Causes and Symptoms (Magill’s Medical Guide, Sixth Edition)
The most common defects in human vision are nearsightedness (myopia), farsightedness (hyperopia), and astigmatism. All three of these conditions are called refractive errors because the cornea-lens focusing system of the eye bends light rays either too much or too little, so that the image formed on the retina is blurred. Fortunately, refractive errors can be corrected by means of eyeglasses or contact lenses. In the United States, almost 60 percent of the population, or about 150 million people, use some form of vision correction, and more than 27 million of them wear contacts.
Myopia and hyperopia are caused by a mismatch between the focusing power of the cornea-lens combination and the length of the eyeball. For a nearsighted person, the incoming light comes to a focus in front of the retina; a diverging lens is needed to move the image farther back. For a farsighted person, the situation is reversed; a converging lens is prescribed to provide extra focusing power.
The problem of astigmatism is attributable to a difference in the focal length of the eye for two perpendicular directions, which can occur if the eyeball is slightly deformed (like a grape being squeezed between two fingers). The curvature of the corneal surface would be different in two perpendicular planes. An optometrist can correct for astigmatism by prescribing glasses with different focal lengths in the two planes. The prescription must...
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Treatment and Therapy (Magill’s Medical Guide, Sixth Edition)
During an eye examination, the optometrist tries to detect any deviations from normal vision. If the patient is nearsighted or farsighted or has astigmatism, appropriate corrective lenses can be prescribed. If cataracts, glaucoma, or a retinal problem exists, the patient will be referred to an ophthalmologist, who has received specialized medical training in eye surgery.
The history of eyeglasses has been traced back to the thirteenth century, when Roger Bacon, a Catholic scholar, wrote about using convex glass to make writing appear larger. Some medieval paintings show elderly noblemen wearing eyeglasses. No significant innovations were made until Benjamin Franklin invented bifocals in 1780, to aid people whose eyes did not focus properly at either near or far distances. Until the late 1940’s, prescription eyeglasses were always made out of glass. Then plastic lenses were introduced; they had the advantages of lighter weight and greater resistance to breakage. The main problem with plastic is that it scratches more easily, but coatings have been developed to overcome this drawback. More than 80 percent of the eyeglasses worn in the United States are now made of plastic.
An alternative to eyeglasses came in the 1950’s with the development of contact lenses. They were made out of a hard plastic and covered the front of the cornea, floating on a thin layer of tears. They provided good vision but were...
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Perspective and Prospects (Magill’s Medical Guide, Sixth Edition)
The human eye is the most important sense organ for individuals to gather information about their environment. An amazingly high 40 percent of all nerve fibers going to the brain come from the retina of the eye. Any defect or deterioration from normal vision is a serious limitation. During the Middle Ages, few people learned to read and write, so the need for seeing at close range was not important. In modern society, however, people with poor eyesight are greatly handicapped. For example, students, computer operators, airplane pilots, and athletes cannot function without good vision.
Society is gradually becoming more sympathetic to people with handicaps, including blindness. Braille printing, guide dogs, and books recorded on audiotape are helpful developments for the blind. The U.S. Congress in 1992 passed the Americans with Disabilities Act, which mandates improved access for the visually impaired in facilities that serve the general public. Nevertheless, retaining good vision and preventing further deterioration will continue to be a vital part of overall health care.
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For Further Information: (Magill’s Medical Guide, Sixth Edition)
Anshel, Jeffrey. Healthy Eyes, Better Vision: Everyday Eye Care for the Whole Family. Los Angeles: Body Press, 1990. A good overview of eye problems typical during childhood, midlife, and old age. The advantages and disadvantages of various types of contact lenses are evaluated. Other topics include eye hazards from work or sports and eye emergencies.
Berns, Michael W. “Laser Surgery.” Scientific American 264 (June, 1991): 84-90. An excellent article explaining how lasers are used as medical tools. Photographs of blood vessels in the retina of the eye before and after surgery are shown, along with other applications of this technology.
Buettner, Helmut, ed. Mayo Clinic on Vision and Eye Health: Practical Answers on Glaucoma, Cataracts, Macular Degeneration, and Other Conditions. Rochester, Minn.: Mayo Foundation for Medical Education and Research, 2002. A helpful handbook on all the medical, social, and emotional facets of vision impairment.
Cassel, Gary H., Michael D. Billig, and Harry G. Randall. The Eye Book: A Complete Guide to Eye Disorders and Health. Baltimore: Johns Hopkins University Press, 2001. With particular attention on degeneration of the eye, the combined expertise of three eye care professionals produces a primer on the physiology of the eye and its dysfunctions.
National Foundation for Eye Research. http://www.nfer.org. Provides consumers...
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Vision Disorders (Encyclopedia of Public Health)
Human vision is dependent on the successful interaction of optical structures in the eye. When these structures malfunction, vision disorders occur. The key to treatment and resolution of these disorders is early detection through regular eye exams and prompt consultation with an ophthalmologist when problems occur.
The best way to describe how vision works is to use the analogy of a camera. The pupil manages the incoming light rays, opening and closingike a camera shutterccording to the amount of light available. These light rays are progressively refracted and focused by three structures: the cornea, a transparent, convex cover over the iris and pupil in front of the eye; the lens, a spherical body behind the cornea, and the vitreous humor, a gelatinous substance that fills the back of the eyeball. It is important that the rays be in sharp focus when they reach the retina, a sensory membrane that lines the back of the eye and acts like film in a camera. The retina converts the light rays into electrical signals that are sent to the brain by way of the optic nerve. The brain then translates these electrical signals into what we know as sight.
Refractive Errors. The most common vision disorders are refractive errorspecifically nearsightedness, farsightedness, and astigmatism. In each case, the eye does not refract the incoming light properly, so the image is blurred. While they are not diseases, refractive errors affect every age range and comprise the largest treatment effort of ophthalmologists. Refractive errors can be successfully corrected with eyeglasses, contact lenses, and laser refractive surgery.
Cataract. A cataract results when the normally transparent lens of the eye clouds, blurring vision. Most cataracts are age-related, advancing slowly and progressively until functional blindness occurs. Cataract cannot be prevented or cured with medication or optical devices, but it can be successfully treated through a surgical procedure that removes the damaged, natural eye lens and replaces it with a permanent, intraocular lens implant. The procedure has over a 90 percent success rate. After refractive errors, cataract is the most common vision disorder.
Macular Degeneration. Located in the retina, the macula is responsible for central vision. When people have macular degeneration, they can no longer bring the center of the picture they see into focus. The most common type of the disease is agerelated, and there are two forms: "wet" and "dry." Whereas the wet form comprises only about 10 percent of cases, it causes the greatest vision loss, striking quickly and without warning as a result of erupting blood vessels. The dry form is characterized by a slow, progressive loss of vision from the thinning and tearing of the macula. Although both forms are being extensively researched, definitive causes and treatments have not yet been identified. Age-related macular degeneration is the leading cause of blindness in most developed countries.
Glaucoma. Glaucoma is a disease of the optic nerve. If the aqueous humor (the clear fluid that fills the front of the eye) does not drain properly, intraocular pressure builds, damaging the optic nerve and causing blind spots to develop. When the entire nerve is destroyed, blindness results. If glaucoma is detected and treated in the early stages, loss of vision can be averted. However, the disease is chronic and cannot be cured or reversed. Unfortunately, the early stages are symptomless. Once symptoms occur, usually manifested by loss of peripheral or side vision, irreversible vision loss has already taken place. Treatment consists of medication and/or surgery, depending on the type of glaucoma, the patient's medical history, and the stage of the disease. Glaucoma is the leading cause of blindness worldwide and the second-leading cause in developed countries.
Diabetic Retinopathy. Retinopathy is a side effect of diabetes and occurs as a result of fluctuations in the body's blood sugar, a daily problem for diabetics. When blood sugar fluctuates over time, it affects the blood vessels in various parts of the body, including the retina of the eye, where the blood vessels can break and bleed, causing blurred vision. The longer a person has diabetes, the higher the risk of retinopathy; good diabetic control can forestall the disease, however. Signs of retinopathy often occur before symptoms appear. Treatment includes the use of laser photocoagulation to seal leaking blood vessels. Often undetected and untreated, diabetic retinopathy is the leading cause of visual disability among working-age people.
Retinal Degenerations. Retinal degeneration is an umbrella term for a number of hereditary and degenerative disorders that range from mild to profound vision loss and blindness. Retinitis pigmentosa is the most common type of retinal degeneration, affecting one in three thousand people. Its many forms have widely varied symptoms, and onset and progress of the disease can be slow or rapid. In general, symptoms occur in childhood or young adulthood. Patients complain of night blindness followed by loss of visual field. There is no treatment, though researchers are hopeful that genetic therapies may be possible in the future.
Strabismus. Unlike most other vision disorders, strabismus is a physical defect. One or both of the eyes are misaligned and point in different directions. One eye may look ahead while the other eye points up, down, in, or out. Strabismus is more common in children than in adults. In adults it can be a side effect of head trauma or brain disorder. Treatment may involve eyeglasses, an eye patch (in some cases), or surgery on the eye muscles.
BARBARA L. PAWLEY
(SEE ALSO: Diabetes Mellitus)
O'Toole, M. (1997). Miller-Keane Encyclopedia & Dictionary of Medicine, Nursing, & Allied Health, 6th edition. Philadelphia, PA: W.B. Saunders.