Myopia

Myopia, or nearsightedness, is the ability to see close objects clearly, but distant objects appear blurry. Myopia is a defect of the eye in which the focusing system, the cornea and lens, is optically too powerful, or the eyeball is too long (axial myopia). As a result, the focal point is too near the lens, and the image is focused in front of the retina.

Myopia is corrected by a concave lens that counters the refractive power of the eye. Concave lenses spread out the light rays coming to the eye and are therefore called "minus" lenses.

Another technique for correcting myopia is radial keratotomy, which consists of making a series of four to eight radiating cuts in the cornea. The cuts are intended to slightly weaken the dome of the cornea so that it becomes more flattened and eliminates the myopia. One problem with the technique is that it is difficult to predict exactly how much flattening will occur. In one study of 400 patients 5 years after the surgery, 55% had normal vision, 28% were still somewhat myopic, and 17% had become hyperopic. Another problem is that some patients are bothered by glare following radial keratotomy because the slits apparently don't heal evenly.

An alternative procedure that is currently being investigated is laser corneal sculpturing, where a thin portion of the cornea is etched away to make the cornea areas convex. The advantage of this procedure is that the results can be more accurately predicted than those from radial keratotomy.

Hyperopia

Hyperopia, or farsightedness, is the ability to see distant objects clearly, but close objects appear blurry. Hyperopia is a disorder in which the cornea and lens systems is optically too weak or the eyeball is too short. The image is focused behind the retina.

Hyperopia can be corrected by convex lenses that cause light rays to converge as they approach the eye. Such lenses are called "plus" lenses.

Presbyopia

Presbyopia is the normal, presently unavoidable, degeneration of the accommodation power of the eye that occurs as a consequence of aging. It occurs because the lens becomes sclerotic and less flexible. The eye is presbyopic when the near point of vision has increased beyond 9 inches. The average age for onset of presbyopia is the mid-forties. Avid readers or people engaged in fine, close work may develop the symptoms earlier.

Presbyopia can be corrected by the use of "reading glasses" that are worn only for close work and are removed when the person wants to see at a distance. However, it is sometimes an annoyance to keep removing and replacing glasses because reading glasses hamper vision of only a few feet away. This problem may be corrected by the use of half glasses, or by bifocals, which have a different lens in the top and the bottom.

Astigmatism

Astigmatism is a type of refractive error in which the quality of focus is affected. If the cornea or lens is not uniformly curved, the light rays do not focus at a single point but fall as a blurred circle. Regular astigmatism can be corrected by glasses that are formed with the opposite curvature gradation. Irregular astigmatism is a situation in which the abnormal form of the cornea fits no specific pattern and is very difficult to correct with glasses.

Strabismus

Strabismus is a lack of parallelism of light paths through the eyes. Strabismus can involve only one eye or both eyes, and the eyes may turn in (convergent) or out (divergent). In concomitant strabismus, the most common congenital type, the angle between visual axes remains constant, regardless of the direction of the gaze. In noncomitant strabismus, the angle varies, depending on the direction of the gaze, and deviates as the gaze changes.

In some cases, the image that appears on the retina of one eye may be considerably different from that appearing on the other eye. This problem is called diplopia (double vision) and is often the result of weak or abnormal eye muscles.

Retinal Detachment

Retinal detachment is a relatively common problem that can result in complete blindness. The integrity of the retina depends on the vitreous humor, which keeps the retina pushed against the other tunics of the eye. If a hole or tear occurs in the retina, fluid may accumulate between the sensory and pigmented retina. This separation may continue until the sensory retina has become totally detached and folded into a funnellike form around the optic nerve. When the sensory retina becomes separated from its nutrient supply in the choroid, it degenerates, and blindness follows. Other causes of retinal detachment include a severe blow to the eye or head; a shrinking of the vitreous humor, which may occur with aging; or diabetes. The space between the sensory and pigmented retina, called the subretinal space, is apparently also very important in keeping the retina from detaching, as well as in maintaining the health of the retina. The space contains a gummy substance that glues the sensory retina to the pigmented retina.

Color Blindness

Color blindness results from the disfunction of one or more of the three photopigments involved in color vision. If one pigment is disfunctional and the other two are functional, the condition is called dichromatism. An example of dichromatism is red-green color blindness.

The genes for the red and green photopigments are arranged in tandem on the X chromosome, which explains why color blindness is over eight times more common in males than in females. There are six exons for each gene. The red and green genes are 96% to 98% identical and, as a result, the exons may be shuffled to form hybrid genes in some people. Some of the hybrid genes produce proteins with nearly normal function, but others do not. Exon five is the most critical for determining normal red-green function. If a red pigment gene with the fifth exon is replaced by the fifth exon from a green gene, the protein made from the gene responds to wavelengths more toward the green pigment range. The person has a red perception deficiency and is not able to distinguish between red and green. If a green pigment gene with the fifth exon is replaced by the fifth exon from a red gene, the protein made from the gene responds to wavelengths more toward the red pigment range. The person has a green perception deficiency and is also not able to distinguish between red and green.

Apparently only about three of the over 360 amino acids in the color opsin proteins (those at positions 180 in exon 3 and those at 277 and 285 in exon 5) are key to determining their wavelength absorption characteristics. If those amino acids are altered by hydroxylation, the absorption shifts toward the red end of the spectrum. If they are not hydroxylated, the absorption shifts toward the green end.

Night Blindness

Everyone sees less clearly in the dark than in the light. However, a person with night blindness may not see well enough in a dimly lit environment to function adequately. Progressive night blindness results from general retinal degeneration. Stationary night blindness results from nonprogressive abnormal rod function. Temporary night blindness can result from a vitamin A deficiency.

Patients with night blindness can now be helped with special electronic optical devices. These include monocular pocket scopes and binocular goggles that electronically amplify light.

Glaucoma

Glaucoma is a disease of the eye involving increased intraocular pressure caused by a buildup of aqueous humor. It usually results from blockage of the aqueous veins or the canal of Schlemm, restricting drainage of the aqueous humor, or from over-production of aqueous humor. If untreated, glaucoma can lead to retinal, optic disc, and optic nerve damage. The damage results from the increased intraocular pressure, which is sufficient to close off the blood vessels, causing starvation and death of the retinal cells.

Glaucoma is one of the leading causes of blindness in the United States, affecting 2% of people over age 35, and accounting for 15% of all blindness. Fifty thousand people in the United States are blind as the result of glaucoma, and it occurs three times more often in black people than in white people. The symptoms include a slow closing in of the field of vision. There is no pain or redness, and there are no light flashes.

Glaucoma has a strong hereditary tendency, but may develop after surgery or with the use of certain eyedrops containing cortisone. Everyone over age 40 should be checked every 2 to 3 years; those over age 40 who have relatives with glaucoma should have an annual checkup. During a checkup, the field of vision is checked, and the optic nerve is examined. Ocular pressures can also be measured. Glaucoma is usually treated with eyedrops, which do not cure the problem but keep it from advancing. In some cases, laser or conventional surgery may be used.

Cataract

Cataract is a clouding of the lens resulting from a buildup of proteins. The lens relies on the aqueous humor for its nutrition. Any loss of this nutrient source leads to degeneration of the lens (that is, a cataract). A cataract may occur with advancing age, infection, or trauma.

A certain amount of lens clouding occurs in 65% of patients over the age of 50 and 95% of patients over the age of 65. The decision as to whether or not to remove the cataract depends on the extent to which light passage is blocked. Over 400,000 cataracts are removed in the United States each year. Surgery to remove a cataract is actually the removal of the lens. The posterior portion of the lens capsule is left intact. Although light convergence is still accomplished by the cornea, with the lens gone the rays cannot be focused as well, and an artificial lens must be supplied to help accomplish focusing. In most cases, an artificial lens is implanted into the remaining portion of the lens capsule at the time that the natural lens is removed. The implanted lens helps to restore normal vision, but glasses may be required for near vision.

Macular Degeneration

Macular degeneration is very common in older people. It does not cause total blindness but results in the loss of acute vision. This degeneration has a variety of causes, including hereditary disorders, infections, trauma, tumor, or most often, poorly understood degeneration associated with aging. No satisfactory medical treatment has been developed. Therefore optical aids, such as magnifying glasses, are used to improve visual function.

Diabetes

Loss of visual function is one of the most common consequences of diabetes because a major complication of the disease is dysfunction of the peripheral circulation. Defective circulation to the eye may result in retinal degeneration or detachment. Diabetic retinal degeneration is one of the leading causes of blindness in the United States.

Infections

Trachoma is the leading case of blindness worldwide. There are 500 million cases of trachoma in the world, and 7 million people are blind or visually impaired as a result of trachoma. It is caused by an intracellular microbial infection of the corneal epithelial cells, resulting in scar tissue formation in the cornea. The bacteria are spread from one eye to another eye by towels, fingers, and other objects.

Neonatal gonorrheal ophthalmia is a bacterial infection of the eye that cause blindness. If the mother has gonorrhea, which is a sexually transmitted disease of the reproductive tract, the bacteria can infect the newborn during delivery. The disease can be prevented by treating the infant's eyes with silver nitrate, tetracycline, or erythromycin drops.

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