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Blindness

Visual impairment, also known as vision impairment or vision loss, is a decreased ability to see to a degree that causes problems not fixable by usual means, such as glasses. Some also include those who have a decreased ability to see because they do not have access to glasses or contact lenses. Visual impairment is often defined as a best corrected visual acuity of worse than either 20/40 or 20/60. The term blindness is used for complete or nearly complete vision loss. Visual impairment may cause people difficulties with normal daily activities such as driving, reading, socializing, and walking.The definition of visual impairment is reduced vision not corrected by glasses or contact lenses. The World Health Organization uses the following classifications of visual impairment. When the vision in the better eye with best possible glasses correction is:Central visual acuity of 20/200 or less in the better eye with corrective glasses or central visual acuity of more than 20/200 if there is a visual field defect in which the peripheral field is contracted to such an extent that the widest diameter of the visual field subtends an angular distance no greater than 20 degrees in the better eye.An individual shall be considered to be blind for purposes of this title if he has central visual acuity of 20/200 or less in the better eye with the use of a correcting lens. An eye which is accompanied by a limitation in the fields of vision such that the widest diameter of the visual field subtends an angle no greater than 20 degrees shall be considered for purposes of the first sentence of this subsection as having a central visual acuity of 20/200 or less. An individual shall also be considered to be blind for purposes of this title if he is blind as defined under a State plan approved under title X or XVI as in effect for October 1972 and received aid under such plan (on the basis of blindness) for December 1973, so long as he is continuously blind as so defined.Visual impairments may take many forms and be of varying degrees. Visual acuity alone is not always a good predictor of the degree of problems a person may have. Someone with relatively good acuity (e.g., 20/40) can have difficulty with daily functioning, while someone with worse acuity (e.g., 20/200) may function reasonably well if their visual demands are not great.The most common causes of visual impairment globally in 2010 were:It is important that people be examined by someone specializing in low vision care prior to other rehabilitation training to rule out potential medical or surgical correction for the problem and to establish a careful baseline refraction and prescription of both normal and low vision glasses and optical aids. Only a doctor is qualified to evaluate visual functioning of a compromised visual system effectively. The American Medical Association provides an approach to evaluating visual loss as it affects an individual's ability to perform activities of daily living.The World Health Organization estimates that 80% of visual loss is either preventable or curable with treatment. This includes cataracts, onchocerciasis, trachoma, glaucoma, diabetic retinopathy, uncorrected refractive errors, and some cases of childhood blindness. The Center for Disease Control and Prevention estimates that half of blindness in the United States is preventable.Aside from medical help, various sources provide information, rehabilitation, education, and work and social integration.The WHO estimates that in 2012 there were 285 million visually impaired people in the world, of which 246 million had low vision and 39 million were blind.To determine which people qualify for special assistance because of their visual disabilities, various governments have specific definitions for legal blindness. In North America and most of Europe, legal blindness is defined as visual acuity (vision) of 20/200 (6/60) or less in the better eye with best correction possible. This means that a legally blind individual would have to stand 20 feet (6.1 m) from an object to see it—with corrective lenses—with the same degree of clarity as a normally sighted person could from 200 feet (61 m). In many areas, people with average acuity who nonetheless have a visual field of less than 20 degrees (the norm being 180 degrees) are also classified as being legally blind.Approximately fifteen percent of those deemed legally blind, by any measure, have no light or form perception. The rest have some vision, from light perception alone to relatively good acuity. Low vision is sometimes used to describe visual acuities from 20/70 to 20/200.A 2008 study tested the effect of using gene therapy to help restore the sight of patients with a rare form of inherited blindness, known as Leber's congenital amaurosis or LCA. Leber's Congenital Amaurosis damages the light receptors in the retina and usually begins affecting sight in early childhood, with worsening vision until complete blindness around the age of 30.Statements that certain species of mammals are 'born blind' refers to them being born with their eyes closed and their eyelids fused together; the eyes open later. One example is the rabbit. In humans, the eyelids are fused for a while before birth, but open again before the normal birth time; however, very premature babies are sometimes born with their eyes fused shut, and opening later. Other animals, such as the blind mole rat, are truly blind and rely on other senses.

[ "Diabetes mellitus", "Ophthalmology", "Surgery", "Pathology", "Blinding eye", "Bilateral blindness", "Congenital blindness", "Trichiasis", "blindness prevention" ]
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