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Achromatopsia (ACHM), is a medical syndrome that exhibits symptoms relating to at least five separate individual diseases. Although the term may refer to acquired disorders such as color agnosia and cerebral achromatopsia, it typically refers to an autosomal recessive congenital color vision disorder, the inability to perceive color AND to achieve satisfactory visual acuity at high light levels (typically exterior daylight). The syndrome is also present in an incomplete form which is more properly defined as dyschromatopsia. The only estimate of its relative occurrence of 1:30,000 in the general population dates from the 1960s or earlier.
There is some discussion as to whether achromats can see color or not. As illustrated in “The Island of the Colorblind” by Oliver Sacks, some achromats cannot see color. With 5 different genes currently known to cause similar symptoms, it may be that some do see marginal levels of color differentiation due to different gene characteristics. With such small sample sizes and low response rates, it is difficult to accurately diagnose the ‘typical achromatic conditions’. If the light level during testing is optimized for them, they may achieve corrected visual acuity of 20/100 to 20/150 at lower light levels. One common trait is hemeralopia or blindness in full sun.
The five symptoms associated with achromatopsia/dyschromatopsia are:
A sixth symptom associated with achromatopsia/dychromatopsia is seldom reported. Many sufferers are unaware of the three-dimensional aspect of their visual system. They frequently fail to observe any of the stereographic features of a scene.
The syndrome of achromatopsia/dyschromatopsia is poorly described in current medical and neuro-ophthalmological texts. It became a common term following the popular book by the neuroscientist Oliver Sacks, “The Island of the Colorblind,” in 1997. Up to that time most color-blind subjects were described as achromats or achromatopes. Those with a lesser degree of color perception abnormality were described as either protanopes, deuteranopes or tetartanopes (historically tritanopes).
Achromatopsia has also been called rod monochromacy and total congenital color blindness. Individuals with the congenital form of this disorder show complete absence of cone cell activity via electroretinography at high light levels. There are at least four genetic causes of congenital ACHM, two of which involve cyclic nucleotide-gated ion channels (ACHM2/ACHM3), a third involves the cone photoreceptor transducin (GNAT2, ACHM4), and the last remains unknown.
The syndrome is frequently noticed first in children around six months of age by their photophobic activity and/or their nystagmus. The nystagmus becomes less noticeable with age but the other symptoms of the syndrome become more relevant as school age approaches. Visual acuity and stability of the eye motions generally improve during the first 6-7 years of life (but remain near 20/200). The congenital forms of the disease are considered stationary and do not worsen with age.
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