Congenital achromatopsia: An hereditary disorder of vision characterized by a lack of cone vision (sight provided by the cone photoreceptors in the retina). People with achromatopsia (achromats) are completely or almost completely colorblind. They have poor visual acuity, and their eyes do not adapt normally to higher levels of illumination and are very light sensitive (photophobic). At higher levels of illumination, the vision of achromats decreases unless they make use of tinted lenses. In moderately bright indoor spaces or outdoors just after dawn or just before dusk, some achromats adapt to their reduced level of visual functioning without resorting to tinted lenses, by using visual strategies such as blinking, squinting, or positioning themselves in relation to the light source. Others routinely wear medium tinted lenses in such settings. In full sunlight outdoors or in very bright indoor spaces, almost all achromats use very dark tinted lenses to have a reasonable amount of vision, since their retinas do not possess the photoreceptors needed for seeing well in such settings.

In normal eyes there are some 6 million cone photoreceptors that are located mainly in the center of the retina. Lacking cones, persons with achromatopsia have to rely on their rod photoreceptors for vision. In the normal eye there are some 100 million rod photoreceptors. Rods are located mostly at the periphery of the retina. Rods saturate at higher levels of illumination. They do not provide color vision or good detail vision.

There are many degrees of severity of symptoms among achromats. Of all achromats, those who are complete rod monochromats have the most severely impaired vision. There are also incomplete rod monochromats and blue cone monochromats who are less severely affected.

Achromatopsia is the subject of the book "The Island of the Colorblind" by the neurologist/writer Oliver Sacks (Alfred Knopf/Vintage Press publisher -- in the U.S.). Dr. Sacks also did a TV documentary film, "Island of the Colour Blind." The form of chromatopsia described by Dr. Sacks in Pingelapese people of the eastern Caroline Islands in the Pacific is known as achromatopsia 3 (ACHM3), Pingalese blindness, total colorblindness with myopia and achromatopsia with myopia. From 4 to 10% of Pingelapese people from infancy have a severe ocular abnormality manifested by horizontal nystagmus (recurrent flickering back-and-forth eye movements), photophobia, amaurosis, colorblindness, severe myopia (nearsightedness) and gradually developing cataract. This condition is due to recessive inheritance of a gene that has been mapped to chromosome 8q21-q22. The same gene has been identified in the Irish and it has been suggested that sailors of English/Irish descent may have introduced the 8q gene into the South Pacific Island of Pingelap. The high gene frequency has been attributed to reduction of the population there to about 9 surviving males by a typhoon (about 1780), combined with subsequent isolation. This phenomenon is called founder effect, the growth of a population from a relative small number of founding fathers (and mothers).

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