Legal blindness, described in the United States as vision less than 20/200 in the better seeing eye, affects 3.3 million Americans over age 40, according to the Centers for Disease Control. Many more lose vision only in one eye. A number of conditions cause vision loss in just one eye.
Amblyopia
People with amblyopia lose vision in one eye because the brain "turns off" signals from that eye to the brain. Refraction issues such as nearsightedness, farsightedness and strabismus, an imbalance in the way the eyes are positioned, can cause amblyopia, according to the CDC. Amblyopia, also called lazy eye, according to the Mayo Clinic, is the most common cause of lost vision in children, affecting two or three out of 100. Patching the eye with better vision to force the brain to use the weaker eye, corrective glasses and, in some cases, surgery, prevent permanent vision loss if amblyopia is diagnosed and treated early.
Traumatic Injury
Traumatic injury to one eye often causes blindness in just one eye. Penetrating trauma, blunt trauma, chemical splashes, corneal abrasions, or scratches and infection, or inflammation after injury can all cause blindness in one eye, according to All About Vision. Any injury to the eye needs immediate medical evaluation and treatment to prevent possible complete vision loss in the eye.
Age-Related Wet Macular Degeneration
Age-related wet macular degeneration, or AMD, a condition where abnormal blood vessels--that leak and swell--form in the retina, often occurs in just one eye, although the better eye needs frequent evaluation to detect signs of early disease. Dry AMD, the early form of the disease, most often affects both eyes. Wet AMD, the more serious form of the disease, affects 1.8 million Americans over age 40, and is the leading cause of vision impairment in people over age 65, according to the CDC.
Cancer
More than 2,300 new cases of eye cancer were diagnosed in 2009, according to the American Cancer Society. In adults, nine out of 10 eye cancers are melanomas, most of which arise from a nevus, a small mole or freckle, inside the eye. Treatments for eye cancer, which include laser, radiation or removal of part or all of the eye, often result in blindness in one eye. Retinoblastoma, an eye cancer usually found in children under age 5, usually affects only one eye unless the child has an inherited form of the disease, according to the National Cancer Institute. Treatment for retinoblastoma, which may include removal of the eye, radiation, cryotherapy or chemotherapy, often leads to blindness in one eye.
Cataracts
A cataract, a clouding of the lens of the eye, can affect just one eye and lead to blindness in that eye if not surgically removed. About 20 million Americans have cataracts, according to the CDC.
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