Encyclopedia
Blindness is the condition of lacking
visual perception due to physiological or
psychological 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 light perception and is clinically recorded as "NLP", an abbreviation for "no light perception"ximately ten percent of those deemed legally blind, by any measure, are fully sightless. The rest have some vision, from light perception alone to relatively good acuity. Those who are not legally blind, but nonetheless have serious visual impairments, possess low vision.
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.
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 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."
The
United States Congress included this definition as part of the Aid to the Blind program in the
Social Security Act passed in 1935
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 65002, the WHO estimated there were 161 million visually impaired people in the world, of whom 124 million had low vision and 37 million 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.
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. 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 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 cannot see.
Recent advances in mapping 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 which is found in methylated spirits which is sometimes used by alcoholics as a cheap substitute for regular
alcoholic beverages.
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:
- Adaptations of banknotes so that the value can be determined by touch. For example:
- In some currencies, such as the euro, pound sterling and Australian dollar, 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 is not standard Braille .
- 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 have been visually impaired for long enough, devise their own adaptive strategies in all areas of personal and professional management.
For corrective surgury 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, 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 do not carry these kinds of canes, opting instead for the shorter, lighter identification cane. Still others require a support cane.
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 cannot 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, about one percent, employ
guide dogs. These companions are trained to lead blind individuals around obstacles on the ground and overhead. Though highly intelligent, guide dogs cannot interpret street signs. Visually impaired people who employ these animals must already be competent travelers.
Reading and magnification
Most blind and visually impaired people read print, either of a regular size or enlarged through the use of magnification devices. A variety of
magnifying glasses, some of which are handheld, and some of which rest on desktops, can make reading easier for those with decreased visual acuity.
The rest read
Braille and
Moon type or rely on talking books and readers. They use computers with special hardware such as scanners and
refreshable Braille displays as well as software written specifically for the blind, like optical character recognition applications and screen reading software.
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 Freedom Scientific's JAWS for Windows screen reading software enable the blind to use mainstream computer applications. Most legally blind people do not 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.
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 such as the seeing with sound project 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.
Social attitudes towards blindness
Historically, blind and visually impaired people have either been treated as if their lack of sight was an outward manifestation of some internal lack of reason, or as if they possessed extrasensory abilities. Stories such as
The Cricket on the Hearth by
Charles Dickens provided yet another view of blindness, wherein those affected by it were ignorant of their surroundings and easily deceived.
The authors of modern educational materials , as well as those treating blindness in literature, have worked to paint a truer picture of blind people as three-dimensional individuals with a range of abilities, talents, and even character flaws. Certain individuals are gifted, and others licentious, but nothing definitive can be said of the blind as a class except that they cannot see well.
References
See also
...
- List of eye diseases
- Low vision
- Night blindness
- Scotoma
- Snow blindness
- Visual loss
- Web accessibility
External links
