achromatopsia

   Also referred to as monochromatism, monochromatopsia, and total colour blindness. The term achromatopsia comes from the Greek words achromatos (colourless) and opsis (seeing). It refers to the inability or strongly diminished ability to perceive colour. Traditionally two types of achromatopsia are distinguished, called typical achromatopsia (or rod monochromatism) and atypical achromatopsia (also referred to as incomplete achromatopsia or cone monochro-matism). Typical achromatopsia is attributed to the congenital absence of normal cones in the retina, forcing the affected individual to depend on the rods for all visual sense perception. Individuals with typical achromatopsia are believed to see all things in shades of grey. They are therefore referred to as (typical) achromats or 'totally colour blind' persons. Typical achromatopsia is associated with photophobia, low visual acuity, and nystagmus. Atypical achromatopsia, on the other hand, is attributed to the presence of rods plus one type of cone (instead of the three types present in individuals with normal trichromatism), entailing a rudimentary type of colour vision, typically in a single hue (i.e. red, green, or blue). Additional symptoms such as low visual acuity and nystagmus tend be absent in cone monochromatism. Full-field achromatopsia, with or without visual agnosia, is rare. Its lifetime prevalence has been estimated at around 1 in 100,000. Etiologically, achromatopsia is associated with a variety of peripheral and central conditions. Although various accurate empirical studies of the centre for colour vision in the CNS have been available from the 1880s onwards, achromatopsia has long been associated either with peripheral conditions such as a lack of retinal cones, retinal trauma, and optic neuritis or with toxic disturbances such as lead poisoning and carbon disulphide poisoning. The notion of a central aetiology of achromatopsia was introduced by the Swiss ophthalmologist Louis Ver-rey (1854-1916). Today cerebral achromatopsia is conceptualized as a severe or absolute loss of colour perception due to a lesion affecting the lingual and fusiform gyri of the cerebral cortex. This variant of a chromatopsia is believed to affect no more than 1 in 10 million people. As colour processing takes place in both hemispheres, fullblown cerebral achromatopsia always involves bilateral cerebral damage. When the lesions extend into the calcarine cortex or the optic radiations, the condition may be accompanied by superior * quadrantanopsia. A variant of cerebral achromatopsia in which half the visual field is seen in shades of grey and the other half in colour is referred to as * hemiachromatopsia. The term * dyschromatopsia is used to refer to all instances of defective or residual colour perception. Both hemiachromatopsia and hemidyschromatopsia appear to be more prevalent than full-field achromatopsia. They are probably under-reported, however, as they frequently go unnoticed by both doctor and patient. The term pseudoachromatop-sia is used to denote an individual's failure to perform well on colour vision tests, due to conditions other than achromatopsia proper (such as visuo-spatial neglect and amnesic colour blindness). Conceptually, achromatopsia constitutes a subclass of the group of * colour vision deficiencies. The term is used in opposition to the terms * normal trichromatism, * anomalous trichroma-tism, * dichromatism, and - from a different vantage point - also in opposition to the notion of a * selective sparing of colour vision. While there would seem to be a certain phenomenological overlap between achromatopsia and conditions such as * scieropia and * scierneuropsia, it is as yet unclear whether these conditions are also related in a pathophysiological sense.

   References

   Hsia, Y., Graham, C.H. (1965). Color blindness. In: Vision and visualperception.EditedbyGra-ham, C.H. New York, NY: John Wiley & Sons.

   Zeki, S. (1990). A century of cerebral achromatopsia. Brain, 113, 1721-1777.

   McIntyre, D. (2002). Colour blindness. Causesand effects. Chester, PA: Dalton Publishing.