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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.〔(【引用サイトリンク】url=http://www.who.int/blindness/Change%20the%20Definition%20of%20Blindness.pdf?ua=1 )〕 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 most common causes of visual impairment globally are uncorrected refractive errors (43%), cataracts (33%), and glaucoma (2%).〔 Refractive errors include near sighted, far sighted, presbyopia, and astigmatism.〔 Cataracts are the most common cause of blindness.〔 Other disorders that may cause visual problems include age related macular degeneration, diabetic retinopathy, corneal clouding, childhood blindness, and a number of infections. Visual impairment can also be caused by problems in the brain due to stroke, prematurity, or trauma among others.〔 These cases are known as cortical visual impairment. Screening for vision problems in children may improve future vision and educational achievement. Screening adults may also be beneficial.〔 Diagnosis is by an eye exam.〔(【引用サイトリンク】url=http://www.cdc.gov/healthcommunication/ToolsTemplates/EntertainmentEd/Tips/Blindness.html )〕 The World Health Organization estimates that 80% of visual impairment is either preventable or curable with treatment.〔 This includes cataracts, the infections river blindness and trachoma, glaucoma, diabetic retinopathy, uncorrected refractive errors, and some cases of childhood blindness.〔(【引用サイトリンク】url=http://www.who.int/blindness/causes/en/ )〕 Many people with significant visual impairment benefit from vision rehabilitation, changes in their environmental, and assistive devices.〔 As of 2012 there were 285 million people who were visually impaired of which 246 million had low vision and 39 million were blind.〔 The majority of people with poor vision are in the developing world and are over the age of 50 years.〔(【引用サイトリンク】url=http://www.who.int/mediacentre/factsheets/fs282/en/ )〕 Rates of visual impairment have decreased since the 1990s.〔 Visual impairments have considerable economic costs both directly due to the cost of treatment and indirectly due to decreased ability to work. ==Classification== 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: * 20/30 to 20/60 : is considered mild vision loss, or near-normal vision * 20/70 to 20/160 : is considered moderate visual impairment, or moderate low vision * 20/200 to 20/400 : is considered severe visual impairment, or severe low vision * 20/500 to 20/1,000 : is considered profound visual impairment, or profound low vision * More than 20/1,000 : is considered near-total visual impairment, or near total blindness * No light perception : is considered total visual impairment, or total blindness Blindness is defined by the World Health Organization as vision in a person's best eye of less than 20/500 or a visual field of less than 10 degrees.〔 This definition was set in 1972, and there is ongoing discussion as to whether it should be altered to official include uncorrected refractive errors.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Visual impairment」の詳細全文を読む スポンサード リンク
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