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’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

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

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 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 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

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- ments 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. Electroretino- graphy (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 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. 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

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- green defects (efficiency 90-95%). Other tests have 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 with children and persons who are illiterate. They are only suitable for screening purpose, however, they neither provide a quantitative evaluation of color vision nor distinguish the type and severity of the color vision defect. Plate tests are designed to distinguish congenital color-defective from color-normal observers, but they do not evaluate the wide range of abilities and aptitudes of observers with normal color vision to distinguish colors. Given individual differences in prereceptoral filters and normal photo pigment polymorphisms, no plate test can be 100% effective in screening. When used improperly (nonstandard illuminant, binocular viewing, colored lenses not removed from observer), 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 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 meandering path of connected dots between two 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 perspec- tive 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 both for demonstration only, while in the smaller 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

The plates are designed to be appreciated correctly in a room which is lit adequately by daylight. Introduction of direct sunlight or the use of electric light may produce some discrepancy in the results 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. The plates are held 75 cm from the subject and tilted at right angles to the line of vision. A missed/ misread plate must be reread (may be in a random order). The findings should be recorded on the Ishihara color vision test and interpretation 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