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Refractive Errors

DESCRIPTION: In a normal eye, parallel rays of light focus exactly on the fovea (the central area of the macula), when the eye is in a state of rest (i.e., the lens does not have to accommodate). In a farsighted (hyperopic) eye, under the same conditions, the eyeball is too short, and the light rays focus (theoretically) behind the fovea. In a nearsighted (myopic) eye, under the at-rest conditions, the eyeball is too long, and the light rays come to a focus before they reach the fovea. Astigmatism is an irregular curvature of the cornea in one or more of its meridians. The lens may also contribute to astigmatism (as in old age, when it may become somewhat irregular in shape because of cataractous changes). Astigmatism may be simple(i.e., not combined with hyperopia or myopia), or compound; when an eye has both myopia and astigmatism or hyperopia and astigmatism, it is a compound defect. Astigmatism may also be "mixed" (when myopia is combined with hyperopic astigmatism, or when hyperopia is combined with myopic astigmatism). In middle age (beginning anytime past age 40), the lens becomes less flexible and less able to accommodate for nearpoint viewing; this condition is called "presbyopia" and is described as "when arms aren't long enough." In addition to simple myopia or hyperopia and the variations of astigmatism, and because the human organism has two eyes which must have coordinated visual reception for good vision to occur, a multitude of refractive variations are possible. Anisometropia refers to different refractive errors in each eye, and aniseikonia denotes a difference in the image size in the two eyes.

Refractive errors tend to be inherited, but there is no pattern of inheritance. Size of the eyeball, shape of the cornea, shape of the lens, and depth of the anterior chamber are all variables in refractive errors. These variables increase the possible ocular combinations for refractive errors.

Symptoms of myopia include squinting and frowning; hyperopia may cause a lack of interest in reading, rubbing of the eyes, or even headache, dizziness, or nausea. Astigmatism may cause visual fatigue, headaches, frowning, and squinting.

Degenerative myopia (sometimes also called progressive myopia) is similar to simple myopia except that the degenerative changes occur in the optic disk, choroid and retina, sclera, and vitreous, and are not related to the degree of myopia (i.e., the myopia does not increase; the structure of eye parts changes in such a way that visual function is negatively affected). Loss of central vision, retinal detachment, and vitreous opacities are typical; cataracts and secondary glaucoma may be additional complications. Progressive myopia is genetically determined as a recessive trait.

TREATMENT: Myopia and hyperopia are treated by the use of spherical concave and convex lenses, respectively. Astigmatic corrections are cylindrical and are added to any prescription for myopia or hyperopia. Presbyopia necessitates the use of bifocals or trifocals. In the absence of disease or other ocular abnormalities, glasses or contact lenses are the only treatment needed for refractive errors.

IMPLICATIONS: The wearing of glasses does not "strengthen" or "weaken" eyes, or affect the degree and progress of myopia. Eye exercises are of no benefit in improving refractive errors, since they cannot alter the size of the eyeball or the refractive power of the lens.

Current interest in radial keratotomy (the surgical changing of the shape of the cornea) suggests possible "cures" for high myopia and astigmatism, but the surgical techniques are still in experimental stages. Long-term effects have yet to be observed, analyzed, and documented.

Contact lenses have revolutionized the corrective lens industry, however, both hard and soft contact lenses continue to be somewhat controversial. As with surgical solutions to refractive errors, there are precautions and problems yet to be solved. Contact lenses are preferable to glasses in some kinds of corneal anomalies and in unilateral aphakia, but their general use still assumes user integrity. Severe ocular injury can occur if contact lenses are used carelessly. Moreover, certain occupations (e.g., beauticians) should avoid the use of the soft lenses, since they tend to absorb chemicals that could be harmful to the eye. The decision to wear contact lenses must be made with care; they are not "for everybody."

About 80% of children are hyperopic at birth because of the shortness of the eye; approximately 5% are myopic. During the years between ages 2 and 25, there is a gradual decrease in hyperopia; myopia usually increases somewhat during the teen years, and levels off at around age 25, regardless of lighting, rest, amount of close work, or vitamins taken. Most astigmatism remains fairly constant throughout life (in the absence of keratoconus).

Educational implications for refractive errors focus primarily on identification through early and continual, adequate vision screening, referral, and follow-up. Most refractive errors found in school children can be corrected with lenses. The major exception is the child with high myopia, who may be unable to be fully corrected. Magnification, higher levels of illumination, and/or telescopic aids may be indicated. Certainly, a low vision evaluation should be pursued for these children.

In the case of degenerative myopia, the associated problems should be addressed (e.g., central vision loss). Educational adjustments may need constant re-evaluation as the ocular condition progresses.

 

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