Clustering for health

When functioning appropriately, the immune system protects against multiple threats such as pathogens, disease-causing microbes, and tumors. However, when the immune system is inappropriately activated, it attacks the body, ...

Aug 15, 2017
popularity6 comments 0

Blindness is the condition of lacking visual perception due to physiological or neurological factors. Various scales have been developed to describe the extent of vision loss and define blindness. Total blindness is the complete lack of form and visual light perception and is clinically recorded as NLP, an abbreviation for "no light perception." Blindness is frequently used to describe severe visual impairment with residual vision. Those described as having only light perception have no more sight than the ability to tell light from dark and the general direction of a light source.

In order to determine which people may need special assistance because of their visual disabilities, various governmental jurisdictions have formulated more complex definitions referred to as 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 ten percent of those deemed legally blind, by any measure, have no vision. 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.

By the 10th Revision of the WHO International Statistical Classification of Diseases, Injuries and Causes of Death, low vision is defined as visual acuity of less than 20/60 (6/18), but equal to or better than 20/200 (6/60), or corresponding visual field loss to less than 20 degrees, in the better eye with best possible correction. Blindness is defined as visual acuity of less than 20/400 (6/120), or corresponding visual field loss to less than 10 degrees, in the better eye with best possible correction.

Blind people with undamaged eyes may still register light non-visually for the purpose of circadian entrainment to the 24-hour light/dark cycle. Light signals for this purpose travel through the retinohypothalamic tract and are not affected by optic nerve damage beyond where the retinohypothalamic tract exits.

This text uses material from Wikipedia licensed under CC BY-SA

Latest Spotlight News

Firing of neurons changes the cells that insulate them

Through their pattern of firing, neurons influence the behavior of the cells that upon maturation will provide insulation of neuronal axons, according to a new study publishing 22 August in the open access journal PLOS Biology ...

Study suggests serotonin may worsen tinnitus

Millions of people suffer from the constant sensation of ringing or buzzing in the ears known as tinnitus, creating constant irritation for some and severe anxiety for others. Research by scientists at OHSU shows why a common ...

Brain's self-regulation in teens at risk for obesity

In a small study that scanned the brains of teenagers while exposing them to tempting "food cues," researchers report that reduced activity in the brain's "self-regulation" system may be an important early predictor of adult ...