Loss of vision can occur suddenly or develop gradually over time. Vision loss may be complete (involving both eyes) or partial, involving only one eye or even certain parts of the visual field. Vision loss is different from blindness that was present at birth, and this article is concerned with causes of vision loss in an individual who previously had normal vision. Vision loss can also be considered as loss of sight that cannot be corrected to a normal level with eyeglasses. The causes of loss of vision are extremely varied and range from conditions affecting the eyes to conditions affecting the visual processing centers in the brain. Impaired vision becomes more common with age. Common causes of vision loss in the elderly include diabetic retinopathy, glaucoma, age-related macular degeneration, and cataracts.

Source: http://www.rxlist.com