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

viii Diagnostic Procedures in Ophthalmology

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

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

Contributors ix

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

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 the form sense, the ability to distinguish the shape 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

Fig. 1.3: Snellen chart

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

Visual Acuity 3

Fig. 1.4: ETDRS chart

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

6 Diagnostic Procedures in Ophthalmology

Fig. 1.6: Broken C, letter E and pictures of familiar objects for testing visual acuity in illiterates and children

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 letter in the alphabet to read and “B” is considered the most difficult. To obviate this difficulty, 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, he or she is asked to move toward the chart or a chart can be brought closer. The maximum distance from which he or she recognizes the top 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 movementsarenot seenwehavetorecordwhether the 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, neuroophthalmic diseases, and retinal disorders. A patient with a low contrast threshold has a high degree of sensitivity; therefore, a healthy young subject may have a threshold of 1%, and a contrast 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.

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. Visual acuity decreases if pupils are smaller than 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.

It is worth while to discuss the role of a pinhole 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 as we proceed down the chart to record the vision.

Visual Acuity 7

A

B

Figs 1.7A and B: Occluder with multiple holes

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 with pinhole one knows the problem is a refractive 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.

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 minutes of arc (20/120) by three months. A steady but modest improvement to approximately three minutes of arc (20/60) occurs by 12 months of age. One minute of arc (20/20) is usually obtained 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.

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 lightdark transition at a border or an edge of an image that delineates the existence of a pattern or object.

Visual Acuity 9

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).

Summary

Both distance and near visual acuities are recorded for each eye with and without spectacles. 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 testing charts should be used as part of the routine assessment of the visual acuity. Visual acuity measurement is often taken for granted. Many pitfalls make this most important assessment subject to variability.10Ambient illumination, aging bulbs, dirty charts or slides, small pupils, and poorly standardized charts are just

Visual Acuity 11

some of the factors that can lead to erroneous results. A little care in ensuring the proper environment for testing can significantly improve 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.

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 explanatory attribute, for example, yellow by color is light.

Illumination

Illumination affects color vision of low illuminances, the errors increase due to poorer discrimination for most of the hue range while

Color Vision and Color Blindness 13

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. Aperture colors are colors that alter due to change 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

14Diagnostic Procedures in Ophthalmology

the card. The after image tends toward the complementary of the primary image (StilesCrawford 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 indistinguishable from each other. Three reference 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

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 referred to as long, middle, and short wavelengthsensitive (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 wavelengthsensitive (MWS) cones (“green” or “green-

Color Vision and Color Blindness 15

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 have been named opponent color cells and double 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

16Diagnostic Procedures in Ophthalmology

field. Double opponent cells are concerned with simultaneous color contrast.

Simple, complex and hypercomplex cells: In rhesus 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 arrangement to their receptive fields. Complex color coded cells respond to color boundaries of the appropriate orientation and the response is independent of the part of the receptive field being stimulated. 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 centersurround 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 (chromaticity type). Mitarai in 1961 regarded horizontal cells as responsible for S-potentials of L-type and Muller’s fibers for those of C-type. The properties of S-potentials support the Herings opponent color theory more than the trichromatic 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 confusions, 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 one of the pigments in photoreceptors. Congenital color deficits may be divided into classes according to whether the patients are red deficient (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

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 monochromatics and rod monochromatics. Blue cone monochromatics have normal blue cone pigment but no red or green cone pigment. In rod monochromatism only 500 nm pigment is present in the retina and all three cones pigments are absent.

Genetics of congenital color deficiencies

The protans and deuterons are commonly sexlinked 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 clinical 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 deficiency. In severe cases of diabetic retinopathy 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).

TABLE.2.3: DRUGS CAUSING COLOR

DEFICIENCY

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 blueyellow deficiency.

b.Turner’s syndrome: Red-green color deficiency 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 whether the color deficiency is likely to be a source 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

Color Vision and Color Blindness 19

A

B

C

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

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 pseudoisochromatic 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 not designed to detect tritan disorders or acquired color defects unless the optic neuropathy is severe.

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 comparison schemes of yarn are selected to match 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 PickfordNicolson 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 anomaloscope is the most widely used. It consists of a spectroscope in which two halves of a circular field are illuminated respectively by monochromatic 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

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 requirements for lighting, viewing distance, and viewing time. It is important for the examiner to be familiar with the test requirements and score sheets before 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. Electroretinography (ERG) and microspectrophotometry may be used in special circumstances.

Test Conditions

Lantern testing is performed after dark adaptation but all other tests require artificial daylight conditions. Light adaptation is critical for anomaloscopy 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

Color Vision and Color Blindness 21

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. Newly printed books sometimes have differential 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 a memorized response. Cheating can be prevented by covering these identifying numbers with a secret label.

Clinical Significance of the Various

Tests

Lantern testing is entirely vocational since around 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

The most common use of plate tests is to identify

22Diagnostic Procedures in Ophthalmology

persons with congenital color defects. Pseudoisochromatic 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 Achromatopsia test), to differentiate incomplete achromatopsia from complete achromatopsia (Berson blue cone monochromatism plates), to detect acquired defects (SPP-2), or to detect color 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 of the color vision defect. Plate tests are designed 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 pseudoisochromatic 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 hesitation can be a sign of a slight color deficiency.

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 not as easy or reliable to score, but they are based 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 perspective there are four different types of test plate employed in both the 38 and 24 plate series preceded by a demonstration plate that is not for scoring. In the large series plates 1 and 38 are bothfordemonstrationonly,whileinthesmaller series plates 1 and 24 are for demonstration. If the subject fails viewing the demonstration plate 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 observers give different responses to color normal 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

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 digit design cannot be seen by a normal trichromat 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 introductory 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.

Color Vision and Color Blindness 23

•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 redgreen 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

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 singleand 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 test for protan and deutan defects. The test booklet 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.

Color Vision and Color Blindness 25

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 psychotechnical test, which quantifies a person’s ability to discriminate hues of pigment color. This simple 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

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

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-

Fig. 2.5: Farnsworth D-15 color test kit

Color Vision and Color Blindness 27

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 crisscrossing 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.

28 Diagnostic Procedures in Ophthalmology

Fig. 2.6: Lanthony desaturated D-15

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, yellowred, 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, who see the colors as grays of different 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. 2.7: Nagel anomaloscope

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 the smallest, 0.48 cm (3/16 inch). Different colors 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 examining acquired color defects. The Engelking equation is used for diagnosis of the blue - yellow or tritan color defects. Individual variability in density of the macular pigment and lens pigmentation affects both the Engelking and PickfordLakowski equations and, accordingly, confounds the interpretation of an individual result.

Lantern Tests

In marine, rail, and airline transportation, and in the armed forces, colored signals and

Color Vision and Color Blindness 29

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 are two types of lantern tests, those that use actual 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 possible combinations of three colors—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

30Diagnostic 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 built to simulate the light of railway traffic signals, 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.

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.

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. The most direct evidence of Young’s trichromatic theory (3 classes of cones) comes from spectrophotometry. The results of microspectrophotometry 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

color naming, coding, and matching. Color vision testing should be done for all children as early as possible, and certainly prior to starting school.

Ifacolordeficiencyispresent,thechild’sschool, teacher, and parents should be informed so that methodsofinstructioncanbemodifiedtomeettheir visual needs. Teachers and parents can help the child in a number of different ways. First, images andutensilssuchascrayons,pencil andpenscan be labeled with words or symbols. Second, discriminationbetweenitemsofdifferentcolorcan befacilitatedbytheuseofhighluminancecontrast. Forexample,itwouldbebettertousewhitechalk on a black or green chalkboard or a dark marker onawhiteboardthancombinationsthatprovide less luminance contrast. The level of luminance contrast in colored materials can be determined quiteeasilybymakingablackandwhitephotocopy ofthemorbyconvertingthemtoblackandwhite onyourcomputer.Third,childrenshouldbetaught commonobjectsbytheirusualcolor(e.g.”bananas are normally yellow and the sky is blue”). Occupations vary in their requirement of color identification. For some, good color judgment is desirablebutnotnecessary.Forothers,knowledge of one’s color vision is critical. Examples where good color judgment can be critical for careers include a painter, safety officer, dermatologist, pharmacist,cartographer,coroner,chemist,buyer oftextiles,foodinspector,electrician,andmarine navigator. Color perception failures in such jobs could be costly, even disastrous.

Enhancing Performance with Filters

The color performance of the patients with color deficiency can be sometimes enhanced using colored filters. By absorbing wavelengths selectively, these filters help the observer to differentiate stimuli based on their relative brightness. For example, a red object viewed through a green filter or a green object viewed through a red filter will appear much darker.

Color Vision and Color Blindness 31

For example the X-chrom lens is a red contact lens worn on one eye that absorbs shorter wavelengths and passes longer ones. By comparing the relative brightness in eye with the X-chrom lens to that in the eye without it, adichromat’sabilitytodistinguishredfromgreen can be enhanced. While such monocular comparisons may be useful in specific applications, the user remains a dichromat and is unlikely to find the approach practical for everyday use.

Summary

Ophthalmic personnel are frequently asked to perform color vision testing. Knowing whether a congenital or acquired defect is suspected can help determine which color vision test should be administered. All color vision tests have specific requirements for lighting, viewing distance, viewing time, and scoring. It is important to be familiar with the various testing and scoring guidelines in order to provide the requesting doctor with accurate and useful information.

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