Sunday, January 15, 2006

myopia (near-sightedness)

Myopia is a condition in which parallel rays of light come to focus at a point in front of the retina. The myopic eye has basically too much plus power for its size. Elderly patients with cataracts will become more near-sighted as their cataracts develop. Frequently, they will discover they can read without glasses. This is sometimes referred to as "second sight. "
In axial myopia the eyeball is too long for
the normal refractive power of the lens and the cornea. In curvature myopia the eye is of normal size but the curvature of the cornea and the lens is increased. In index myopia a change in the index of refraction of the lens is present. This is witnessed in two pathologic states, diabetes mellitus and cataract. In diabetes the lens loses water because of the high level of blood sugar in the anterior chamber and therefore its index of refraction increases. In the cataract patient the center of the lens becomes increasingly hard. This hard inner core increases the index of refraction of the entire lens structure, thereby increasing the converging power. The chief symptom of myopia is an inability to see at a distance. Treatment of myopia usually involves the use of glasses or contact lenses to correct the refractive error. Radial keratotomy is a procedure in which multiple radial cuts are made in the cornea to help correct the near-sightedness. An older adult also may need to wear reading glasses if he or she does not wear glasses for distance following this procedure.
In spite of extensive research and clinical investigations the cause of myopia is poorly understood. Myopia is familial in nature and is passed from one generation to another as a dominant trait.
See also ASTIGMATISM; HYPEROPIA; PRESBYOPIA.
Newell, F. W. Ophthalmology Principles and Concepts, 6th ed. St. Louis; C. V. Mosby Co., 1986
Slatt, B. I., and Stein, H. A. The Ophthalmic Assistant Fundamentals and Clinical Practice, 4th ed. St. Louis: C. V. Mosby Co., 1983.

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