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.
Decreased ability to see
Uncorrected refractive errors, cataracts, glaucoma, poisoning etc.
Cataracts describes the greying or opacity of the crystalline lens, which is most commonly caused by intrauterine infections, metabolic disorders, and genetically transmitted syndromes.
The best way to reduce the risk of developing cataracts is to avoid smoking and extensive exposure to sun light.
Cataracts are the leading cause of child and adult blindness that doubles in prevalence with every ten years after the age of 40. People face higher chances of developing cataracts as they age.
Glaucoma characterized by increased pressure within the eye.
Glaucoma causes visual field loss as well as severs the optic nerve.
One of the most common causes of pediatric glaucoma is cataract removal surgery, which leads to an incidence rate of about 12.2% among infants and 58.7% among 10-year-olds..
Rarely, blindness is caused by the intake of certain chemicals. A well-known example is methanol, which is only mildly toxic and minimally intoxicating, and breaks down into the substances formaldehyde and formic acid which in turn can cause blindness, an array of other health complications, and death. When competing with ethanol for metabolism, ethanol is metabolized first, and the onset of toxicity is delayed. Methanol is commonly found in methylated spirits, denatured ethyl alcohol, to avoid paying taxes on selling ethanol intended for human consumption. Methylated spirits are sometimes used by alcoholics as a desperate and cheap substitute for regular ethanol alcoholic beverages.
People with albinism often have vision loss to the extent that many are legally blind, though few of them actually cannot see. Leber’s congenital amaurosis can cause total blindness or severe sight loss from birth or early childhood.
Recent advances in mapping of the human genome have identified other genetic causes of low vision or blindness. One such example is Bardet-Biedl syndrome.
Eye injuries, most often occurring in people under 30, are the leading cause of monocular blindness (vision loss in one eye) throughout the United States. Injuries and cataracts affect the eye itself, while abnormalities such as optic nerve hypoplasia affect the nerve bundle that sends signals from the eye to the back of the brain, which can lead to decreased visual acuity.
Classifications of Visual impairment
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 (NLP) : is considered total visual impairment, or total blindness
Blindness is defined by the World Health Organization as vision in a person’s best eye with best correction of less than 20/500.