Patients may present with color vision defects, or dyschromatopsia, that are either static or progressive. Static dyschromatopsia is usually, although not always, congenital. Whereas congenital color vision defects are stationary and usually affect both eyes equally, acquired color vision defects may be progressive and/or uniocular. This chapter describes the nonprogressive forms of color vision loss. For further discussion of optic neuropathies that may lead to acquired forms of vision loss, please refer to BCSC Section 5, Neuro-Ophthalmology.
Congenital Color Deficiency
Congenital color vision defects are traditionally classified by an individual’s performance on color-matching tests. A person with normal color vision (trichromatism, or trichromacy) can match any colored light by varying a mixture of 3 different-colored lights, or primary colors (eg, a long-wavelength red, middle-wavelength green, and short-wavelength blue light).
Individuals with anomalous trichromatism make up the largest group of color-deficient persons; they include approximately 5%–6% of males. These individuals can also use 3 primary colors to match a given color; however, because 1 of their cone photopigments has an abnormal absorption spectrum, they use different proportions of colors than those used by persons with normal color vision. Anomalous trichromacy ranges in severity. Some individuals have only a mild abnormality and may, for example, fail some of the subtle Ishihara test plates but have no trouble naming colors or passing the less-sensitive screening tests such as the Farnsworth Panel D-15 hue test (see Chapter 3 in this volume). Others have poor color discrimination and may appear to have dichromacy on some of the color vision tests.
Hereditary congenital color vision defects are most frequently X-linked-recessive, red-green abnormalities; they affect 5%–8% of males and 0.5% of females. Acquired defects are more frequently of the blue-yellow, or tritan, variety and affect males and females equally. Table 12-1 shows the traditional classification of color vision deficits based on color-matching test results.
Table 12-1 Classification and Male-Population Incidence of Color Vision Defects
Individuals who need only 2 primary colors to make a color match have dichromacy. It is assumed that such individuals lack 1 of the cone photopigments. Approximately 2% of males have dichromacy.
An absence of color discrimination, or achromatopsia, means that any spectral color can be matched with any other solely by intensity adjustments. The congenital achromatopsias are disorders of photoreceptor function. Essentially, there are 2 forms of achromatopsia: (1) rod monochromatism, and (2) S-cone monochromatism (blue-cone monochromatism). Both disorders typically present with congenital nystagmus, poor visual acuity, and photophobia. Electroretinography (ERG) testing helps differentiate achromatopsia from congenital motor nystagmus or ocular albinism, both of which are associated with normal cone ERGs (see Chapter 3, Fig 3-2).
Rod monochromatism (complete achromatopsia) is the most severe form; affected individuals have normal rod function but no detectable cone function and see the world in shades of gray. Patients may have full to partial expression of the disorder, with visual acuity ranging from 20/80 to 20/200. Nystagmus is usually present in childhood and may improve with age. Characteristically, the ERG pattern in patients with rod monochromatism shows an absence of cone-derived responses and normal rod responses. Dark adaptometry shows no cone plateau and no cone–rod break. The disorder exhibits autosomal-recessive inheritance.
In S-cone (or blue-cone) monochromatism, the function of rods and S cones is normal, but L- and M-cone function is absent. The condition is usually X-linked and can be difficult to distinguish clinically from rod monochromatism in the absence of a family history or results from specialized color or ERG testing. Individuals with S-cone monochromatism exhibit preserved S-cone ERG responses, severely reduced cone flicker ERGs, and normal rod ERGs. These individuals typically have a visual acuity of approximately 20/80, which is better than the visual acuity found in individuals with typical rod monochromatism.
Dingcai C. Color vision and night vision. In: Schachat AP, Wilkinson CP, Hinton DR, Sadda SR, Wiedemann P, eds. Ryan’s Retina. Vol 1. 6th ed. Philadelphia: Elsevier/Saunders; 2018: 325–339.
Wu DM, Amani AF. Abnormalities of cone and rod function. In: Schachat AP, Wilkinson CP, Hinton DR, Sadda SR, Wiedemann P, eds. Ryan’s Retina. Vol 2. 6th ed. Philadelphia: Elsevier/Saunders; 2018:1006–1017.
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.