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1. Inability to see. The leading causes of blindness in the U.S. are age-related macular degeneration, diabetic retinopathy, and glaucoma.

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.

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.

The symptoms and signs of visual impairment include blurring of vision, difficulty reading, and inability to see in bright light or in darkness.

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.

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.

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