Senile cataract, pictured here, may cause blindness by obstructing the passage of light. This most common form of cataract usually occurs in people over 50. The only treatment is extraction of the lens of the eye.
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 19 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 5 million people are blind from trachoma. 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 second leading cause of blindness worldwide, affecting approximately 105 million people. Of these, some 6.7 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. Between 90,000 and 120,000 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, 450,000 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. Some 2.7 million children under five years of age suffer from vitamin A deficiency related blindness. 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.
Macular degeneration is a leading cause of severe vision loss, affecting about 25 to 30 million people worldwide. 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 in the United States about 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.