A healthy human retina has 3 cone types, each containing a different outer segment visual pigment: short-wavelength sensitive (S cone; formerly, blue), medium-wavelength sensitive (M cone; formerly, green), and long-wavelength sensitive (L cone; formerly, red) cones. The integrative cells in the retina and higher visual centers are organized primarily to recognize contrasts between light or colors, and the receptive fields of color-sensitive cells typically have regions that compare the intensity of red versus green or blue versus yellow.
The classification and the testing of dysfunctional color vision are based upon this contrast-recognition physiology. Red-green color deficiency, which is common in males through X-linked inheritance (6%–8% incidence), is traditionally separated into protan and deutan types, referring to absent or defective long-wavelength–sensitive or medium-wavelength–sensitive pigment, respectively. These distinctions have value in terms of patients’ perception, even though individuals with normal color vision often have a duplication of pigment genes, and individuals with color-vision deficiency may not have single or simple gene defects. Blue-yellow color deficiency is rarely inherited and can be an important early marker for acquired disease. Inherited color vision defects are described in Chapter 12.
Testing of color vision
The most accurate instrument for classifying congenital red-green color defects is the anomaloscope, but it is not widely used. The patient views a split screen and is asked to match the yellow appearance of one half by mixing varying proportions of red and green light in the other half. Individuals with red-green color deficiency use abnormal proportions of red and green to make the match.
The most common tests of color vision use colored tablets or diagrams. These tests must be performed in appropriate lighting, usually illumination that mimics sunlight. Pseudoisochromatic plates, such as the Ishihara plates (which assess color discrimination along protan [red] and deutan [green] axes only) and Hardy-Rand-Rittler plates (which also assess the tritan [blue] axis), present colored numbers or figures against a background of colored dots (Fig 3-8). The colors of both figure and background are selected from hues that are difficult for a color-deficient person to distinguish. Individuals with defective color vision see either no pattern at all or an alternative pattern based on brightness rather than hue. These tests are quick to perform and sensitive for screening color vision, but they are not effective in classifying the deficiency.
Figure 3-8 Pseudoisochromatic plates.
(Courtesy of Carl Regillo, MD.)
Panel tests, including the Farnsworth-Munsell 100 and the Farnsworth Panel D-15 hue tests, are more accurate in classifying color deficiency. The Farnsworth-Munsell 100-hue test is very sensitive because the difference in hues between adjacent tablets approximates the minimum that a typical observer can distinguish (1–4 nm). The spectrum is divided into 4 parts of 25 colored tablets each, and the patient is asked to discriminate between subtle shades of similar colors. However, the test is tiring and time-consuming.
Consisting of only 15 colored tablets, the Farnsworth Panel D-15 test (Fig 3-9) is quicker and more convenient for routine clinical use. The hues are more saturated, and they cover the spectrum so that patients will confuse colors for which they have deficient perception (such as red and green). The patient is asked to arrange the tablets in sequence, and errors can be quickly plotted to define the color deficiency. The D-15 test may miss mildly affected individuals, but it is still deemed useful because of its speed. The relative insensitivity may also be an asset in judging the practical significance of mild degrees of color deficiency. For example, individuals who fail the Ishihara plates but pass the D-15 test will probably not have color discrimination problems under most circumstances and in most occupations. Desaturated versions of the D-15 test, such as the L’Anthony D-15, which recognize more subtle degrees of color deficiency, are perhaps more clinically useful.
Figure 3-9 Panel D-15 test.
(Courtesy of Luneau Ophtalmologie.)
Individuals with major congenital color deficiencies typically show a distinct protan or deutan pattern on the D-15 scoring graph, whereas those with acquired optic nerve or retinal disease show an irregular pattern of errors. Tritan axis errors (blue-yellow confusion), which usually signify acquired disease, are readily detected using the D-15 test. Enlarged versions (PV-16 tests) are available for testing patients with reduced visual acuity.
Neitz M, Green DG, Neitz J. Visual acuity, color vision, and adaptation. In: Albert DM, Miller JW, Azar DT, Blodi BA, eds. Albert & Jakobiec’s Principles and Practice of Ophthalmology. 3rd ed. Philadelphia: Saunders; 2008:chap 123.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.