What is myopia?
Nearsightedness (myopia) occurs when light from distant objects reaches a focal point in front of the retina, the photoreceptive tissue of the eye. Consequently, vision of distant objects is blurred on the retina. The primary cause of myopia is an eyeball that is too long from front to back. People whose parents have myopia are more likely to have it, indicating a genetic cause; a few studies have also shown correlations between higher testosterone levels in the womb and later incidence of myopia. Research has also found that prolonged eyestrain, especially from long periods of reading or other close work, can distort the shape of the eye. This may be one reason why more highly educated people manifest higher rates of nearsightedness than individuals with less formal education.
All children are born nearsighted; by the age of six months, however, vision begins to improve. Myopia is an uncommon problem in younger school-age children but begins to increase in prevalence as children move into their teenage years. From the twenties until the forties, the rate of visual deterioration tends to slow down. As people enter middle and old age, however, the rate of visual decline accelerates again. People past the age of seventy are fourteen times as likely to experience myopia resulting in legal blindness as those in their twenties.
For several centuries, nearsightedness has been corrected by the use of a concave lens, which moves the focal point of light in myopic eyes closer to the retina. The first eyeglasses, invented in late-thirteenth-century Italy, had convex lenses that corrected for farsightedness; not until the fifteenth century did glasses with concave lenses appear. As the twentieth century drew to a close, innovative surgical approaches were developed to correct for myopia. Most of these procedures, such as laser surgery, move the focal point of light closer to the retina by changing the shape of the cornea.
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