Everything about Blindness totally explained
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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." 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 from an object to see it—with vision correction—with the same degree of clarity as a normally sighted person could from . 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 6/18, but equal to or better than 3/60, 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 3/60, or corresponding visual field loss to less than 10 degrees, in the better eye with best possible correction.
It should be noted that 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 (RHT), so a damaged optic nerve beyond where the RHT exits it's no hindrance.
Legal blindness
In 1934, the
American Medical Association adopted the following definition of blindness:
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's 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.
The
United States Congress included this definition as part of the
Aid to the Blind program in the
Social Security Act passed in 1935. In 1972, the Aid to the Blind program and two others combined under Title XVI of the Social Security Act to form the
Supplemental Security Income program which currently states:
An individual shall be considered to be blind for purposes of this title if he's 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's 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's continuously blind as so defined.
Kuwait is one of many nations that share the same criteria for legal blindness.
Epidemiology
In 1987, it was estimated that 598,000 people in the United States met the legal definition of blindness. Of this number, 58% were over the age of 65.
In November 2004 article
Magnitude and causes of visual impairment, the
WHO estimated that in 2002 there were 161 million (about 2.6% of the world population) visually impaired people in the world, of whom 124 million (about 2%) had low vision and 37 million (about 0.6%) were blind.
Causes of blindness
Serious visual impairment has a variety of causes:
Diseases
Most visual impairment is caused by
disease and
malnutrition. According to
WHO estimates in 2002, the most common causes of blindness around the world are:
People in developing countries are significantly more likely to experience visual impairment as a consequence of treatable or preventable conditions than are their counterparts in the developed world. While vision impairment is most common in people over age 60 across all regions, children in poorer communities are more likely to be affected by blinding diseases than are their more affluent peers.
The link between
poverty and treatable visual impairment is most obvious when conducting regional comparisons of cause. Most adult visual impairment in
North America and
Western Europe is related to age-related macular degeneration and diabetic retinopathy. While both of these conditions are subject to treatment, neither can be cured. Another common cause is
retinopathy of prematurity.
In developing countries, wherein people have shorter life expectancies, cataracts and water-borne parasites—both of which can be treated effectively—are most often the culprits (see
River blindness, for example). Of the estimated 40 million blind people located around the world, 70–80% can have some or all of their sight restored through treatment.
In developed countries where parasitic diseases are less common and
cataract surgery is more available, age-related macular degeneration, glaucoma, and diabetic retinopathy are usually the leading causes of blindness.
Abnormalities and injuries
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.
People with injuries to the
occipital lobe of the
brain can, despite having undamaged eyes and
optic nerves, still be legally or totally blind.
Genetic defects
People with
albinism often suffer from visual impairment to the extent that many are legally blind, though few of them actually can't 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.
Poisoning
A small portion of all cases of blindness are caused by the intake of certain chemicals. A well-known example is
methanol
, found in
methylated spirits, which are sometimes used by
alcoholics as a cheap substitute for regular
alcoholic beverages.
Willful actions
Blinding has been used as an act of vengeance and
torture in some instances, to deprive a person of a major sense by which they can navigate or interact within the world, act fully independently, and be aware of events surrounding them. An example from the classical realm is
Oedipus, who gouges out his own eyes after realizing that he fulfilled the awful prophecy spoken of him.
Treatments
A 2008 study published in the New England Journal of Medicine tested the effect of using gene therapy to help restore the sight of patients with a rare form of inherited blindness, known as Leber Congenital Amaurosis or LCA. Leber 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.
The study used a common cold virus to deliver a normal version of the gene called RPE65 directly into the eyes of affected patients. Remarkably all 3 patients aged 19, 26 and 26 responded well to the treatment and reported improved vision following the procedure. Due to the age of the patients and the degenerative nature of LCA the improvement of vision in gene therapy patients is encouraging for researchers . It is hoped that gene therapy may be even more effective in younger LCA patients who have experienced limited vision loss as well as in other blind or partially blind individuals.
Adaptive techniques
Visually impaired and blind people have devised a number of techniques that allow them to complete daily activities using their remaining senses. These might include the following:
Adaptive computer and mobile phone software that allows people with visual impairments to interact with their computers and/or phones via screen readers or screen magnifiers.
Adaptations of banknotes so that the value can be determined by touch. For example:
- In some currencies, such as the euro, the pound sterling and the Norwegian krone, the size of a note increases with its value.
- Many banknotes from around the world have a tactile feature to indicate denomination in the upper right corner. This tactile feature is a series of raised dots, but it isn't standard Braille (External Link
).
- It is also possible to fold notes in different ways to assist recognition.
Labeling and tagging clothing and other personal items
Placing different types of food at different positions on a dinner plate
Marking controls of household appliances
Most people, once they've been visually impaired for long enough, devise their own adaptive strategies in all areas of personal and professional management.
For corrective surgery of blindness, see acquired vision.
Tools
Designers, both visually impaired and sighted, have developed a number of tools for use by blind people.
Mobility
Many people with serious visual impairments can travel independently assisted by tactile paving and/or using a white cane with a red tip - the international symbol of blindness.
A long cane is used to extend the user's range of touch sensation, swung in a low sweeping motion across the intended path of travel to detect obstacles. However, some visually impaired persons don't carry these kinds of canes, opting instead for the shorter, lighter identification (ID) cane. Still others require a support cane. The choice depends on the individual's vision, motivation, and other factors.
Each of these is painted white for maximum visibility, and to denote visual impairment on the part of the user. In addition to making rules about who can and can't use a cane, some governments mandate the right-of-way be given to users of white canes or guide dogs.
A small number of people employ guide dogs. The dogs can be trained to navigate various obstacles. The human half of the guide dog team does the directing, based upon skills acquired through previous mobility training. The handler might be likened to an aircraft's navigator, who must know how to get from one place to another, and the dog is the pilot, who gets them there safely.
Orientation and mobility specialists are professionals who are specifically trained to teach people with visual impairments how to travel safely, confidently, and independently in the home and the community.
Reading and magnification
Most visually impaired people who are not totally blind read print, either of a regular size or enlarged by magnification devices. Many also read large-print, which is easier for them to read without such devices. A variety of magnifying glasses, some handheld, and some on desktops, can make reading easier for them.
The rest read Braille (or the infrequently used Moon type), or rely on talking books and readers or reading machines. They use computers with special hardware such as scanners and refreshable Braille displays as well as software written specifically for the blind, such as optical character recognition applications and screen readers.
Some people access these materials through agencies for the blind, such as the National Library Service for the Blind and Physically Handicapped in the United States, the National Library for the Blind or the RNIB in the United Kingdom.
Closed-circuit televisions, equipment that enlarges and contrasts textual items, are a more high-tech alternative to traditional magnification devices. So too are modern web browsers, which can increase the size of text on some web pages through browser controls or through user-controlled style sheets.
Computers
Access technology such as screen readers and Screen magnifiers enable the blind to use mainstream computer applications. Most legally blind people (70% of them across all ages, according to the Seattle Lighthouse for the Blind) don't use computers. Only a small fraction of this population, when compared to the sighted community, have Internet access. This bleak outlook is changing, however, as availability of assistive technology increases, accompanied by concerted efforts to ensure the accessibility of information technology to all potential users, including the blind. Later versions of Microsoft Windows include an Accessibility Wizard & Magnifier for those with partial vision, and Microsoft Narrator, a simple screen reader. Linux distributions (as Live CDs) for the blind include Oralux and Adriane Knoppix, the latter developed in part by Adriane Knopper who has a visual impairment. The Macintosh OS also comes with a built-in screen reader, called VoiceOver.
The movement towards greater web accessibility is opening a far wider number of websites to adaptive technology, making the web a more inviting place for visually impaired surfers.
Experimental approaches in sensory substitution are beginning to provide access to arbitrary live views from a camera.
Other aids
People may use talking thermometers, enlarged or marked oven dials, talking watches, talking clocks, talking scales, talking calculators, talking compasses and other talking equipment.
Sports
Blind and partially sighted people participate in sports such as swimming, snow skiing and athletics. Some sports have been invented or adapted for the blind such as goalball, cricket and golf. . The worldwide authority on sports for the blind is the International Blind Sports Federation (IBSA). People with vision impairments have participated in the Paralympic Games since 1976.
Blindness in animals
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, but 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.
Metaphorical uses
The word "blind" (adjective and verb) is often uses for various types of failing to notice something, for example in this page "For years I [=a gyrfalcon breeder] was impressed and blinded by the white color, but after I bred gyrs for a while I noticed that demand for darker falcons was very strong.".
A "blind spot" is an area where someone can't see, for example where a car driver can't see because parts of his car's bodywork are in the way.
Further Information
Get more info on 'Blindness'.
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