Why We're Losing Sight
May 1, 2001 -- Warning: Reading this article could be bad for your eyesight. Eye care specialists still don't know what causes myopia -- nearsightedness -- or why more and more nearsighted people seem to need frequent changes in their eyeglass prescriptions. But there is evidence to suggest that a combination of heredity and environment may be at work, say researchers who peer into the workings of the eye.
Not too long ago, people who were nearsighted generally found that their eyesight didn't get any worse once they were past puberty. The general thinking was that the eye had done all the growing it was going to do, and that no more changes in refraction (the degree of visual correction needed to bring vision close to 20/20) would occur until they reached middle age, when the inevitable shift toward farsightedness begins to occur.
But as several studies presented at the annual meeting of the Association for Research in Vision and Ophthalmology in Ft. Lauderdale, Fla., suggest, what we look at, how we do it, and under what conditions we are looking can influence how quickly nearsighted people will need new glasses.
"There's certainly lots of evidence that how we use our eyes will effect a refractive outcome," Douglas R. Frederick, MD, assistant professor of ophthalmology at Dartmouth Medical School in Lebanon, N.H., tells WebMD. "People who go to graduate school tend to be more myopic and tend to get myopic during graduate school, so it's a process that continues through the 20s, and doesn't stop in the teens or adolescence. Then there has been a whole body of [animal studies], ... and in these ... how the eyes are focused and what they're focused upon determine the length of the eye and the eventual refractive outcome," Frederick tells WebMD.
In an eye with normal or 20/20 vision, light focuses as a sharp image on the retina, the thin tissue that lines the back of the eye and is responsible for transmitting visual images to the brain. Myopia occurs when the eyeball is too long, instead of round, or when the cornea -- the transparent outer membrane in the center of the eye -- is too steep, causing light to focus in front of the retina, rather than on it. Glasses or contact lenses correct for myopia by refracting or bending light so that it focuses on the retina as it should.
Although studies of eyesight in identical twins have suggested that there may be some genetic component to myopia, heredity seems to play less of a role than environment, says Richard A. Stone, MD, vice chair for research at the Scheie Eye Institute and professor of ophthalmology at the University of Pennsylvania in Philadelphia.
"There is a tremendous increase in myopia generally as societies change, moving from rural to urban industrialized societies," he tells WebMD. "That's what happened in Asia: There's very good data showing a tremendous increase in myopia prevalence from about 10% to 50%-80% over just a few decades, so obviously there's a tremendous environmental component."