(blīnd'nĕs )

1. Inability to see. The leading causes of blindness in the U.S. are age-related macular degeneration, diabetic retinopathy, and glaucoma. Incidence: The prevalence of blindness in wealthy nations like the U.S. increases with aging. According to the National Eye Institute, the prevalence is less than 1% until a person reaches his or her 70s. Seven percent of the U.S. population over the age of 80 is blind.Causes: 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 neonatal conjunctivitis, trachoma, and onchocerciasis are relatively common causes of severe visual impairment. In malnourished people, vitamin A deficiency is an important cause of blindness. Retinopathy of prematurity is an important cause of blindness in neonates in both resource-replete and impoverished nations.Symptoms and Signs: The symptoms and signs of visual impairment include blurring of vision, difficulty reading, and inability to see in bright light or in darkness.Diagnosis: Blindness is identified with measurements of both visual acuity, e.g., with a Snellen vision test, and peripheral vision (with formal visual field testing). On Snellen testing, a visual acuity of less than 20/200 in the better functioning eye defines legal blindness.Treatment: Treatments for blindness depend on its underlying causes. Refractive errors, e.g., myopia or astigmatism, can be corrected with eyeglasses, which focus light on the retina; blindness resulting from opacification of the lens is surgically treatable by removing cataracts; and primary open-angle glaucoma is treatable with topical medications.Patient Care: A variety of free services are available for the blind and physically hand icapped. Talking Books Topics, published bimonthly in large-print, cassette, and disk formats, is distributed free to the blind and physically hand icapped 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.

color b.1A genetic or acquired abnormality of color perception. Complete color blindness (achromatopsia) is rare. Red-green color blindness, which affects about 8% of the male population, is an X-linked trait. Color blindness is the term most commonly used, but it is inaccurate: color deficiency and color vision deficiency are preferred.

SEE: illus.

cortical b.Blindness due to lesions in the left and right occipital lobes of the brain. The eyes are still able to move, and the pupillary light reflexes remain, but the blindness is as if the optic nerves had been severed. The usual cause is occlusion of the posterior cerebral arteries. Transitory cortical blindness may follow head injury. SYN: cerebral visual impairment.

day b.Hemeralopia.

eclipse b.Blindness due to burning the macula while viewing a solar eclipse without using protective lenses. Looking directly at the sun at any time can damage the eyes. SYN: solar blindness; solar maculopathy.

functional b.Blindness caused by psychological disorders rather than by demonstrable organic pathology. It is also inaccurately called hysterical blindness.

hysterical b.

SEE: functional blindness.

legal b.A degree of loss of visual acuity that prevents a person from performing work requiring eyesight. In the U.S., legal blindness is defined as corrected visual acuity of 20/200 or less, or a visual field of 20° or less in the better eye. In the U.S., there are about three quarters of a million blind people, and about 8 or 9 million people with significant visual impairment.

letter b.A form of aphasia marked by an inability to understand the meaning of letters.

night b.Nyctalopia (1).

object b.A disorder in which the brain fails to recognize things even though the eyes function normally.

SEE: apraxia.

psychic b.Sight without recognition, caused by a brain lesion.

red-green b.Red-green color blindness

red-green color b.Inability to see red hues. It is the most common kind of color blindness.SYN: red-green blindness.

river b.

SEE: onchocerciasis.

snow b.Blindness, usually temporary, due to the glare of sunlight on snow. It may result in photophobia and conjunctivitis, the latter resulting from the effects of exposure to ultraviolet radiation.

solar b.Eclipse blindness.

taste b.An inability to taste certain substances such as phenylthiocarbamide (extremely bitter). This inability is attributable to an autosomal recessive trait.

transient monocular b.A temporary loss of vision affecting one eye. In older adults, it is usually a form of transient ischemic attack, caused by carotid atherosclerosis, and is therefore a harbinger of stroke. In young adults, it may be caused by migraine.SYN: amaurosis fugax.

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.