Here's one good reason to turn off the Wii or Game Boy: Eye experts increasingly believe that time spent outdoors could reduce the likelihood that children will develop myopia, or nearsightedness, a condition in which distance vision is blurred.
"Your mother was doing the right thing when she said, 'Go outside and play,' " says Earl Smith, dean of the College of Optometry at the University of Houston.
Myopia is on the rise around the world. A recent study found that in Americans ages 12 to 54, the prevalence of myopia increased 66% between 1970 and 2000. Asia has also experienced a sharp jump in nearsightedness in urban areas. "Nearsightedness is showing up at younger ages and at higher progression rates," says Thomas Aller, an optometrist based in San Bruno.
Though myopia has a strong genetic component, genes alone cannot explain these increases.
"It's not all your family history, it's not all your outside time, it's not all your near work," says Susan Vitale, a research epidemiologist at the National Eye Institute of the National Institutes of Health. "All those things work together in a complicated way that we don't really understand yet."
"Near work" activities, such as reading and computer use, have long been considered the most likely culprits. But recent studies indicate that the amount of time children spend outdoors could play an important role as well. One recent example: A 2008 study in the journal Ophthalmology found that 12-year-olds who spent more than 2.8 hours outside per day on average were less likely to have myopia than those who spent less time outside, regardless of the amount of time they spent doing near work.
One possibility is that the eyes need exposure to a certain amount of light intensity; another is that spending time outside exposes the eyes to objects that are consistently focused in the distance.
Optometrists and researchers are developing a new arsenal of treatments that they hope will slow myopia's progression. These include specialized eyeglasses and contact lenses, new types of bifocals, specialized eye drops and contact lenses to be worn at night to reshape the cornea.
But even with these promising technologies, there is no way yet to prevent nearsightedness or any drug to reverse it. Ultimately, scientists hope that through better understanding of the interplay of genetics, environmental factors and eye function, the effects of nearsightedness on lifelong vision can be reduced.
Nearsightedness usually develops during childhood or early adolescence, often between the ages of 8 and 10, though it may not show up until early adulthood. For some people, the condition grows progressively worse over time. For those with extreme myopia, the structural changes in the eye associated with the condition can lead to cataracts, glaucoma and blindness.
Myopia occurs because the eye grows too long. As a result, images become focused in front of the retina, rather than squarely on it. Traditional eyeglasses and contact lenses bend light to redirect the focal point back to the retina, bringing distant objects into focus. But traditional lenses have had limited success in slowing the rate of growth of the eye and preventing myopia's progression.
One possible reason for the limitations of traditional lenses is that they correct only the image in the center of the eye. They don't address peripheral vision. Some experts believe that signals from the eye's periphery may cause the eye to elongate, leading to nearsightedness.
Bifocal eyeglasses and contacts, with two corrective powers in each lens, have been shown to reduce the rate of myopic progression in certain children. Researchers recently reported that children who wore one type of bifocal lens, called a "prismatic executive bifocal," had a 58% reduction in the rate of myopic progression, compared with children wearing single-vision lenses. Even standard bifocals slowed myopic progression by 38% compared with single-vision lenses.
Bifocal lenses are particularly effective in children who tend to cross their eyes or have focusing problems. And studies have shown that bifocal soft contact lenses work more effectively than bifocal eyeglasses. But it's not yet clear whether the benefit of bifocals is due to their effect on peripheral or central vision, or both.
Another treatment for myopia, orthokeratology, uses rigid, gas-permeable contact lenses that are worn at night to flatten the corneas. The contacts are removed during the daytime. The effect is to eliminate nearsightedness during the day. Studies have shown that corneal reshaping slows the rate of myopic progression by about half.
"Even though they're relatively small, all of the [orthokeratology] studies so far have shown very positive benefits," says Christine Wildsoet, an expert on nearsightedness at UC Berkeley.