Blindness is the total or partial inability to see. It can be described in various ways. People who are totally blind cannot tell light from dark. Those who are partially blind have some sight that may be useful for certain purposes. People born unable to see are congenitally blind. Others lose their sight because of disease or injury and become victims of acquired blindness.
Blindness can also be described according to how it affects a person’s life. A man or woman who is economically blind cannot see well enough to have a job that requires sight. People are vocationally blind if the loss of sight prevents them from continuing to work at their present job. A person who is educationally blind cannot see well enough to be educated without special materials or methods.
In the United States, a legally blind person is one who has visual acuity (sharpness of vision) of 20/200 or worse with glasses or contact lenses. A person with 20/200 vision sees from a distance of 20 feet (6 meters) what someone with normal vision—20/20 vision—sees from 200 feet (61 meters). People are also legally blind if they have an extremely limited field of vision. A person’s field of vision is what is seen—straight ahead and all around—when looking steadily at an object. About 500,000 people in the United States are legally blind. Almost half of them are 65 years old or older.
Other conditions of blindness involve the inability to see under special circumstances or to tell colors apart. People with night blindness cannot see normally in reduced light, though they have normal vision in good light. Night blindness may be caused by various eye diseases or by a lack of vitamin A in the diet. Snow blindness is the temporary loss of vision due to bright sunlight reflected by snow. Color blindness, an inherited condition, is the inability to tell certain colors apart.
Causes of blindness
Diseases cause about 95 per cent of all blindness, and injuries cause the rest. A disease or injury can affect one or both of the eyes, the visual center of the brain, or the nerves that connect the eyes and the brain.
Diseases.
Many diseases can cause blindness. The chief causes in the United States are cataract, diabetic retinopathy, glaucoma, and macular degeneration. This section also discusses the other major causes of blindness throughout the world.
Cataract
is the clouding of the lens of the eye. Blindness results if the lens becomes opaque (nontransparent). The most common form of this disease, senile cataract, appears gradually as a result of aging. However, some babies are born with cataracts. Cataracts can be removed by surgery. After the operation, the patient wears strong glasses or contact lenses, or artificial lenses that are permanently placed in the eyes. In most cases, cataract surgery can restore useful vision.
Diabetic retinopathy
occurs in some people who have diabetes mellitus over a period of years. Diabetes causes changes in the blood vessels of the retina, the part of the eye that absorbs light rays. In some cases, these blood vessels may burst and cause an intraocular hemorrhage (bleeding in the eye) that may cloud a person’s vision. In other cases, fluid leaks from capillaries in the retina and causes the retina to thicken. Occasionally, the retina becomes detached from the back of the eye. Retinal detachment or bleeding into the vitreous humor, the clear fluid that fills the center of the eye, can cause blindness. Surgery can usually correct an intraocular hemorrhage or a detached retina. Early treatment with a laser can usually prevent these two problems. Laser treatment can also eliminate excess fluid in the retina and prevent a gradual loss of vision.
Glaucoma
is a disease in which the fluid in the eyeball does not drain properly. Pressure builds up in the eye and may damage the optic nerve, which connects the eye with the sight center in the brain. The most common type of glaucoma occurs in people more than 40 years old. The field of vision gradually narrows, and total blindness may result. Another type of glaucoma occurs in some babies at birth. Most glaucoma cases can be treated with drugs or surgery.
Macular degeneration
causes people to lose their ability to see things at the center of the field of vision. This loss of vision most often occurs as people age. Macular degeneration causes hemorrhaging, or cuts off the circulation of blood, in the macula, the center of the retina. The person loses central vision but retains peripheral (side) vision. People who have this condition may become legally blind. In its early stages, macular degeneration can be successfully treated with a laser.
Amblyopia
is a condition that may occur if one eye is much stronger than the other, or if the two eyes are not lined up together. The most common form of amblyopia appears in children. If a child has better vision in one eye than in the other, he or she may begin to lose vision in the weaker eye. Physicians treat many cases of amblyopia by putting a patch over the strong eye in order to strengthen the weak one. If the condition is not corrected, the weak eye may become blind.
Corneal opacity
may occur if the cornea, the clear tissue that covers the colored part of the eye, becomes scarred. Certain infections or injuries can cause such scarring. The cornea may also become opaque as a result of age. Corneal damage can cause blurred vision or blindness. In most cases of corneal infection, medication can prevent permanent damage to vision. Even if the cornea becomes permanently opaque, a surgeon may be able to perform a corneal transplantation. In this operation, the cornea is replaced with that of a person who recently died. Physicians obtain unscarred corneas from an agency called an eye bank (see Eye bank ).
Ophthalmia neonatorum
is an eye infection that strikes newborn babies. It is caused by bacteria that pass from the mother’s birth canal into the infant’s eyes. These bacteria include the ones that cause gonorrhea, a sexually transmitted disease. The symptoms of ophthalmia neonatorum—inflammation of the eyelids and cornea—appear two or three days after birth in most cases. The infection can produce blindness if it is not treated. In many countries, doctors prevent ophthalmia neonatorum by dropping an antibiotic ointment into the eyes of newborn babies. This procedure is required by many U.S. states. This procedure is required by most U.S. states.
Retinitis pigmentosa
destroys the retina over a period of years. Night blindness is one of the first symptoms of this disease, and clumps of pigment (color) appears in the retina. The field of vision gradually narrows, and many victims become totally blind. The disease is hereditary. There is no treatment.
Retinoblastoma
is cancer of the retina. It is usually hereditary. In most cases, a tumor forms on the retina during early childhood. X-ray treatment, drugs, or surgery may prevent the cancer from spreading. But if treatment fails, the eye must be removed.
Sympathetic ophthalmia
is an unusual condition that occurs after one eye has been badly injured. The other eye may lose its sight even though it has not been damaged. In some cases, the injured eye must be removed to prevent blindness in both eyes. Physicians do not know the causes of sympathetic ophthalmia.
Trachoma
is caused by a virus that affects the cornea and the conjunctiva, the lining of the eyelid. Unlike most eye diseases, trachoma is contagious. It is spread by some kinds of flies and by contact with towels and other articles used by people who have the disease. Physicians can cure most cases in their early stages, but blindness results in many advanced cases. Trachoma is a leading cause of blindness in India, the Middle East, and other developing regions. It rarely occurs in the United States or other countries that have good sanitation.
Injuries.
Many kinds of injuries can cause blindness. Any particle that enters the eye may carry germs that can produce an infection leading to blindness. Certain chemicals can burn the eye. A blow to the head may damage the sight center of the brain and result in total blindness. Eye or head injuries can cause retinal detachment or bleeding into the vitreous humor. A wound that damages the optic nerve can also cause blindness.
Safety regulations help prevent industrial accidents that could cost workers their sight. In a number of industries, employees who have certain jobs are required to wear protective glasses. Many steelworkers wear them to prevent injury from flying metal fragments. Welders wear masks with special lenses to protect their eyes from the harmful light of the welding process.
Many hobbyists face the risk of eye injury when working at home. They also should use protective eyewear.
Overcoming blindness
Many schools and agencies provide education and training for people who are born blind or become blind later in life. These people learn special ways to read and to get around alone, and they are employed in a number of fields. Blind men and women hold jobs as computer programmers, electrical engineers, factory workers, lawyers, musicians, social workers, teachers, and X-ray technicians, and as many other kinds of specialists.
Aids for blind people
include braille and talking books (recordings on records, tapes, or compact discs), with which blind individuals can read. Many libraries and schools provide books and magazines that are published in braille or have been recorded. The Library of Congress loans such books through its National Library Service for the Blind and Physically Handicapped. Such organizations as American Printing House for the Blind, Inc., and Learning Ally prepare braille and talking books.
Such devices as the Optacon and optical scanners enable people who are blind to read printed material. An Optacon forms an enlarged image of each letter, and the user reads by feeling the images with the fingers. Optical scanners are computers with a voice simulator that read printed material aloud.
Many motion pictures, videotapes, and television programs provide audio descriptions of action, scene changes, and other visual elements for people who are blind or partially sighted. At movie theaters, patrons hear a description of the onscreen action through a headset. TV and video programs have a descriptive soundtrack that runs along with the spoken dialogue.
Many blind people use a special cane when walking alone. The cane is painted white to identify the user as a blind person. It is longer than a regular cane to enable a blind person to feel his or her way. Experienced cane users can get around confidently in unfamiliar places. Some blind people use trained dogs to guide them.
Engineers have developed aids to help blind individuals get around. One such aid is a sonar device built into a pair of glasses. This device sends out sonar waves that are reflected back in the form of sound after hitting an object in a person’s path (see Sonar ). Another aid consists of a light-sensitive transmitter implanted onto the retina of a blind person. When light hits it, the transmitter sends signals to the brain through nerve channels.
Education and training.
The first school for blind people was established in Paris in 1784. The first such school in the United States, the New England Institution for the Education of the Blind (now Perkins School for the Blind) opened in Boston in 1832.
Until the mid-1900’s, most people believed that blind children should be taught in special boarding schools. But today, more than 60 percent of blind schoolchildren in the United States go to a regular day school. Some attend special classes for students who are blind or partly sighted, but others attend regular classes with sighted students. Many blind students go on to college.
Many agencies offer occupational training programs that teach blind people such skills as typing and operating various industrial machines. A number of these organizations provide job placement services for people who complete their programs. However, many blind people have difficulty finding someone who will hire them. Many employers believe that all people who are blind—even skilled workers—are completely helpless.
Some agencies offer programs in such skills as cooking and personal grooming. Staff members also teach blind people how to use a cane. These experts, called peripatologists, help many blind people become familiar with their neighborhoods and teach them the best route to and from their jobs. Many U.S. government agencies offer help to blind people. They include the Office of Special Education Programs. In Canada, the Canadian National Institute for the Blind, usually called CNIB, and the Canadian Federation of the Blind provide many services.
Such organizations as the American Foundation for the Blind (AFB) and the National Federation of the Blind (NFB) inform blind people of the services available to them.
