Causes and Symptoms
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. Millions of people use some form of vision correction.
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 specify the angle at which the deformation of the eyeball is maximized.
A vision problem that is common among older adults is the formation of cataracts, in which the lens of the eye becomes cloudy. Cataracts are a normal part of the aging process, like wrinkled skin or gray hair. In rare cases, however, children have them at birth or after an eye injury. Cataracts form on the inside of the lens capsule, not on the surface of the eye. Once started, their growth is irreversible. While vision can often helped through the use of eyeglasses and stronger lighting, the only entirely effective treatment is surgical removal of the defective eye lens, followed by implantation of an artificial (plastic) replacement. With developments in ophthalmology, such microsurgery has a success rate of better than 95 percent. What causes eye cataracts in the elderly is not yet well understood. One suggested explanation is the Maillard reaction, in which glucose and protein molecules combine when heated to form a brown product. This chemical reaction is responsible for the browning of bread or cookies during baking. The same process is thought to occur even at body temperature, but very slowly over a period of years. It has been suggested that the onset of cataract formation can be delayed by a good diet, regular exercise, and a generally healthy lifestyle.
Glaucoma
is a vision problem that afflicts about 2 percent of the adult population, normally after the age of forty. Excessive fluid pressure develops inside the eye, causing damage to the optic nerve. Peripheral vision gradually decreases—a decrease that the patient may not even notice until it is detected by an optometrist during an eye examination. The usual treatments are medicated eyedrops to reduce pressure and laser surgery to improve fluid drainage. Glaucoma has nothing to do with red or watery eyes because these symptoms occur on the exterior of the eyeball.
The retina is a paper-thin membrane at the back of the eye, nourished by a network of tiny blood vessels. A frequent problem encountered by diabetics is the enlargement and possible hemorrhaging of these blood vessels. For older adults,
macular degeneration is a condition associated with arteriosclerosis, sometimes leading to retinal bleeding. The most sensitive, central region of the retina deteriorates, causing an irreversible loss in reading ability that cannot be corrected with glasses.
Another retinal problem is its detachment from the back wall of the eye. This is an emergency situation requiring immediate medical attention. A
detached retina can be caused by an accumulation of fluid behind the retina resulting from leakage through a small tear in the membrane. It can also come from a blow to the eye, as with a sports injury. Laser surgery has become an effective treatment for the various types of retinal damage.
Treatment and Therapy
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 1940s, 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.
An alternative to eyeglasses came in the 1950s 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 uncomfortable to insert. Also, hard contacts cannot transmit oxygen and carbon dioxide to nourish the surface of the cornea, causing dryness and irritation for the wearer. Such lenses are now virtually obsolete. Daily-wear soft contact lenses became available in the 1970s. They were much more comfortable than the hard plastic material and were gas-permeable. The soft lenses had an affinity for infection-causing bacteria, however, requiring a tedious, nightly sterilizing procedure with heat or chemicals. The technology of contact lenses continues to evolve. More recent developments are soft contacts for extended wear (up to two weeks without removal), bifocal gas-permeable contacts, and inexpensive, disposable contacts (to be discarded after two or three weeks). Contact lens wearers are cautioned to have regular eye checkups to make sure that the cornea is not being damaged.
Starting in the 1970s, eye specialists began to investigate the possibility of reshaping the eyeball to do away with eyeglasses completely. The first attempt utilized a hard lens pressing directly against the cornea to flatten it, in much the same way that orthodontic braces are used to straighten teeth. The change induced in the shape of the eye generally was only temporary, so lenses still were needed afterward.
A Soviet physician, Svyatoslav Fyodorov, developed radial keratotomy (RK), a surgical procedure to flatten the cornea permanently. A series of shallow incisions is made in the outer part of the cornea in a radial pattern, like the spokes of a wheel. The center of the cornea is not touched. As the incisions heal, the cornea bulges slightly near the edges, thus reducing its curvature in the middle. In this way, a permanent cure for nearsightedness can be accomplished. While thousands of patients underwent RK surgery between 1980 and 1993, the procedure remained controversial. The main problem was that the number of incisions and their depth could overcorrect or undercorrect the original refractive error. Also, some ophthalmologists were concerned about possible long-term aftereffects of scars on the cornea. RK soon decreased in popularity.
Another technique to alter the shape of the cornea is called keratomileusis. The outer half of the patient’s cornea is removed and frozen, and then reshaped with a computer-controlled lathe to a predetermined curvature. After thawing, the cornea is sewn back into place, where it acts as a permanent contact lens. Keratomileusis can correct both myopia and hyperopia.
The laser, invented by physicists in the 1960s, is a very intense beam of light that can be adapted particularly well for surgery on the retina of the eye. The light beam passes successively through the transparent cornea, aqueous fluid, and lens without being absorbed. Its energy is then concentrated into a tiny spot on the retina, causing localized vaporization, or “welding,” to occur. Laser-assisted in situ keratomileusis (LASIK) became quite popular at the end of the twentieth century. In LASIK surgery, the cornea is reshaped to help patients overcome myopia, hyperopia, or astigmatism. The procedure is done with a cool beam laser that removes thin layers of tissue from selected sites on the cornea to change its curvature. Success rates are high: 90 to 95 percent of the patients get 20/40 vision, and 65 to 75 percent of the patients get 20/20 vision or better. Lasers can also be used to excise leaking blood vessels and to repair or reattach a damaged retina.
Another surgical technique is to use a corneal transplant from an organ donor. The new cornea is shaped to the proper curvature with a lathe and is sewn on top of the patient’s own cornea.
The standard treatment for cataracts is surgical removal of the defective lens, followed by implantation of an artificial, plastic lens. Ophthalmologists routinely perform cataract surgery using only local anesthetic, so that the patient can go home without an overnight hospital stay.
Glaucoma, a condition of excess pressure in the eye, affects millions of people and has caused thousands of cases of
blindness. The first line of treatment is the use of daily medication in the form of eyedrops to reduce the fluid pressure. Eventually, surgery may be necessary. The procedure used enlarges an opening at the edge of the iris to allow for better drainage of the aqueous fluid between the lens and cornea. The incision can be made with either a miniature scalpel or a laser. Glaucoma damage to the optic nerve cannot be repaired, but prompt treatment can prevent further deterioration of vision.
Perspective and Prospects
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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