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Optometry => Vision => Blindness
Blindness
INTRODUCTION
Blindness, total or partial inability to see because of disease or disorder of the eye, optic nerve, or brain. The term blindness typically refers to vision loss that is not correctable with eyeglasses or contact lenses. Blindness may not mean a total absence of sight, however. Some people who are considered blind may be able to perceive slowly moving lights or colors.
The term low vision is used for moderately impaired vision. People with low vision may have a visual impairment that affects only central vision-the area directly in front of the eyes-or peripheral vision-the area to either side of and slightly behind the eyes. Some people with low vision are able to function with their remaining sight while others need help to learn to use their sight more efficiently with training and special tools.
The term blindness is used somewhat misleadingly to designate certain visual conditions. Color blindness, for example, does not reduce visual acuity and should more accurately be called color-perception deficiency. Color blindness occurs almost exclusively in males, and the most common form is the inability to differentiate between certain shades of red and green. Night blindness, the inability to see in low levels of light, is commonly associated with a lack of vitamin A in the diet or with inherited diseases such as retinitis pigmentosa, a condition involving progressive degeneration of the eye's retina and abnormal deposits of pigment.
DEFINING BLINDNESS
Vision experts assess an individual's sight using two measurements: visual acuity and visual field. Visual acuity is the ability to see details, such as symbols or letters of specific sizes. Normal vision is described as 20/20. A person with any degree of sight loss has a visual acuity with a higher second number, such as 20/200. Visual field refers to the space around the center of vision-the peripheral area. A normal visual field is said to be 180 degrees in diameter, or half a circle.
In the United States, legal blindness is defined as a person with a visual acuity of 20/200 or less in the better eye with the best optical correction, such as eyeglasses or contact lenses, or a visual field whose widest diameter is no greater than 20 degrees. An individual with a visual acuity of 20/200 must stand at 6 m (20 ft) to see objects that a person with normal sight can see at 60 m (200 ft). An individual with a visual field of 20 degrees or less has a limited visual range sometimes referred to as tunnel vision that is likened to viewing the world through a toilet paper roll. A person declared legally blind in the United States is eligible for government benefits.
PREVALENCE AND INCIDENCE
About 750,000 Americans are considered legally blind and about 50,000 new cases of blindness occur each year. Approximately 8.9 million people have low vision. Of these, about 1.5 million cannot read ordinary newspaper type even with the aid of eyeglasses or other optical aids that provide significant magnification . Although it is very difficult to determine the global prevalence of blindness, the World Health Organization (WHO) estimates that approximately 38 million people are blind worldwide and an additional 110 million individuals suffer from low vision.
MAJOR CAUSES OF BLINDNESS
The three major causes of blindness in the world are cataract, trachoma, and glaucoma, accounting for over 70 percent of all cases of sightlessness. Cataract is an opacity, or cloudiness, in the normally clear lens of the eye that interferes with vision. Although regarded by many people as an unavoidable effect of advancing age, cataract may develop at any time in life-even before birth. Worldwide, cataract causes about 16 million cases of blindness. In Africa and Asia, cataract accounts for nearly half of all blindness. About 60 percent of Americans between the ages of 65 and 74 show some signs of cataract, and about 3.3 million are visually impaired by this disorder. At least 43,000 of these people are blind from cataract, making it the third leading cause of legal blindness in the United States. Surgery to remove the opaque lens is the only effective way of treating cataract. About 90 to 95 percent of the estimated 600,000 cataract extractions performed on individuals each year in the United States provide these people with useful vision when eyeglasses, contact lenses, or artificial lens implants are subsequently used. Worldwide, however, only 10 to 20 percent of all cataracts are removed.
Although it is not a serious problem in the economically advanced nations of Europe and North America, trachoma afflicts as many as 146 million people worldwide, mostly in Africa, the Middle East, the Indian subcontinent, Southeast Asia, and in localized populations in central Australia and Latin America. At least 6 million people are blind from trachoma and about 100 million people with trachoma have potentially serious visual impairment that may eventually lead to blindness. Trachoma is a contagious disease of the conjunctiva (the mucous membranes that line the inner eyelid and cover the front of the eyeball) and the cornea (the membrane that covers the pupil and iris of the eyeball), caused by the bacteria Chlamydia trachomatis. It is spread easily by eye-hand contact, by certain flies, or by contact with contaminated articles such as towels. Trachoma can be treated effectively with topically or orally administered antibiotics and other drugs, although it may recur. In 1998 the WHO launched a program to fight trachoma by distributing a long-acting antibiotic to people in five nations where trachoma is particularly common. Chlamydia trachomatis is so common that eradication of trachoma is not thought possible, but the WHO hopes to eliminate trachoma as a major cause of blindness by 2020.
Glaucoma, characterized by an abnormally high level of pressure within the eye, initially causes progressive destruction of peripheral vision due to irreversible damage to the optic nerve. Early signs of glaucoma are difficult to detect and the disease often goes untreated, leading to blindness. Glaucoma is the third largest cause of blindness worldwide, after cataract and trachoma, affecting approximately 105 million people. Of these, about 5.2 million are blind. In the United States, glaucoma is the leading cause of preventable blindness and the most common cause of blindness in African Americans. About 80,000 Americans are legally blind from this condition and more than a million are at risk for vision loss because their disease remains undetected.
Onchocerciasis, or river blindness, ranks second only to trachoma as an infectious cause of blindness in the world. It has been estimated that this parasitic disease, caused by a minute nematode worm, Onchocerca volvulus, infects from 17 to 18 million people worldwide. Of these, 270,000 are blind and 500,000 are visually impaired. It is especially virulent in the central sub-Saharan region of Africa, along the major rivers. The disease is spread by the Simulium damnosum black fly, whose bites transmit the worms from person to person. Since 1987 the drug ivermectin, which halts the progression of the disease, has been provided for free to treat those individuals infected with onchocerciasis. To date, approximately 18 million people have been treated with the drug.
A lack of vitamin A in the diet, almost always associated with malnutrition, is the chief cause of xerophthalmia (extreme dryness of the conjunctiva) and the more advanced condition known as keratomalacia (deterioration and ulceration of the cornea). One of the most devastating effects of severe vitamin A deficiency produces cornea destruction and perforation of the eyeball. It is the major cause of childhood blindness in developing countries. In Asia alone, about 5 million children each year develop signs of xerophthalmia, and in approximately 250,000 of these children the disease causes blindness. Keratomalacia is particularly tragic, not only because it strikes children, but because it is preventable. The worldwide prevalence of keratomalacia has been estimated at 20 per 10,000 children between the ages of 1 and 6 years. It afflicts at least 100,000 children each year in Asia, and it is also widely prevalent in the Middle East, Africa, and South America. Improved worldwide nutrition would significantly help to reduce the incidence of xerophthalmia and keratomalacia.
In the United States, macular degeneration is a leading cause of severe vision loss. In this disorder, the macula, the small area of the retina responsible for sharp central vision, is progressively destroyed, producing a blind spot or empty area in the center of focus. Each year an additional 165,000 persons, age 75 or older, develop macular degeneration.
Diabetic retinopathy, a disease of the small blood vessels that nourish the retina, is the most common eye complication of diabetes mellitus, a disease in which glucose, or sugar, is not properly used by the body, allowing high levels of sugar to build up in the blood and urine. More than 32,000 Americans are blind from diabetic retinopathy, and each year an estimated 300,000 diabetics are seriously at risk for blindness from this disease. Laser treatment has proven highly effective in forestalling severe visual loss at certain stages of the disease. Even lost vision can, in some instances, be restored, at least partially, through a surgical procedure called vitrectomy, in which the semisolid, normally clear gel in the center of the eye is removed.
Other retinal disorders affect more than 85,000 Americans each year. Retinopathy of prematurity (retrolental fibroplasia) in its advanced stages causes an abnormal fibrous condition in the eye of a premature infant . Sickle-cell retinopathy involves blockages and hemorrhages of retinal blood vessels that occur in association with sickle-cell anemia.
No worldwide estimates are available on the incidence of blindness and visual loss caused by eye injury, but some figures are available from individual countries. In Nigeria, for example, 25 percent of those accidentally blinded are schoolchildren. In such developing countries, where medical care may be minimal, a slight abrasion of the cornea often leads to ulceration, severe infection, and ultimately loss of the eye. In the United States about 19,000 people are blind because of eye injury, and nearly one million have some degree of injury-caused visual impairment. Each year in the United States, 300,000 eye injuries occur on the job, 160,000 in schools, and 40,000 in sports and recreational activities; yet it is estimated that 90 percent of all eye injuries could be avoided by practicing eye safety and using protective eyewear.
EDUCATION OF THE VISUALLY IMPAIRED
Efforts to educate people who are blind are of comparatively recent date. No records can be found of attempts to provide systematic education for blind children in the days of antiquity or during medieval times. By the end of the 18th century, enlightened humanitarians became convinced that it was possible for the blind to adapt themselves to the conditions of normal living. Valentin Haüy, a French government employee, began the pioneering work of teaching the blind in the latter half of the 18th century, founding the first institution for the education of blind children in Paris in 1784. Haüy also taught his blind pupils to read, by touch, raised letters embossed on paper. He not only provided school training in formal subjects in his school, but also in music and in several trades.
In 1824 Louis Braille, a French student and later teacher who was blind himself, invented the system that enables blind persons to read. It was based on the night writing principle of Charles Barbier, a captain in the French cavalry, who used a combination of 12 dots that were embossed, or pressed, into paper to allow military communications to be read at night. Braille's system used six raised dots arranged in cells of three rows of two. Dots were arranged in different combinations that blind people feel with their fingertips. The patterns formed a code that spelled out letters and numbers and symbolized concepts.
Although the Braille system was published in 1829, it was not officially accepted in the United States until 1916. During that time, Braille advocates argued in favor of three different embossed-type systems: English Braille, which closely followed the original French Braille; American Braille, which assigned Braille signs to the letters of the alphabet on the basis of how often each letter was used in the language; and New York Point, in which dots were arranged in cells two dots high and one to four dots long. The United States eventually accepted English Braille as its standard. In 1932 English Braille also became the universal system for the English-speaking world.
Braille was first printed using a pointed sharp instrument that elevated small dots on a sheet of heavy paper. A metal template that clamped over the paper kept it secure while the dots were inscribed by hand. Printing later was done using presses consisting of two stereotype plates, each imprinted with the dots for the Braille text being printed. These dots had to be hammered by hand into the plates or the plates were cast in a custom-designed mold. In 1893 the stereotyping machine was developed, which automated the process of transferring the dots onto the stereotype plates. A further printing innovation was interpoint printing, which enabled Braille to be printed on both sides of a page. It was introduced in the United States in the 1920s. These developments made producing Braille books faster and easier.
In 1858 the American Printing House for the Blind was founded in Louisville, Kentucky, to produce and distribute books to blind children. In 1879 Congress permanently funded this national printing house. It produced its first raised-print book in 1866, in which letters were embossed on the surface of the page, and its first Braille book in 1893. Today publications in Braille are produced using computer programs that translate print into Braille. The files containing the Braille text are sent to electronically driven machines that make the printing plates.
TOOLS FOR VISUAL REHABILITATION
Up until the 1970s, many children with visual impairments in the United States continued to be educated only in schools for the blind. Today, however, visually impaired children attend regular classes with their sighted peers in both private and public schools. In part, this is due to the many technological innovations for teaching visually impaired people. Braille can now be automated and printed rapidly using special typewriters or word processors and Braille printers. Students may also have access to machines that translate Braille into speech. Audiotape recordings of book texts permit visually impaired people to learn by listening.
Computers equipped with synthetic speech systems-a sound board and microphone for inputting spoken words and a program to translate those words into a form the computer can use-are also becoming available. These systems are usually combined with a voice output system to enable the user to verify that the computer has correctly interpreted the input. These systems are faster than Braille and audiotapes for accessing information. Another innovation is the voice output print scanner. These systems scan a book into a computer and then convert the text into voice output-that is, the computer reads the material out loud in one of a choice of voices. These devices are useful not only in the classroom, but also in the workplace, permitting blind adults to be productive in any job.
Other devices also exist to help persons with reduced sight perform day-to-day tasks. Close-circuit television (CCTV) systems consist of a still or video camera and a television or video monitor. A printed page is placed in front of the camera, which then projects a highly magnified image of the page onto the monitor. A recent development for CCTV is the mouse-cam, a handheld camera that can be plugged into any TV monitor and then rolled along lines of print on a printed document. The print is then displayed on the monitor. Magnification programs for computer screens enlarge the text and graphics of any software program run on the computer on which the magnification program is installed. These programs can amplify a screen image from 2 to 32 times its normal size, and they work as if the user is moving a magnifying glass over the computer screen.
Additional advances for the visually impaired include aids that use sonar and radar to convey information about objects and obstacles in the user's vicinity, infrared viewing systems for help with night vision, and devices such as Braille note-takers, portable handheld computers with Braille keyboards that function as notepads, date and address books, and diaries. Many low-tech aids devices are also available, including eyeglasses equipped with telescopic lenses, lighted hand-held magnifiers, and specially tinted glasses to reduce glare and provide proper color balance.
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