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Depending on its brightness, light is perceived by either of two sets of visual cells located in the retina of the eye. One set, the cones, perceive bright light primarily; the other set, the rods, perceive dim light primarily. Dim light produces a change in a pigment called rhodopsin in the rods. This change causes nerve impulses to travel to the brain, where they register as visual impressions. Night blindness occurs when the rods lack rhodopsin.
One cause of night blindness is a deficiency of vitamin A—the primary source of rhodopsin. The defect in vision usually can be cured by proper diet plus therapeutic doses of the deficient vitamin.
In the elderly, there is sometimes a diminution of rhodopsin, with resulting night blindness. Other losses in vision may follow. Diminished blood supply to the eyes is thought to be a cause of this form of the condition. Treatment generally is only of limited effectiveness.
Night blindness sometimes accompanies glaucoma.
Synonym(s): night blindness.
Blindness may be caused by diseases of the lens, retina, or other eye structures; diseases of the optic nerve; or lesions of the visual cortex or pathways of the brain. A small number of infants are born blind, but far more people become blind during life. In the U.S., blindness due to infection is rare, but worldwide diseases like trachoma and onchocerciasis are relatively common causes of severe visual impairment. In malnourished people, vitamin A deficiency is an important cause of blindness.
A variety of free services are available for the blind and physically handicapped. Talking Books Topics, published bimonthly in large-print, cassette, and disc formats, is distributed free to the blind and physically handicapped who participate in the Library of Congress free reading program. It lists recorded books and magazines available through a national network of cooperating libraries and provides news of developments and activities in library services. Subscription requests may be sent to Talking Books Topics, CMLS, P.O. Box 9150, Melbourne, FL 32902-9150.
amnesic color blindness
night blindnessNyctalopia (1).
red-green blindnessRed-green color blindness
red-green color blindness
river blindnessSee: onchocerciasis
solar blindnessEclipse blindness.
transient monocular blindness
In older adults, causes of carotid atherosclerosis include smoking, diabetes mellitus, hypertension, obesity, and hypercholesterolemia. When atherosclerotic plaques form within the carotid artery, they may ulcerate. The exposed endothelium within the artery becomes a focus of inflammation and blood clotting. Blindness occurs when tiny clots from the carotid arteries embolize to the ophthalmic arteries.
Patients often describe a dark shade descending into the field of vision. At the same time they may have other stroke symptoms, e.g., difficulty with speech or weakness of the hand on the side opposite the affected eye.
A patient who may have carotid atherosclerosis should begin taking aspirin or other antiplatelet drugs if these are tolerated. Blood pressure and lipid levels should be controlled. The patient should be referred for noninvasive evaluation of blood flow through the carotid arteries, e.g., ultrasonography. If the carotid arteries are significantly blocked, the patient and physician should consider the risks and benefits of carotid endarterectomy.