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."
What's more, the stereotype of the nearsighted intellectual who polishes his eyeglasses with the end of his tie may not be too far off the mark, Stone says.
Other studies show a very strong association between myopia and increased education and increased intelligence. "The theory is that [the myopia] is caused by near work, reading, using the eyes for stuff up close, but it's proved extraordinarily difficult to get convincing evidence that actually using the eyes up close is what's responsible," says Stone.
Stone and colleagues examined another possibility: That exposure to light, or, more accurately, less time spent in darkness, might have an effect on myopia progression. Stone tells WebMD that decades of animal studies have suggested that alterations in normal cycles of light and darkness can have an adverse effect on vision, and that this might account for at least some myopic changes.
To test this, they looked at myopia progression and various potential myopia risk factors in third-year law students at the University of Pennsylvania. They found that while having nearsighted parents didn't seem to make much difference, burning the midnight oil did. Of the 96 students who were nearsighted before law school, myopia progressed in 86% during law school, and among 75 students with normal vision at the beginning of law school, 19% became myopic. Students who were exposed to less than about 5.5 hours of darkness each day were more likely to experience worsening of myopia than those getting more darkness -- presumably in sleep.
"This study confirms high myopia prevalence and a high rate of myopia progression among law students. The strongest association, especially in those with myopia onset before college, was a relation of myopia progression during law school with less daily exposure to darkness, a potential risk factor previously identified in childhood myopia," the researchers write.
Other researchers, however, argue that it's not too much light exposure, but too much time spent hunched over textbooks that has studious students squinting.
Karla Zadnik, OD, PhD, and colleagues at the Ohio State University College of Optometry in Columbus found in a study of eighth graders that children who had at least one myopic parent, who did more near work and/or less sports, and who scored higher in reading on a standardized test were more likely to be myopic. Significantly, increasing the amount of near work doubled the risk of becoming myopic, but that was less significant an effect than having a myopic parent. The finding suggests both heredity and environment play a role, the researchers say.
A third study found that among 71 nearsighted schoolchildren, myopia progressed more during the school year, when the kids were more likely to be doing the three "R's" or working at a computer, than it did during the summer months, when they were more likely to use their eyes for tasks requiring distance vision.
Researcher George W. Fulk, PhD, OD, a professor of optometry at Northeastern State University in Tallequah, Okla., tells WebMD that when they measured the eyes of these students, they found that the change in nearsightedness was much greater after three months of school than after three months of vacation, and that the changes could be explained by the need for the eyes to adapt to close work during the academic year.
The bottom line is that when your mom yelled, "Get your nose out of that comic book or you'll ruin your eyes!" -- she may just have been right.