WHAT IS THE EYE PROBLEM CALLED MYOPIA?
It is estimated by the United States National Eye Institute that every fourth adult around the world is affected to some degree by myopia, the medical name for nearsightedness. Myopia is a condition in which parallel rays of light are focused in front of the retina, the consequence of an error in refraction or of elongation of the globe of the eye, causing nearsightedness.
To explain more fully: In normal vision, light rays entering the eye are bent by the cornea, bent and inverted by the lens, and come to focus on the retina, the sensitive
membrane at the back of the eye that receives images and transmits them through the optic nerve to the brain, where the image is turned right side up. In nearsighted eyes, images fall short, coming to focus in front of the retina.
Myopia is expressed medically in terms of diopters. If a patient has one diopter (-1.00 D) of nearsightedness, this means the farthest focus of the eyes is one meter (about three feet) from his or her eyes. If the patient has two diopters (-2.00 D) of nearsightedness, his or her farthest point of vision is half a meter from the eyes; with three diopters (-3.00 D) of such myopia, the far point is a third of a meter; with four diopters (-4.00 D), the far point is one-fourth of a meter. In other words, the person having nearsightedness of-4.00 D or one-fourth of a meter as his or her far vision can see clearly only to a distance of approximately nine inches. Between -1.00 D and -4.00 D is in the average or "usual" range of myopia.
Myopia consists essentially of two types; (1) usual myopia, in which the eyeball is longer than normal and with a cornea that is too refractive for which the patient may be wearing concave lenses in order to see at a distance, (2) the pathologic higher degree of myopia in which the patient may have approximately -7.00 diopters of "high myopia." Then unusual changes begin to become apparent to the examining ophthalmologist, due to excessive stretching and elongation of the eyeball.
Using an instrument called the ophthalmoscope, the eye doctor sees the interior of your eye by using a beam of light directed inside. He observes along the spot where the beam falls and checks for a white crescent (conus) around the optic nerve. When stretching of the eyeball from high myopia becomes too great for the retina and choroid to withstand, the choroid becomes thinned and a large, whitish, yellow area of tissue degeneration is seen. The stretching produces cracks and even areas of wasting in the macula. At times a small red or black spot resulting from hemorrhage in the fovea may be observed. This is bad because it abolishes central vision and is referred to as a "Fuch's dot." Thus, while nearsightedness is a mechanical condition of the eyes, it can bring on an abnormal physiological change resulting in disease.
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