Vision loss is most accurately described in terms of a range of function, from the ability to see relatively clearly to diminishing eyesight levels and total blindness. The World Health Organization (WHO) specifies four levels of visual function: normal, moderate visual impairment, severe visual impairment, and blindness.
With moderate to severe vision impairment, a person’s eyesight cannot be corrected to a normal level, even with eyeglasses, contact lenses, medicine, or surgery. As described by the U.S. Centers for Disease Control, such impairment can be distinguished by a loss of visual acuity, where the eye does not see objects as clearly as usual, or a loss of visual field, where the eye cannot see as wide an area as usual without moving the eyes or turning the head. In the United States, legal blindness is defined as a visual acuity of 20/200 (normally 20/20) or worse with the best possible correction, or a visual field of 20 degrees (normally 160 to 170 degrees) or less.
The latest WHO figures state about 314 million people worldwide are visually impaired; 45 million are blind. Common causes of vision loss and blindness include:* Cataract (a clouding of the eye lens that hinders passage of light)* Uncorrected refractive errors (near-sightedness, far-sightedness, astigmatism (abnormal curvature of the cornea), or presbyopia (a focusing problem that makes it difficult to see near objects))* Glaucoma (a slow build-up of fluids in the eye producing excessive pressure that damages the optic nerve)* Macular degeneration (a breakdown of the retina, which involves loss of the central field of vision)* Diabetic retinopathy (changes in the blood vessels of the light-sensitive retina tissue in the back of the eye)* Retinitis pigmentosa (a hereditary, progressive loss of vision, often beginning with “night blindness,” with narrowing visual field leading to “tunnel vision” and, frequently, total blindness)
Other causes of blindness include stroke, accidents or injury to the eye, blocked blood vessels, optic neuritis, tumors, complications of premature birth, vitamin A deficiency in children, and infections.
Age is a common factor in vision loss. In the United States, according to the National Institutes of Health (NIH), irreversible vision loss is most prevalent among people aged 65 and older. A 2007 NIH/Blindness Prevention America report states blindness affects more than one million Americans aged 40 and older. The visually impaired, including those who are blind, total more than 3.6 million older Americans. According to the report, the prevalence of blindness and vision impairment increases rapidly in the later years, particularly after age 75.
WHO refers to studies that indicate women have a significantly higher risk of visual impairment than men, in every region of the world and at all ages.
About three-fourths of all visual impairment globally is preventable, according to WHO. Early detection, diagnosis, and treatment can help alleviate or forestall increased loss of vision. For example, timely correction can help prevent severe vision impairment in individuals with eyesight impaired by refractive errors. In addition, treatment of diabetes through diet, exercise, careful control of blood-sugar levels, and avoidance of smoking can help prevent blindness from diabetic retinopathy. Recommending regular comprehensive eye exams, the NIH advises being alert to subtle, small changes in vision, e.g., difficulty in focusing on near or distant objects, unusual sensitivity to light or glare, squinting, or inability to recognize familiar faces.