts. Near visual acuity can be recorded using reduced Snellen or equivalent cards at 40 cm. Acuity performance, like any other human performance, is subject to impairment depending on ocular and general health, emotional stress, boredom, and a variety of drugs acting both peripherally and centrally. The examiner must provide encouragement and must have patience. For clinical studies the ETDRS charts are recommended because near vision is often more important in the daily life of older or infirm patients. Reading charts or other near vision testingchartsshouldbeusedaspartoftheroutine assessment of the visual acuity. Visual acuity measurement is often taken for granted. Many pitfalls make this most important assessment subject to variability.10 Ambient illumination, aging bulbs, dirty charts or slides, small pupils, and poorly standardized charts are just some of the factors that can lead to erroneous results. A little care in ensuring the proper environmentfortestingcansignificantlyimprove accuracy. References 1. Newell FW. Ophthalmology Principles and Concepts. St Louis, Mosby, 1969. 2. Moses RA (Ed). Adlers Physiology of the Eye. St Louis, Mosby, 1970. 3. Scheie H. Textbook of Ophthalmology. Philadelphia, WB Saunders, 1977. 4. Duane TD. Clinical Ophthalmology. New York, Harper and Row, 1981. 5. Michaels DD. Visual Optics and Refraction. St Louis, Mosby, 1985. 6. Vander J. Ophthalmology Secrets. Hanley and Belfus. 7. Borish I. Clinical Refraction. Professional Publisher, 1970. 8. Owsley C. Contrast Sensitivity. Ophthalmic Clinics of North America 2003;16:173. 9. Colebrander A. Preservation of Vision or Prevention of Blindness? Am J Ophthalmol 2003; 133:263. 10. Kniestedt, Stamper RL. Visual Acuity and its Measurements. Ophthalmic Clinics of North America 2003; 16:155.
  25. 25. 12 Diagnostic Procedures in Ophthalmology HARINDER SINGH SETHI Color Vision and Color Blindness2 Color vision examination is an essential part of screening before a person is taken up for a job. A person who is color vision defective may go through life quite unconscious of his color deficiency and without making any incrimi- nating mistakes, differentiating objects by their size, shape and luminosity, using all the time a complete color vocabulary based on his experience which teaches him that color terms are applied with great consistency to certain objects and to certain achromatic shades, until circumstances are arranged to eliminate these accessory aids and then he realizes that his sensations differ in some way from the normal. Various tests have been developed to enable screening of anomalous subjects with color deficiency from a much larger group of normal subjects. Color Vision Color is a sensation and not a physical attribute of an object. Color is what we see and is result of stimulation of retina by radiant energy in a small band of wavelengths of the electromagnetic spectrum usually considered to span about one octave, from 380 nm to 760 nm. There are three main characteristics of color namely hue, saturation, and brightness. Hue is a function of wavelength. It depends on what the eye and brain perceive to be the predominant wavelength of the incoming light. An object’s “hue” is its “color.” Saturation refers to the richness of a hue as compared to a gray of the same brightness. Saturation is also known as “chroma.” Brightness correlates to the ease with which a color is seen, other factors being equal. Brightness is a subjective term referring to the sensation produced by a given illuminance on the retina. The spectral wavelengths of different colors are as follows: violet 430 nm; blue 460 nm; green 520 nm; yellow 575 nm; orange 600 nm and red 650 nm. The concept of white light is vague, most agreeable definition is, white surface is one which has spectral reflection factors independent of wavelength (in the visible spectrum) and greater than 70%. Factors Affecting Color Vision Crystalline Lens The lens absorbs shorter wavelengths; in young, wavelengths of less than 400 nm and in old people up to 550 or 600 nm are absorbed by
  26. 26. 13Color Vision and Color Blindness the lens resulting in defective color vision on shorter wavelength side. Retinal Distribution of Color Vision The center of the fovea (1/8 degree) is blue blind. Trichromatic vision extends 20-30° from the point of fixation. Peripheral to this red-green become indistinguishable up to 70-80° and in far peripheral retina all color sense is lost although cones are still found in this region. In the central 5°, macula contains carotenoid pigment, xanthophyll. The molecules of the pigment are arranged in such a way that they absorb blue light polarized in the radial direction. If one looks at a white card through linear polarizer, one will see two blue sectors separated by two yellow sectors the figure is called Haidinger’s brushes. Macular pigment may also be seen as in homogeneity in the field of blue or white light called Maxwell’s spot. Wavelength Discrimination The normal observer is able to detect a difference between two spectral lights that differ by as little as 1 nm in wavelength in the regions of 490 nm and 585 nm. In the region of violet and red a difference of greater than 4 nm is necessary. Hue, Saturation and Lightness Hue is the extent to which the object is red, green, blue or yellow. Saturation is the extent to which a color is strong or weak. Lightness is self explanatoryattribute,forexample,yellowbycolor is light. Illumination Illumination affects color vision of low illuminances, the errors increase due to poorer discrimination for most of the hue range while testing color vision. An illuminance of 400 lux (± 100 lux) would be practical value for most clinical applications. Bezold-Burcke Effect von Bezold (1873) and Burcke (1878) discovered independently the phenomenon named after them, that variation of the luminance levels modifies hues. Color Constancy; Aperture Colors and Surface Colors Color constancy is a phenomenon in which color of the objects can be recognized unchanged in spite of possible differences in the illumination. Aperturecolorsarecolorsthatalterduetochange in illumination. Surface colors do not vary with illumination. Extrafoveal vision favors the appearance of aperture colors and foveal vision that of surface colors. Complementary Wavelengths Complementary wavelengths are those which, when mixed in appropriate proportions, give white. Simultaneous Color Contrast Color contrast is visually demonstrated by observing the color of a spot in a surround. The general rule is that the color of the spot tends toward the complementary of the color of the surround. Successive Color Contrast Successive color contrast is more commonly described as colored after images, when one stares at a red spot for several seconds and then looks at a gray card one sees a green spot on
  27. 27. 14 Diagnostic Procedures in Ophthalmology the card. The after image tends toward the complementary of the primary image (Stiles- Crawford effect). The light entering near the edge of the pupil is less effective than light entering at the center of the pupil because of the shape of the receptors and the fact that they are embedded in a medium of different refractive index. This effect is wavelength-dependent. Color Triangle Color triangle can be drawn to describe the trichromacy of color mixtures and is useful for deciding which bands of wavelength are indistinguishablefromeachother.Threereference wavelengths are chosen, i.e. 450 nm, 520 nm and 650 nm and are placed at vertices of X, Y and Z of a triangle, the position of other wavelengths is determined. A color triangle does not describe the color of a band of wavelengths unless other circumstances are defined. Theories of Color Vision This is a complex topic as no theory explains the phenomenon of color vision fully. Few important theories are given below: Young-Helmholtz Theory (Trichromatic Theory) Young’s concept is that there are three types of retinal receptors with different spectral sensitivities. Young’s principal colors are red, green and violet. Young’s hypothesis was not followed up until it was revived by Helmholtz in 1852. The Young’s theory may be summarized as follows: a. At some stage of visual receptor mechanism there are three different types of sensory apparatus G1, G2, G3. These receptors must be same for everyone but they may not be same at the fovea as at the periphery. b. Each of these receptors is characterized from the spectral point of view by particular function of wavelengths which may be denoted by G and the response G1 of a receptor for radiation with a spectral energy distribution Eλ may be supposed to have the form. G1 = Sgi Eλ dλ. c. Sensation of color is a function of the relative values of the three responses G1. d. Sensation of light is a function of a linear combination of the three responses. Fundamental sensations By determining approximately the coordinate of the confusion points of dichromats Arthur Konig in1893 established a system of fundamental sensations and identified red, green and violet as fundamental colors. Blue was also identified as fundamental color in addition to red and green by Gothelin. Granit’s Theory of Color Vision Granit divides retina into receptor units, each unit comprising groups of cones and rods which are connected with a single ganglion cell or several ganglion cells which synchronize their discharges. These units are classified as “dominators” or “modulators”. The dominators which are numerous have a spectral sensitivity curve which indicates that they are responsible for the sensations of luminosity. Modulators show a selective sensitivity which makes them responsible for color discrimination. Granit’s theory does not explain the fact of trichromatism. Hering’s Theory of Color Vision (Opponent Color Theory) Hering assumed six distinct sensations arranged in three opposing pairs: white-black; yellow-blue and red-green; he explains three pairs as being
  28. 28. 15Color Vision and Color Blindness due to opposing actions of light on three substance of the retina, a catabolism producing warm sensation (white, yellow, red) and an anabolism the cold ones. This theory is clearly a psychological concept and aims at explaining complex percepts than the intermediate effect of the stimuli. Anatomy of Color Vision The understanding of visual pathway is complex and not evident fully. There are two types of photoreceptors in the retina: rods and cones. Approximately 120 million rods are responsible for night and peripheral vision. Rods contain a photopigment called rhodopsin, a chemical variant of vitamin A and a protein called opsin that serves at very low levels of illumination. Rods have their maximum density about 5 degrees from the fovea and cannot distinguish one color from another. The fovea itself is essentially rod-free containing only cones. Approximately 7 million cones are responsible for central and color vision. Cones have their maximum density within 2 degrees of the center of the fovea. Both types of receptors diminish in number toward the retinal periphery. Cones In the retina three types of cones responsible for the red, green and blue sensations have been isolated. Three types of cone pigments in the human retina absorb photons with wavelengths between 400 nm and 700 nm. Color vision is mediated by these three cone photoreceptors referredtoaslong,middle,andshortwavelength- sensitive (LWS, MWS, SWS) cones. The long wavelength-sensitive (LWS) cones (sometimes called “red” or “red-catching”) contain a pigment called erythrolabe, which is best stimulated by a wavelength near 566 nm. Medium wavelength- sensitive (MWS) cones (“green” or “green- catching”) contain the pigment chlorolabe, which has a maximal sensitivity to a wavelength near 543 nm. Short wavelength-sensitive (SWS) cones (“blue” or “blue-catching”) contain cyanolabe, which have maximal sensitivity at 445 nm. The blue cones are absent in the center of the macula. Trichromatic vision perception occurs in central 30º field. It is not uncommon to hear the cones referred to as blue, green, and red cones, but such nomenclature is misleading because the L-cones are more sensitive to blue lights than they are to red lights. The spectral sensitivities of the three cone pigments overlap somewhat. For example, light of 540 nm and 590 nm stimulate both green (MWS) and red (LWS) receptors yet we can easily distinguish between these two wavelengths as “green” and “yellow.” If the human retina contains all three cone pigments in normal concentrations, and has normal retinal function, the subject is a trichromat. Any color the trichromat sees can be matched with a suitable mixture of red, green, and blue light. Color Coded Cells Two types of color coded cells are found at peripheral levels (ganglion cells and lateral geniculate body) of the visual system and they havebeennamedopponentcolorcellsanddouble opponent color cells. More complex types are found at more central levels (striate cortex). Opponent color cells: An opponent color cell is one that gives only polarity of response for some wavelengths and opposite polarity of response for other wavelengths. Opponent color cells are concerned with successive color contrast. Double opponent color cells: These are cells opponent for both color and space. The response may be onto red light, off to green light in the center of the receptive field and off to red light, onto green light in the periphery of the receptive
  29. 29. 16 Diagnostic Procedures in Ophthalmology field. Double opponent cells are concerned with simultaneous color contrast. Simple,complexandhypercomplexcells:Inrhesus monkey striate cortex there are a variety of cells that are specific for both color and orientation. They have been categorized as color sensitive simple, complex and hypercomplex cells. Simple cells have a bar-flank double opponent arrange- menttotheirreceptivefields.Complexcolorcoded cells respond to color boundaries of the appro- priate orientation and the response is indepen- dent of the part of the receptive field being sti- mulated. The edge of hypercomplex cells must be short. Opponent color cells are found among ganglion cells of the retina and lateral geniculate body. Double opponent cells with center- surround or flank receptive fields are present in the input layer IV of the striate cortex. Complex and hypercomplex color coded cells are also found in the striate cortex in layers II, III, V and VI. Vaetichin in 1953 recorded a negative slow potential from fish retinae called “S-potential” of two types: L-type (luminosity type) and C- type(chromaticitytype).Mitaraiin1961regarded horizontal cells as responsible for S-potentials of L-type and Muller’s fibers for those of C-type. ThepropertiesofS-potentialssupporttheHerings opponentcolortheorymorethanthetrichromatic theory of Young. Anomalies of Color Vision Deficiency of color vision first was described by Dalton in1794, the founder of the atomic theory, who himself was color blind; hence the term daltonism was coined. The color deficiency is of two types: (1) congenital and (2) acquired. In clinical evaluation of color vision it is important to distinguish between acquired and congenital defects. Congenital vs Acquired Color Deficiencies Congenital color vision deficiencies can be distinguished functionally from acquired deficiencies in a number of ways. Congenital deficiencies typically involve red-green confu- sions, whereas acquired deficiencies, more often than not, are a blue-yellow (Köllner’s rule). Also, because some of the most common congenital defects are linked to the X-chromosome, they are more prevalent in males than females. Acquired defects, in contrast, are not related to gender except by gender differences to trauma or toxic exposure. Acquired color deficiencies are more likely to be asymmetric between the two eyes than are hereditary defects; they are also less likely to be stable with time. Congenital defects are usually easier to detect with standard clinical color vision tests, but some acquired ones can be more subtle and thus are difficult to diagnose. Finally, those with acquired color deficiencies are also more likely to display color-naming errors because, unlike those with congenital deficiencies, they lack the life-long experience with defective color perception. Congenital Color Vision Deficiency The color vision anomalies commonly being X-linked are relatively common (8%) in men and rare in women (Fig. 2.1). Nearly all congenital color defects are due to absence or alteration of oneofthepigmentsinphotoreceptors.Congenital color deficits may be divided into classes accordingtowhetherthepatientsarereddeficient (protans), green deficient (deuterans) or blue deficient (tritans). The term anopia is used for absolute deficiency and anomaly for relative deficiency (Tables 2.1 and 2.2). Anomalous trichromats are people who generally require three wavelengths to match
  30. 30. 17Color Vision and Color Blindness TABLE 2.1: CLASSIFICATION OF COLOR BLINDNESS Congenital: Males (8%), Females (0.4%) Acquired classically X-linked recessive inheritance Unilateral Red-green defect pattern, always bilateral Blue-Yellow defect (a) Achromatopsia Bilateral Red-green defect Cone monochromats Blue-Yellow defect Rod monochromats (b) Dyschromatopsia Dichromats - Deuteranopia Disease Acquired defect - Protanopia Glaucoma Blue-Yellow - Tritanopia Hypertensive retinopathy Blue-Yellow Diabetic retinopathy Blue-Yellow Anomalous trichromats AMD Blue-Yellow - Protanomaly Lesions of visual pathway Red-Green - Deuteranomaly Alcohol-nicotine Red-Green - Tritanomaly TABLE 2.2: VARIOUS TYPES OF COLOR DEFICIENCY Red deficient Green deficient Blue deficient Anomalous trichromats Protanomaly Deuteranomaly Tritanomaly Dichromats Protanopia Deuteranopia Tritanopia Monochromats Rod monochromat Blue monochromat Fig. 2.1: Inheritance pattern of congenital color vision defects another wavelength but do not accept the color matches made by normal people, Lord Rayleigh in 1881 discovered trichromacy. Anomalous trichromats have three classes of cones but one is abnormal. Protanomalous people lack the red receptors and instead they have two pigments both peaking in the range of the normal green. Similarly the deuteranomalous people lack green receptors. Dichromats require only two wavelengths to match another wavelength and will accept the color matches made by normal people. The dichromats have two classes of cone receptors with normal spectral sensitivity, the third class being absent. Measurements of their pigments can be made by reflection densitomer and cone processes isolated by colored backgrounds confirm the findings. Protanopes have normal green and blue cones, red cones being absent. Deuteranopes have normal red and blue cones and tritanopes normal red and green cones.
  31. 31. 18 Diagnostic Procedures in Ophthalmology Protans color deficient subjects are easier to test and classify than deuterans and tritans; because the red cone pigment is quite sensitive to green wavelengths and both red and green cone pigments are quite sensitive to blue wavelength covering the green and blue range, in deuterans and tritans, as the sensitivity of visual pigment does not fall off sharply on the short wavelength side of the peak. Monochromatics can be blue cone mono- chromatics and rod monochromatics. Blue cone monochromatics have normal blue cone pigment but no red or green cone pigment. In rod monochromatismonly500nmpigmentispresent in the retina and all three cones pigments are absent. Genetics of congenital color deficiencies The protans and deuterons are commonly sex- linked recessive. About 1% males are protanopes, 1% protanomalous, 1% deutaranopes and 5% deuternomalous. The incidence of color vision deficiency (red-green) in females is 0.4%. The gene for tritans is autosomal incompletely dominant. Rod monochromatism is very rare; occurs 1 in 30,000, autosomal recessive and thus an increased incidence is seen in consanguineous offsprings. Acquired Deficiency of Color Vision Koellner formulated that lesions in the outer layers of the retina give rise to a blue-yellow defect, while lesions in the inner layers of the retina and the optic nerve gives rise to red green defect. However, the correlation is not always true. Some patients with lesions in the cerebral cortex may have color deficits. These may involve naming of the colors or perception of colors. Factors Responsible for Deficiency of Color Vision Ocular Diseases a. Squint amblyopia: Francois by means of clini- cal tests stated that color vision deficiencies in squint amblyopia do not correspond to the classical type of acquired deficiencies but rather approximate the normal color sense of eccentric retinal positions. b. Glaucoma: Primary glaucoma and ocular hypertension cause tritan-type of defect. c. Diabetic retinopathy: Diabetic retinopathy may cause color deficiency which may vary from a mild loss of hue discrimination to moderate blue-yellow color vision defi- ciency.Inseverecasesofdiabeticretinopathy the defect may resemble tritanopia. d. Retinal disorders: Blue-yellow deficits are found in senile macular degeneration, myopia, retinitis pigmentosa, siderosis bulbi and chorioretinitis. e. Optic nerve disorders: In one study about 57% of patients with resolved optic neuritis were found to have color vision defects. Red-green defects have been found in cases of multiple sclerosis and optic atrophy. Tobacco amblyopia causes red-green defect. f. Color vision after laser photocoagulation: After argon-laser photocoagulation there may be overall loss of hue discrimination and color deficiency, mostly of blue-yellow. Drugs Many drugs are known to cause deficiency of color vision. They can cause more than one type of color deficiency (Table 2.3).
  32. 32. 19Color Vision and Color Blindness TABLE.2.3: DRUGS CAUSING COLOR DEFICIENCY Drugs Type of color deficiency Chloroquine, Indomethacin, Blue-yellow oral contraceptives, antihistaminics, estrogens, digitalis and butazolidin. Ethyl alcohol, Ethambutol Red-green Tri- and bicyclic antidepressants Mixed type Systemic Disorders Besides diabetes, a few systemic disorders are known to be associated with defective color vision. Following diseases may cause color deficiency: a. Cardiovascular disease: Patients with heart diseases have been found to have blue- yellow deficiency. b. Turner’ssyndrome:Red-greencolordeficiency is usually encountered in the syndrome. Color Vision Testing The main objective for testing the color blindness is to determine the exact nature of the defect and whetherthecolordeficiencyislikelytobeasource of danger to the community and/or to the individual, if given a particular job. Types of Color Vision Tests Color Confusion Tests Pseudo-isochromatic (PIC) plates are example of color confusion tests (Figs 2.2 and 2.3). PIC Tests are designed on the basis of the color confusions made by persons with color defects. In these a symbol or figure in one color is placed on a background of another color so that the figure and background are isochromatic for the color-defective person. PIC tests are used primarily as screening tests to identify those with an inherited color defect, although, some of the Figs 2.2A to C: A Ishihara pseudo-isochromatic plates, B Transformation plate seen as “3” by patients with anomalous red-green color defect, C “Vanishing” or “disappearing” digit type A B C
  33. 33. 20 Diagnostic Procedures in Ophthalmology Fig. 2.3: City University test tests permit a diagnosis of type and severity. Because the inventory of PIC tests is extensive, only the more commonly used tests are described here. The most widely used test, Ishihara pseudo- isochromatic plates, is a screening test used to determine the presence of X-linked congenital (red/green) color deficiency. Most screening tests are designed to give a quick, accurate assessment of red/green deficiencies. The Ishihara test is notdesignedtodetecttritandisordersoracquired colordefectsunlesstheopticneuropathyissevere. Arrangement Tests The arrangement tests require the observer to place colored samples in sequential order on the basis of hue, saturation, or lightness or to sort samples on the basis of similarity. One of the earliest tests of this nature that is still available but is rarely used today is the Holmgren Wool test. In this matching test, 46 numerically coded comparisonschemesofyarnareselectedtomatch three test colors: yellow-green, pink, and dark red. The comparison schemes differ from the test schemes in being lighter or darker. The test is not accurate for screening or classification and is not recommended for clinical use. It is of historical significance as an early occupational test. The clinical arrangement tests that are in use today are colored papers mounted in black plastic caps. The caps are placed in order according to specific instructions, and the order is recorded as the sequence of numbers printed on the underside of the caps. Results are plotted on score forms for analysis and interpretation and quantitative scores computed. The tests are standardized for CIE standard illuminant C. The Farnsworth-Munsell Dichotomous-15 (D-15) and the FM-100 test are examples of hue discrimination based on arrangement tests utilizing color chips mounted in a circular cap that subtend exactly 1.5 degrees at a test distance of 50 cm. This ensures that the observations of the subject are made with the central rod free retina. The D-15 contains 15 colored chips and the FM-100 contains 85 chips. The chips have identical brightness and saturation and differ from one another. Farnsworth-Munsell tests reveal the type of defect, but not the severity. Color Matching Tests The spectral anomaloscope and Pickford- Nicolson anomaloscope are used for color matching examinations. They can provide the examiner with information on the severity of a particular color vision defect. The Nagel anoma- loscope is the most widely used. It consists of a spectroscope in which two halves of a circular field are illuminated respectively by monochro- matic yellow (589 nm) and a mixture of monochromatic red and green (670 nm and 546 nm, respectively). The observer is asked to match the two halves of the circle with the three primary colors available. The most widely used color vision tests are the pseudo-isochromatic plates and the D-15
  34. 34. 21Color Vision and Color Blindness panel due to their ease of use and relative low cost. The Nagel anomaloscope and FM-100 tests are usually only found in academic or research settings. All color vision tests have specific require- mentsforlighting,viewingdistance,andviewing time.Itisimportantfortheexaminertobefamiliar withthetestrequirementsandscoresheetsbefore conducting a color vision test, otherwise the results may be inaccurate. Lantern Tests Lantern tests are used only for occupational purpose. Different types of lantern tests are in use in different countries. The FALANT is used in the United States by marine and aviation authorities; the Holmes Wright Type A is used in the United Kingdom by aviation authorities; and the Holmes Wright Type B is used in Australia, the United Kingdom and other Commonwealth countries by marine authorities. The Edridge-Green Lantern is included in the United States Coast Guard requirements, but it is surpassed by the FALANT. Electroretino- graphy (ERG) and microspectrophotometry may be used in special circumstances. Test Conditions Lanterntestingisperformedafterdarkadaptation but all other tests require artificial daylight condi- tions. Light adaptation is critical for anomalo- scopy and especially for FM-100 hue testing, but a color neutral glare-free background and correct illumination are more important. Reliable results can be obtained with an artificial daylight source (such as a Macbeth Sol source) or fluorescent lighting with a color temperature between 5850 and 6850 degrees Kelvin and good color rendering index (Ra over 90). If appropriate artificial light is not available then skylight is a good source. The illumination should be between 250 and 350 lux (approximately 1.5 meters below twin fluorescent globe). A failed Ishihara test under incandescent globe is a failure of the examiner to observe basic principles, not a failure of the subject. A pass on the other hand is still a pass and is statistically the more likely outcome. The viewing geometry should be with the light 45 degrees to the surface and the subject viewing the pages at 90 degrees to the surface. Newlyprintedbookssometimeshavedifferential reflectance between pigments so when tilted back and forth in the light by an anomalous observer they may provide luminance clues. Appropriate optical correction for the 65 cm viewing distance must be available if required. Experienced testers know that some people read the small identifying numbers on the bottom of each page and give amemorizedresponse.Cheatingcanbeprevented by covering these identifying numbers with a secret label. Clinical Significance of the Various Tests Lanterntestingisentirelyvocationalsincearound 5% of males fail and these include all those with a severe anomaly but a relatively unpredictable group from those with the milder anomalies. Anomaloscopy is the gold standard for clinical testing, while the D-15 and FM-100 tests have both clinical and vocational applications (diamond sorters and croupiers). A common vocational test battery should consist of: • Ishihara plates 2 - 17 from the 38 plate series • D-15 color sorting test (3 or more cross over errors is a failure) • Lantern testing. Pseudo-isochromatic Color Plates Themostcommonuseofplatetestsistoidentify
  35. 35. 22 Diagnostic Procedures in Ophthalmology persons with congenital color defects. Pseudo- isochromatic plates (for example, AO-HRR, Ishihara, Dvorine,Tokyo Medical College, SPP- 1) provide efficient screening of congenital red- greendefects(efficiency90-95%).Othertestshave been designed to detect achromatopsia (Sloan Achromatopsiatest),todifferentiateincomplete achromatopsia from complete achromatopsia (Berson blue cone monochromatism plates), to detectacquireddefects(SPP-2),ortodetectcolor confusion (City University test). Plate tests have the advantages of being relatively inexpensive, easily available, simple to use, and appropriate withchildrenandpersonswhoareilliterate.They areonlysuitableforscreeningpurpose,however, they neither provide a quantitative evaluation ofcolorvisionnordistinguishthetypeandseverity ofthecolorvisiondefect.Platetestsaredesigned to distinguish congenital color-defective from color-normalobservers,buttheydonotevaluate the wide range of abilities and aptitudes of observerswithnormalcolorvisiontodistinguish colors. Given individual differences in prereceptoral filters and normal photo pigment polymorphisms,noplatetestcanbe100%effective in screening. When used improperly (nonstandard illuminant, binocular viewing, coloredlensesnotremovedfromobserver),their efficiency can diminish dramatically. The viewing distance required for pseudo- isochromatic plates is 75 cm or approximately 30 inches. Proper refractive correction should be provided to the patient in order for them to see the plates clearly. Viewing time for each plate should be no more than 4 seconds. Undue hesitationcanbeasignofaslightcolordeficiency. Ishihara Pseudo-Isochromatic Plates (Confusion Charts) The Ishihara color vision charts are developed by Shinobu Ishihara in 1917. This test is based on the principle of confusion of the pigment color in red-green color defectives (Fig. 2.2B). There are three editions –- a 16 plate series, 24 plate series and a 38 plate series. The 10th edition of Ishihara has 38 plates. It is best to use the larger series because there are relatively few reliable plates in the smaller series. Both 24 set and 38 plate series set consist of two groups of plates — a group for those who are literate / numerate which starts from plate 1 at the front of the book, and a group for illiterates / innumerate in which the colored pattern is a meanderingpathofconnecteddotsbetweentwo X symbols. The second group is arranged so as to commence with the last page of the book and proceed in reverse order. The group of plates for innumerate are seldom used because they are notaseasyorreliabletoscore,buttheyarebased on the same colorimetric principles as the set for numerates. It is not necessary to use both types in the one subject. From a colorimetric perspec- tive there are four different types of test plate employed in both the 38 and 24 plate series precededbyademonstrationplatethatisnotfor scoring. In the large series plates 1 and 38 are bothfordemonstrationonly,whileinthesmaller series plates 1 and 24 are for demonstration. If thesubjectfailsviewingthedemonstrationplate do not proceed with the test. The following description applies to the numerate plates in the 38 plate series. The different types of plates in the test are: Transformation plates (Fig. 2.2B): Anomalous color observersgivedifferentresponsestocolornormal observers. In these plates, one number is seen by a normal trichromat and another (different) number is seen by a color deficient person. Those with true total color blindness cannot read any numeral. These are the plates numbered 2 to 9 inclusive. Disappearing digit (Vanishing) plates (Fig. 2.2C): The normal observer is meant to recognize the colored pattern. On these plates, a number can
  36. 36. 23Color Vision and Color Blindness be seen by a normal trichromat but nothing can be seen by the color deficient person. These are plates 10 to 17 inclusive in the 38 plate series. Hidden digit plates: The anomalous observer should see the pattern. The number on a hidden digitdesigncannotbeseenbyanormaltrichromat but can be seen by most people with red/green deficiencies. Those people with total color blindness cannot see any numeral. These are plates 18 to 21 inclusive. Qualitative plates: These are intended to classify protan from deutan and mild from severe anomalous color perception. The plates are numbered 22 to 25. Procedure of Testing Theplatesaredesignedtobeappreciatedcorrectly in a room which is lit adequately by daylight. Introductionofdirectsunlightortheuseofelectric lightmayproducesomediscrepancyintheresults because of an alteration in the color values of the charts. It is suggested that when it is convenient only to use electric light, it should be adjusted as far as possible to resemble the effect of natural daylight.Theplatesareheld75cmfromthesubject and tilted at right angles to the line of vision. A missed/misreadplatemustbereread(maybein arandomorder).Thefindingsshouldberecorded ontheIshiharacolorvisiontestandinterpretation marking chart (Table 2.4). A correct response to the Ishihara introduc- tory plate is expected and demonstrates suitable visual acuity to perform the test and rules out malingering. • Plates 1-25 have numerals and each answer should be given without more than 3 seconds of delay. • Plates 26-38 are tracings for use in illiterates, and windings lines between the two Xs are traced with a dry soft brush. Each tracing should take less than 10 seconds. • Each eye should be tested separately (as should be done for all color vision tests). The recommendations of the test state that of the first 21 plates if 17 or more plates are read correctly by an individual his color sense should be regarded as normal. If 13 or less plates are correctly read then the person has a red- green color defect. It is rare to have persons who read 14-16 plates correctly. Hardy, Rand, Rittler (H-R-R) Plates Hardy, Rand, Rittler (H-R-R) plates are another type of pseudo-isochromatic (PIC) plate test. This test is similar to the Ishihara test except that the H-R-R plates classify and quantify the type of color defect whether protan, deutran, or tritan (blue/yellow). H-R-R plates have colored symbols/shapes rather than numbers. This makes H-R-R plates a good choice for children and illiterates. Since it is capable of detecting tritan disorders, this test is especially useful when an acquired color vision defect is suspected. Lighting, viewing distance, and viewing time are the same as that of testing with Ishihara plates. The first four (non-numbered) plates of the H-R-R series are for demonstration only (similar to the Ishihara “12”). The first six (numbered) plates are screening plates. Color vision is deemed “normal” and no further testing needs to be done if the subject gives correct responses to the screening plates. If there is an incorrect response to one or more of the screening plates, the examiner must follow the directions on the scoring sheet and show additional plates to the subject in order to specifically classify the color vision defect. City University Color Vision Test The City University test (Fig. 2.3) was developed by Fletcher. It consists of 10 black charts each of which has 5 color dots. One of the dots is
  37. 37. 24 Diagnostic Procedures in Ophthalmology TABLE 2.4: INTERPRETATION AND MARKING OF THE ISHIHARA COLOR VISION TEST Number Normal Person with of plate person Person with red-green deficiency total color blindness and weakness 1 12 12 12 2 8 3 x 3 6 5 x 4 29 70 x 5 57 35 x 6 5 2 x 7 3 5 x 8 15 17 x 9 74 21 x 10 2 x x 11 6 x x 12 97 x x 13 45 x x 14 5 x x 15 7 x x 16 16 x x 17 73 x x 18 x 5 x 19 x 2 x 20 x 45 x 21 x 73 x Protan Deutan Strong Mild Strong Mild 22 26 6 (2)6 2 2(6) 23 42 2 (4)2 4 4(2) 24 35 5 (3)5 3 3(5) 25 96 6 (9)6 9 9(6) The mark x shows that the plate cannot be read. Blank space denotes that the reading is indefinite. The numerals in parenthesis show that they can be read but they are comparatively unclear
  38. 38. 25Color Vision and Color Blindness located in the center being encircled with 4 other dots so that a subject has to match the central color dot with one of the 4 other dots. American Optical Company Plates The American Optical Company (AOC) plates, a screening test for protan and deutan defects, appears to be a composite of other tests. In addition to a demonstration plate, there are 14 test plates that include 6 transformation and 8 vanishing plates. The figures are single- and double-digit Arabic numerals. There are at least two different fonts used on different plates. Five or more errors on the 14 test plates constitute failure of the test. Plates with double-digit numbers are failed if the response to either digit is incorrect. Dvorine The Dvorine is another widely used screening testforprotananddeutandefects.Thetestbooklet contains both PIC plates and a Nomenclature test, which is a unique and valuable feature of this test. The plates are presented in two sections: 15 plates with Arabic numerals and 8 plates with wandering trails, with 1 demonstration plate in each section. Any symbol missed is an error. Three or more errors in the first section constitute a failure. The Dvorine Nomenclature test is used to assess color naming ability. There are eight discs (2.54 cm in diameter) of saturated color and eight discs of unsaturated or pastel colors, which include red, brown, orange, yellow, green, blue, purple, and gray. A rotatable wheel allows the presentation of one disc at a time. Color-naming aptitude adds another dimension to a color vision assessment, and the results are appreciated by patients and employers curious to know the impact of a color defect on the ability to name colors. Tritan Plate (F-2) The Tritan plate, or F-2, is a single plate that Farnsworth designed to screen for tritan color defects. It is a good test and it can also be used for screening for red-green (protan-deutan) defects. The test is performed by a vanishing plate consisting of outlines of two interlocking squares with different chromaticities on a purple background. One square is purple-blue and vanishes for patients with the red-green defects; the other square is green-yellow and vanishes, or is seen less distinctly compared with the purple-blue square, for the tritan. Persons with normal color vision see both squares, but the green-yellow one is more distinct. Arrangement Tests Farnsworth-Munsell 100-Hue Test (Pigment Matching Test) Farnsworth-Munsell test (Fig. 2.4A) is a psycho- technical test, which quantifies a person’s ability todiscriminatehuesofpigmentcolor.Thissimple and useful test consists of 85 colored chips that are designed to approximate the minimum difference between the hues that a normal observer can distinguish (1-4 nm). Color deficient Fig. 2.4A: Farnsworth-Munsell 100-hue test
  39. 39. 26 Diagnostic Procedures in Ophthalmology Fig. 2.4B: Farnsworth-Munsell 100-hue test results from four subjects: A Normal; B Protan defects; C Deutan defects; D Tritan defects persons make characteristic errors in arranging the chips. The results are recorded on a circular graph. The greater the error arranging the chips, the farther the score is plotted from the center of the circle (Fig. 2.4B). Automated score for FM 100-hue test is also available. The currently available standard version consists of 85 knobs with pigment-colored paper on top arranged in 4 horizontal panels. Each panel has 2 knobs fixed at its 2 ends. The subject is required to arrange the knobs in each panel in such a manner that the colors of the knobs appear to be changing gradually from one end of the panel to another. Generally recommended time for arranging each panel is 2 minutes. The time spent on
  40. 40. 27Color Vision and Color Blindness arranging the each panel is recorded. Scores of a knob/cap is the sum of the differences between the number of that cap and the number of the caps adjacent to it on either side. Sum of the scores of the entire set of knob / caps goes to make the total error score (TES). Then, the scores of each knob are plotted on a circular graph. By plotting the scores in a graph, it is seen that characteristic patterns are obtained in specific defects (Fig. 2. 4 B). The test is capable of detecting all types of color deficiencies. The test results show that: 1. Average discrimination lies between 20 to 100 total error score, 2. Superior discrimination is below 20 total error score, and 3. Low discrimination is more than 100 total error score. Farnsworth D-15 Test The Farnsworth D-15 test (Fig. 2.5) consists of single box of 15 colored chips. The test can be carried out more rapidly than the 100-hue test. Viewing distance required is 50 cm or approxi- mately 20 inches. Unlimited testing time is usually allowed but the subject may be told he/ she has two minutes to complete the test in order to prevent dawdling. The object of the test is to arrange the caps in order using the fixed reference cap as a starting point. The subject is instructed to take the cap which most closely resembles the fixed reference cap, and place it next to it; then find the cap that most closely resembles the cap he just placed, and place it next to it. Once the subject has arranged all the caps, the lid is closed and the box flipped over. The examiner then scores the test based on the order in which the subject placed them (the caps are numbered on the bottom). The examiner then connects the numbers on the score sheet in the order in which the patient placed the caps. The score is either “passing” or “failing.” A circular pattern on the score sheet indicates passing, a criss-crossing or lacing pattern indicates failing. The D-15 panel uses only saturated colors, therefore, subtle defects such as those seen with an anomalous trichromat may be missed. The D-15 is useful for detecting dichromacy, in particular, tritan defects which are often associated with eye diseases and drug toxicity. The disadvantage with this test is that minor defects are not detected. Dichromatic subjects will generally form a series of parallel or criss- crossing lines with at least two lines crossing the chart in the same direction. The type of deficiency is indicated by the index line most nearly parallel to the crossover lines. Lanthony Desaturated D-15 Test The Lanthony desaturated D-15 test (Fig. 2.6) is similar to the Farnsworth D-15 except that the color on chips is much less saturated. This makes the hue circle smaller and the arrangement task more difficult. It is especially useful for detecting subtle acquired color deficiencies.Fig. 2.5: Farnsworth D-15 color test kit
  41. 41. 28 Diagnostic Procedures in Ophthalmology The Sloan Achromatopsia Test The Sloan Achromatopsia test is a matching test designed for rod monochromats described by Sloan in 1954. The test consists of seven plates, each with a different color: gray, red, yellow- red, yellow, green, purple-blue, and red-purple. Each plate includes 17 rectangular strips forming a gray scale from dark to light in 0.5 steps of the Munsell value. In the center of each rectangle is a colored disc that has the same Munsell value from one end of the gray scale to the other. The patient’s task is to identify the rectangle that matches the lightness of the colored disc. This is a difficult task for persons with normal color vision because of the color difference, but it is readily and precisely accomplished by complete achromats,whoseethecolorsasgraysofdifferent lightness. There are normative data for both persons with normal color vision and achromats. Anomaloscopes Anomaloscopes are instruments that assess the ability to make metametric matches. The results are used for definitive diagnosis and quantitative assessment of color vision status. Anomaloscopes are much more difficult to administer than pseudo-isochromatic plates and arrangement tests. The first anomaloscope was designed by Nagel and is based on the color match known as the Rayleigh equation, that is, R + G =Y. Because of their relatively high price, anomaloscopes are rarely used in private practice. Nagel Anomaloscope (Spectral Matching Test) Nagel (1970) constructed anomaloscope for studying the color vision defects. It is based on the color match known as the Rayleigh equation, that is Red (R) + Green (G) = Yellow (Y). The Nagel anomaloscope (Fig. 2.7) assesses the observer’s ability to make a specific color match. In anomaloscope, the observer is asked to match a mixture of red and green wavelengths to a yellow. This instrument consists of a source of white light, which is split into spectral colors by a prism. These colors are viewed through a telescope. The field of vision consists of a circle divided into two halves. The lower half projects a spectral Yellow (Sodium line) and this has to be matched by a mixture of Red (Lithium line) and Green (Thallium line) in the other half. The ratio of the two component lights can be controlled by press buttons on the base of the telescope on a scale of 0 – 73, where 0 is pure green, and 73 is pure red. The readings are interpreted as follows: the red/green mix Fig. 2.6: Lanthony desaturated D-15 Fig. 2.7: Nagel anomaloscope
  42. 42. 29Color Vision and Color Blindness proportions can be expressed in the form of an Anomaly Quotient (AQ). Normal observers have AQ between 0.7 and 1.4; higher AQs indicate deuteranomaly (AQ usually >1.7), whereas lower AQs indicate protanomaly. A major advantage of the Nagel anomaloscope is that it can distinguish between dichromatic and anomalous trichromatic vision by measuring the balance of red and green wavelengths in the mixture field. Pickford-Nicolson Anomaloscope The Pickford-Nicolson anomaloscope can be used for three different matches or colorimetric equations: The Rayleigh equation [R + G = Y], The Engelking equation [B + G = CY] and The Pickford - Lakowski equation [B + Y = W]. The matching field is presented on a screen for free viewing at a variety of distances, and there are no intervening optics between the patient and the matching field. The size of the field is changed by selecting different apertures: the largest is 2.54 cm (1 inch) in diameter and thesmallest,0.48cm(3/16inch).Differentcolors are obtained by inserting broadband filters. The Pickford-Lakowski equation is used to assess the consequence of senescent changes in the spectral transmission of the ocular media (yellowing of the lens), it also has value in examiningacquiredcolordefects.TheEngelking equationisusedfordiagnosisoftheblue-yellow or tritan color defects. Individual variability in densityofthemacularpigmentandlenspigmen- tation affects both the Engelking and Pickford- Lakowski equations and, accordingly, con- foundstheinterpretationofanindividualresult. Lantern Tests In marine, rail, and airline transportation, and in the armed forces, colored signals and navigational aids are extensively used. Lantern tests are performance-based, and they do not diagnose, classify, or grade the level of color vision defect. Rather, they attempt to determine whether the person is capable of performing the color signal recognition tasks with adequate proficiency to maintain safety standards. There aretwo typesof lanterntests,thosethat useactual signal light filters and those that use simulations of signal lights. Farnsworth Lantern Test (Falant) In the United States, the Farnsworth Lantern (Falant) is the standard lantern test (Fig. 2.8). It simulates marine signal lights under a variety of atmospheric conditions. Two lights are presented in a vertical display in any of the nine possiblecombinationsofthreecolors—red,green, and white—in the two positions. A subject must average eight out of nine correct responses to Fig. 2.8: Holmes-Wright Lantern
  43. 43. 30 Diagnostic Procedures in Ophthalmology pass the test. White lights are particularly problematic, especially for milder color defects. It is reported that the test is not representative of actual field conditions. Edridge-Green Lantern Test The Edridge-Green Lantern (Fig. 2.9) is an instrument used for testing the ability of a person to recognize color of transmitted light. It was builttosimulatethelightofrailwaytrafficsignals, as they are visible from a distance. The apertures represent the equivalents of five and half-inch railway signals at 600, 800 and 1000 yards, respectively when viewed from 20 feet distance. Usually two apertures 1.3 and 13 mm are used, set of filters showing signal red, yellow, green and blue colors are shown, each color being shown twice for each aperture size. Other Tests Electroretinography Use of electroretinography (ERG) in the modem era is more useful for detection of color vision deficiencies for two reasons: (i) new methods allow to separate and observe accurately the photopic and scotopic components of ERG with the possibility of better study of cone activity and (ii) with the use of computer averaging, picking up of oscillatory potentials is more easy. Microspectrophotometry In spectrophotometry, an individual cone of a dissected retina is aligned under a small spot of light and its absorption is measured at various wavelengths.ThemostdirectevidenceofYoung’s trichromatic theory (3 classes of cones) comes from spectrophotometry. The results of micro- spectrophotometry confirm three groupings with peak sensitivities at 437-458 nm, 520-542 nm and 562-583 nm. Color Vision Deficiencies and Everyday Life Many tasks depend on our ability to discriminate color. Selecting products at the grocery store, matching paint colors or items of clothing, or connecting color-coded wiring all depend on efficient color vision. Color vision deficiencies can seriously affect an individual’s ability to learn, to work at a chosen occupation and move effectively in the world. Young children are expected to learn color names early in their educational experience and color is frequently used to categorize educational materials. Good color vision is also important for students of art, chemistry, biology, geology and geography. A child with deficient color vision will have disadvantage on such tasks as Fig. 2.9: Edridge-Green Lantern The recommendations of the test state that a candidate should be rejected if he calls 1. Red as Green 2. Green as Red 3. White light as Green or Red or vice versa 4. Red-Green or White light as Black. Any candidate who makes any other errors should be tested with other test.