What is achromatopsia?
Congenital achromatopsia is a rare hereditary vision disorder which affects 1 person in 33,000 in the U.S. The incidence varies in different parts of the world.
Persons who have achromatopsia (sometimes called achromatopia) do not have normal "cone vision." In the retinas of normal eyes there are 6 million cone photoreceptors, located mostly at the center of the retina. There are complete and incomplete forms of achromatopsia. Persons with complete achromatopsia must rely on their "rod vision." In the normal eye there are 100 million rod photoreceptors. Rods are located mostly at the periphery of the retina. Rods "saturate" at higher levels of illumination. Therefore, the eyes of achromats, lacking normal cone vision and having only rod vision, are not able to adapt normally to higher levels of illumination. Rods do not provide color vision or good detail vision. Therefore, persons with achromatopsia are either totally colorblind or almost totally colorblind, and they have poor visual acuity. There are many variations in the severity of these manifestations among individual achromats. There are complete rod monochromats, incomplete rod monochromats, and blue cone monochromats. Complete rod monochromats have the most severely impaired vision of all achromats. Blue cone monochromacy is much rarer than rod monochromacy and has entirely different inheritance factors.
Various diagnostic terms have been applied to patients who have congenital achromatopsia. Some of these terms have been applied inaccurately and some of the terms simply demonstrate the fact that the nomenclature in use can vary considerably among different vision care specialists and in different parts of the world. Sometimes a patient with rod monochromacy is diagnosed as having cone dystrophy, because some vision care professionals are accustomed to thinking of all patients who have inherited disorders of the cones as having cone dystrophy. Congenital achromatopsia, however, should never be confused with progressive cone dystrophy or cone/rod dystrophy. Congenital achromatopsia is not progressive, and it does not lead to blindness. Some doctors refer to it as "stationary cone dystrophy."
Misdiagnosis is common. As one example, many achromats have been given the diagnosis of "congenital nystagmus." Nystagmus (involuntary movement of the eyes) is a symptom of achromatopsia, one that is especially noticeable during infancy and childhood, but having this symptom is not the same as having the medical eye condition which is known as "congenital nystagmus."
As levels of illumination increase, the vision of persons with achromatopsia decreases. 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, shielding their eyes, or positioning themselves favorably in relation to light sources. Others routinely wear medium tinted lenses in such settings. However, in full sunlight outdoors or in very bright indoor spaces, almost all achromats use very dark tinted lenses in order to function with a reasonable amount of vision, since their retinas do not possess the photoreceptors needed for seeing well in such settings.
Public awareness of achromatopsia has greatly increased in recent years as a result of the publication of Dr. Oliver Sacks' book, The Island of the Colorblind (hardback edition, Alfred Knopf publisher; paperback edition, Vintage Press publisher -- in the U.S.) and the TV documentary film, "Island of the Colorblind," which has been shown since 1996 in the U.K. and since 1998 in the U.S. and certain parts of Canada. Dr. Sacks usually refers to achromatopsia as "achromatopia," and he refers to a person having this vision disorder as an "achromatope." These terms are variations of the more commonly used terms, "achromatopsia" and "achromat."
The books that are described on The Achromatopsia Network website present a substantial amount of information about many topics of importance to persons affected by achromatopsia, including the hereditary factors, symptoms and manifestations of achromatopsia, ways of coping with achromatopsia, the factors which influence the choices achromats make in terms of tinted lenses, and much more. For example, to cope with their hypersensitivity to light outdoors, some networkers wear very dark amber lenses or very dark brown lenses, some wear deep red lenses, and some wear very dark gray lenses. Some wear wraparound sunglasses, some wear lenses mounted in standard frames with opaque side shields attached, and others wear tinted contact lenses, which are supplemented outdoors with sunglasses.
Congenital, inherited achromatopsia should not be confused with cerebral achromatopsia, which is an acquired form of total colorblindness that can result from trauma, illness, or some other cause. Persons who develop cerebral achromatopsia report that they see a monochromatic world, all in shades of gray. They are able to see gray because they previously experienced color vision, making it possible for them to perceive the absence of color as gray. This is in sharp contrast to the visual perception of congenital, complete achromats (i.e., complete rod monochromats), who report that the concept of "gray" is as mystifying to them as is the concept of any of the other colors. Persons with cerebral achromatopsia are diagnosed by neurologists, rather than eye specialists. Their loss of color perception is not accompanied by severely impaired vision, extreme light sensitivity, or abnormality in the photoreceptors of the retina, as is the case with persons who have congenital, inherited achromatopsia.
This information was developed by The Achromatopsia Network and is herewith used with permission.
The Achromatopsia Network. What is Achromatopsia? Available at: http://www.achromat.org/what_is_achromatopsia.html. Last accessed November 12, 2017.
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