Clinical Optometric Procedures: Color Vision

Clinical Optometric Procedures: Color Vision

CHAPTER 10 – COLOR VISION

• This chapter includes a review of:
• Normal color vision
• Defective color vision
• Color vision testing

 Pseudoisochromatic Plates

• Color Arrangement Tests

INTRODUCTION

Color vision is the visual function that allows one to perceive variation among the physical wavelengths of light that compile the visible spectrum.

NORMAL COLOR VISION

Persons with normal color vision are termed trichromats. Trichromats have 3 different divisions of cone photoreceptors within the retina which contain photopigments to absorb the wavelengths of light within the visible spectrum.

• Erythrolabe refers to the photopigment that absorbs red wavelengths.
• Chlorolabe refers to the photopigment that absorbs green wavelengths.
• Cyanolabe refers to the photopigment that absorbs blue wavelengths.

All the spectral hues can be matched by an additive mixture of the three primary colors taken from the long-wave (red), medium-wave (green), and short-wave (blue) parts of the spectrum (Fig.10.1).

Figure 10.1 Colors of the rainbow viewed by an individual with normal color vision International Centre for Eyecare Education

DEFECTIVE COLOR VISION

• Defective color vision is either congenital or acquired.

Congenital Color Deficiencies

Approximately 4% of the total population has congenital color deficiencies out of which about 95% are males.

Deficiencies may present as a partial abnormality which is more common, or as a complete absence of specific cone photopigments which comprise the retinal pigment system.

• Certain abbreviations are designated to each portion of the retinal pigment system:
• Prot refers to the red or erythrolabe cone photopigment system
• Deut refers to the green or chlorolabe cone photopigment system
• Trit refers to the blue or cyanolabe cone photopigment system

There are three different types of color deficiency that are based on how many colors of the cone pigment system it takes to make a color match on an anomaloscope, a device specifically used to distinguish the various color deficiencies. The patient must adjust the ratio of red and green lights to match a yellow light in terms of hue, brightness and saturation.

• Types of Color Deficiencies

Monochromacy

Monochromats have an absence of two or all three cone photopigments. There are two types, the rod or typical monochromat and the cone or atypical monochromat. The rod monochromat has no functioning photopic system. Persons with rod monochromatism generally have macular dysfunction, nystagmus and low vision. The cone monochromat has one functioning cone photopigment of the photopic system. Persons with cone monochromatism may be further characterized as red, green or blue monochromats. The red or green monochromat has reduced but adequate visual acuity while the blue monochromat has very poor visual acuity.

All types of monochromatism which are extremely rare lead to no available color discrimination (Fig.10.2b).

A B

Figure 10.2 (a) Colors of the rainbow viewed by an individual with normal color vision; (b) Colors of the rainbow viewed by an individual with monochromatism Dichromats have an absolute defect or complete absence of one portion of the cone pigment system.

Dichromatism is further separated into Protanopia, Deuteranopia and Tritanopia. In all dichromats there is a neutral point within the spectrum. Under photopic conditions, it is the point that appears achromatic. There is no hue present at the neutral point because it is the position where the remaining 2 photopigments are balanced.

Protanopia is an abnormality of erythrolabe. There exists a major luminosity loss on the red end of the spectrum. These individuals experience color confusions or difficulty discriminating between green, yellow and red. For example, the color red is perceived as a darker color perhaps similar to brown (Fig. 10.3b). Finally, their spectral sensitivity shifts towards shorter wavelengths. International Centre for Eyecare Education A B

Figure 10.3 (a) Colors of the rainbow viewed by an individual with normal color vision; (b) Colors of the rainbow viewed by an individual with protanopia Deuteranopia is an abnormality of chlorolabe. There is a luminosity loss in the green portion of the spectrum.

These individuals experience color confusions or difficulty discriminating between green, yellow and red (Fig.10.4b). Their spectral sensitivity does not shift.

A B

Figure 10.4 (a) Colors of the rainbow viewed by an individual with normal color vision; (b) Colors of the rainbow viewed by an individual with deuteranopia Tritanopia is an abnormality of the cyanolabe. There exists a major luminosity loss on the blue end of the spectrum (Fig.10.5b). Their spectral sensitivity shifts towards longer wavelengths.

A B

Figure 10.5 (a) Colors of the rainbow viewed by an individual with normal color vision; (b) Colors of the rainbow viewed by an individual with tritanopia Anomalous trichromacy

Anomalous Trichromats have a partial defect or alteration of one portion of the cone photopigment system.

• Anomalous Trichromats are further separated into:
• Protanomalous trichromacy

 Deuteranomalous Trichromacy

• Tritanomalous Trichromacy

The colors matches of anomalous trichromats appear a little different due to the weakness but there are no confusions in color. Persons with these types of defects generally disagree on exact shades of color. International Centre for Eyecare Education Table 10.1 Classification of Congenital Color Deficiency Designation

• (based on number of cone photopigments)
• Prevalence
• Male (female) Hue Discrimination
• 1 Monochromat Typical or Rod 0.003 Absent
• Atypical, Incomplete or Cone 0.000001 Absent

2 Dichromat

Protanope 1 (0.01)

• Deuteranope 1 Severely impaired (0.01)

Tritanope 0.002

• 3 Trichromat
• Anomalous
• (2 normal; 1 defective)
• Protanope 1 (0.01) Variable impairment Deuteranope 5 (0.25)

Tritanope trace

Normal 92 (910.6) Optimum

Acquired Color Vision Deficiencies

Acquired color deficiencies are usually associated with trauma, drug toxicity or disease. These all have the potential of affecting the retina or optic nerve. Persons with acquired color deficiencies most frequently present with a blue-yellow defect. The color deficiency may be present before the patient experiences light sensitivity or a reduction in visual acuity.

COLOR VISION TESTING

Routine color vision testing serves several clinical purposes necessary to proper patient care. Firstly it aides in the diagnosis of color deficiency and secondly it allows for distinction between acquired and inherited color deficiencies. Finally, it contributes to the evaluation and quantification of retinal and optic nerve function.

The present tests work by exploiting the difficulty of color discrimination experienced by persons with color deficiencies. There are two types of tests for practical in-office use:

Pseudoisochromatic Plates

Pseudoisochromatic plate (PIP) tests requires that the patient identifies a symbol made of colored dots of varying size in a colored background. The figure and ground are chosen so that the symbol is confused by color-deficient patients. There are many types available. The most common are the Ishihara, Hardy Rand Rittler (HRR) and Dvorine tests.

• Ishihara pseudoisochromatic plates
• Instrumentation:
• Pseudoisochromatic plate test
• Procedure:
• 1. The patient’s VA must be better than 6/60 (0.1) in order to conduct the test.
• 2. Good illumination is absolutely necessary; The Macbeth Easel Lamp is the standard.
• 3. Testing is performed monocularly.
• 4. The test is held 75 cm in front of patient. International Centre for Eyecare Education
• 5. The plates must be perpendicular to visual axis.
• 6. Viewing time should be at least 3 seconds and not longer than 5 seconds.

7. The patient is asked to identify the digits on the presented plate.

8. Patients can alternatively be asked to trace the digits (or tracing patterns) using a cotton tip applicator if they are unable to identify numbers.

9. The 1st plate is a demonstration which everyone should see. Patients who fail to see the number may be misunderstanding the instructions, malingering or have a significantly decreased VA.

10. Plates 2 to 7 are transformation plates with normals seeing the correct number while color deficient patients read another.

11. Plates 8 to 13 are vanishing plates with normals seeing them but color deficient not being able to see them.

12. Plates 14 to 15 are hidden design plates with normals not being able to see a number but color deficient may.

13. Plates 16 to 17 are diagnostic plates which help determine the severity of the deficiency.

14. Plates 18 to 24 are tracing plates which may be used for individuals who are illiterate or children.

Table 10.2 Ishihara Pseudoisochromatic Score Chart

Plate Normals Red-Green Defect Total Color Blindness

• 1 12 12 12
• 2 8 3 X
• 3 29 70 X
• 4 5 2 X
• 5 3 5 X
• 6 15 17 X
• 7 74 21 X
• 8 6 X X
• 9 45 X X
• 10 5 X X
• 11 7 X X
• 12 16 X X
• 13 73 X X
• 14 X 5 X

15 X 45 X

Protan Deutan

Strong Mild Strong Mild

• 16 26 6 (2) 6 2 2 (6)

17 42 2 (4) 2 4 4 (2)

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.

• Shortened method:
• (Commonly used clinically to save time)
• The 7 left-side plates are used to test the patient’s right eye and vice versa.
• By this method 8 plates are tested on each eye (the first plate is presented to both eyes)

e.g. OD: 1,3,5,7,9,11,13,15

OS: 1,2,4,6,8,10,12,14

• If the patient identifies any plate incorrectly, then the examiner must present all 17 plates.
• Recording the findings:
• The grading criteria is defined as:
• Normal: < or = to 4 errors out of 17 plates

Failure: > or = to 5 errors out of 17 plates

• The examiner records the results by noting the # correct/total # plates followed by type of plates used. The

missed plates & numbers called may also be noted to help determine the color vision defect.

• e.g. 7/7 OD, 7/7 OS Ishihara

e.g. 12/17 OD, 11/17 OS Ishihara

• If possible, the type of color vision defect is determined and noted using the Score Chart provided (Table 10.
• 2).

Hardy Rand Rittler (HRR) test

The HRR test was developed by Hardy, Rand and Rittler and published by American Optical in 1955. Since then it has been revised several times with the currently distributed version being the Richmond HRR test.

• The HRR has been designed to:
• Screen individuals for defective or normal color vision.
• Provide a qualitative assessment color defect by detecting the type of defect as being either protan, deutan

or tritan.

• Provide a quantitative assessment of the color defect by grading the degree of the color defect as being mild,

medium or strong.

While the Ishihara test has probably been the most widely used tests to screen for protan and deutan deficiencies, it is unable to provide an assessment of the blue (tritan) color defects. The HRR test on the other hand while using the same principles of testing as the Ishihara test is slightly superior in its ability to detect protan, deutan and tritan defects. Only moderate or severe tritan defects may be identified. While the grading plates are effective, the test is unable to distinguish dichromats and severe anomalous trichromats.

• Instrumentation:
• HRR pseudoisochromatic plates
• Laminated scoring sheet
• Camel’s hair brush (to avoid patient’s touching the color plates)
• Procedure:

1. The test set up and requirements are similar to that of the Ishihara test.

2. The HRR test comprises of 24 plates, with each plate containing 1 or 2 symbols which can be either cross, circle or a triangle. The symbols comprise colored dots which appear on a background of grey dots.

3. The 1st plate is a demonstration which everyone should see. Patients who fail to see the number may be misunderstanding the instructions, malingering or have a significantly decreased VA.

4. There are 4 demonstration plates with the symbols being seen by all individuals, including those with color deficiencies. One of the plates does not have a symbol so that the patient understands that there may be a plate on which they may not see a symbol.

5. There are 6 screening plates, 4 for protan-deutan deficiencies and 2 for tritan deficiencies. The colors of the symbols lie on the protan, deutan and tritan achromatic confusion loci. Patients with severe specific deficiencies will not see the symbols with colors lying on the specific confusion loci, however, will see other symbols which do not lie on the other confusion loci.

6. There are 14 diagnostic plates which help to grade the severity of the deficiency. Ten plates are for protan, deutan defects, and 4 plates for tritan defects.

Color Arrangement Tests

The most common are the Farnsworth D-15 and Farnsworth-Munsell 100 Hue. They are generally used once the pseudoisochromatic plates are failed or if the examiner would like to further qualify a color deficiency.

Arrangement tests classify color vision deficits as protan, deutan or tritan, but they are unable to distinguish dichromats from anomalous trichromats. A score sheet is used to record the order of the patient’s arrangement of hues in relation to their correct sequence. Once the sequence is recorded it is then placed on an analysis graph.

The color deficiency will parallel the axis of color confusion.

Farnsworth D-15

The Farnsworth D-15 consists of 16 colored caps. The 16 different colors are selected from the Munsell system color circle. The chosen color samples are equally spaced around the Munsell system color circle. Therefore, distinction between bordering caps is quite clear to the person with normal color vision. Persons with severe color vision defects have major color confusions and will make significant arrangement errors when performing the test.

Persons with mild color vision defects have minor color confusions and will generally not make significant arrangement errors when performing the test. Consequently, the Farnsworth D-15 is used to differentiate between severe color deficiencies in which the patient fails and mild to moderate color deficiencies in which the patient passes. International Centre for Eyecare Education Figure 10.6 Recording sheet for the Farnsworth D-15 color vision test Lantony’s Desaturated D-15 test according to Farnsworh is also available. It uses much paler colors so it is more sensitive to subtle or acquired defects. It is performed and interpreted similar to the standard Farnsworth D-15. In addition to the Protan, Deutan and Tritan axis of the standard Farnsworth D-15 analysis graph, the Desaturated D-15 graph contains a Tetartran axis which may be affected in acquired pathology and a Scotopic Axis. The Lantony’s Desaturated D-15 test is intended for use in conjunction with the standard Farnsworth D-15 for comparison.

The Farnsworth D-15 colour vision test is one that is used more frequently in clinical practice and is actually a condensed version of the Farnsworth-Munsell 100 Hue test. It is more commonly used since it is less time consuming than the Farnsworth-Munsell 100 Hue test, however, if defects are noted on this test and more sensitive testing is required, then the Farnsworth-Munsell 100 Hue test is the test of choice.

• Procedure:

1. The test is performed monocularly.

2. The patient is seated with caps placed 50 cm away & views the caps at a 60 angle. The proper light source (preferably a Mac Beth Easel Lamp) illuminates from above the test.

3. The patient is to select the cap that looks the most like the fixed cap (reference cap) and place it next to it in the box. The patient then selects from the remaining caps the one that looks the closest to the one just placed in the box and continue until all caps are in the box. There is no time limit imposed for this test.

4. The examiner scores the test by recording the cap sequence on a standard recording sheet (Fig. 10.6). A failure is recorded as 2 crossings on the diagram. Crossings require the caps to be placed at least 4 numbers apart. Once the graph is plotted, the examiner must determine which index line is closely parallel to the patient’s error crossing(s). International Centre for Eyecare Education

Farnsworth-Munsell 100 Hue

The Farnsworth-Munsell 100 Hue test consists of 85 colored caps, in addition to the 15 from the D-15, which are placed in 4 separate boxes. Both the first and last cap of each box sequence is fixed. The 85 different colors are selected from the Munsell system color circle. The chosen color samples are spaced uniformly around the Munsell system color circle. The distance between bordering colors is quite small. Since there is only a slight variance in colors, the test allows for fine color discrimination. The Farnsworth-Munsell 100 Hue test is a more sensitive test for mild and moderate congenital color deficiencies and early detection & classification of acquired color deficiencies due to ocular disease. Since it is a lengthy exam, it is used as a specialty test in specific clinical situations. International Centre for Eyecare Education

SECTION B – VISUAL FUNCTION ASSESSMENT