Web edition: January 24, 2013
Print edition: February 9, 2013; Vol.183 #3 (p. 22)
Rogers Hornsby, one of the best hitters ever to swing a baseball bat, had a reputation for being standoffish. Teammates complained that he didn’t socialize, even balking at attending movies — prime entertainment during the 1920s. Sitting in a dark theater watching a bright screen made it difficult to hit a baseball, Hornsby used to say. Hard to argue with a guy who reportedly had terrific eyesight and who finished three seasons with a batting average better than .400.
Hornsby might have been onto something that scientists are only now coming to embrace: Too much time spent indoors may contribute to nearsightedness, also called myopia.
Nearsightedness has increased steadily in North America and Europe in recent decades, with one-third of adults in the United States now nearsighted. That figure alone is cause for concern. But the rise of myopia in East Asia is downright alarming. Recent studies of young men in Seoul and college students in Shanghai find that more than 95 percent are nearsighted. Increases also have shown up across other urban centers in the Far East.
Studies first uncovered a link between myopia and limited outdoor time during childhood just a few years ago. At the time, many researchers were taken aback. The notion that child’s play might promote normal eye growth seemed almost magical.
“Certainly, before five years ago, I don’t think anybody had taken much notice of how much time people spent outdoors,” says Jeremy Guggenheim, an optometrist who has researched myopia in Wales and is currently at Hong Kong Polytechnic University. He believes the findings offer a “new and exciting direction” for research.
It’s tantalizing to think that time spent outdoors early in life might fend off the need for eyeglasses, contact lenses or laser surgery in many people. But, Guggenheim notes, it is not yet clear to what extent outdoor exposure can cut risk or how it does so. Some scientists say the benefit could come from exposure to natural light, a relaxation of the eye gained from viewing things at a distance or the visual tableau that reaches the eyes’ peripheries while outdoors. Or it could be a mix of all those factors.
“There are great open questions, and that keeps us from making strong recommendations,” says Donald Mutti, an optometrist at Ohio State University College of Optometry in Columbus.
Aside from spending time outdoors, a person’s other behaviors might matter too. The higher myopia rates documented today coincide with a whole generation of children raised on computers, video games and, especially in the Far East, intense pressure to achieve in school. Some researchers cite a long-debated theory linking myopia to excessive reading or other “near work.”
The issue is far from settled. But in Guangzhou, China, a clinical trial of nearsightedness is under way in which some children have been randomly assigned to spend an extra hour outside at school each day. The strategy is showing a slight benefit as the data trickle in.
Lots of myopia
The human eye is quite possibly the most fine-tuned organ in the body. In someone with 20/20 vision, the eyes adapt effortlessly to the task at hand. To see a nearby object, eye muscles controlling the lens constrict, focusing an image of the object on the retina at the back of the eye. People with good vision can also see faraway images clearly, because the eye muscles relax and the lens changes shape accordingly. Distant images thus land in focus on the retina.
But people with myopia have a slightly elongated eyeball, which means trouble. Their eyes can still focus images of nearby objects. But more distant images arrive slightly in front of the retina, so they end up fuzzy.
The eye’s shape depends on growth that occurs primarily during infancy, and to a lesser extent through adolescence. That growth is ruled in large part by genetic instructions that humans have evolved over many millennia; if the genetic blueprint is defective, eyesight can certainly suffer. But growth of the eye also depends heavily on external cues — what scientists call visual feedback. The bombardment of light, with its colors and contrasts, helps guide proper eye growth.
Scientists are now convinced that something about this external input has changed in recent decades, and those changes are driving the onslaught of nearsightedness seen in teens and young adults.
From the early 1970s to the turn of the century, myopia prevalence in the United States rose from 25 percent to nearly 42 percent among people ages 12 to 54, a substantial shift in just one generation. The rate among U.S. young adults is 38 percent, up from 28 percent in the 1970s. On the other side of the globe, myopia rates in Singapore, which has gone from a sleepy port city to a center for international commerce, have risen from 43 percent among military conscripts (all young men) in the late 1980s to more than 80 percent today.
Meanwhile, older generations haven’t experienced a sharp rise in the disorder. The rate in people over age 40 in China and the United States is at about one-fourth.
Because such increases also have not shown up in rural areas, scientists think the trend reflects new behaviors among young urbanites. With more people moving to cities, the trend is likely to worsen.
For some, nearsightedness will be a mere inconvenience. But others, who develop high-degree myopia, will have worsening vision over time and a greater risk of cataracts, glaucoma or a detached retina later in life. Of those young men in Seoul and students in Shanghai who are nearsighted, roughly one in five already has high-degree myopia.
“There will be an epidemic of pathological myopia and associated blindness in the next few decades in Asia,” says Seang-Mei Saw, a physician and epidemiologist at the National University of Singapore.
The uptick in nearsightedness in recent decades prompted scientists to hunt for a cause, bringing the link between myopia and outdoor exposure into sharp relief. In 2007, Mutti and his Ohio State team identified 514 children who were not nearsighted in the third grade and analyzed differences among the kids up to five years later. By then, one-fifth of them had developed myopia. Even after accounting for any parental nearsightedness, the team found that kids who had spent more time outdoors doing physical activity were less likely to become nearsighted than were children who got out less. The report appeared in Investigative Ophthalmology & Visual Science.
In 2008, Ian Morgan of the Australian National University in Canberra and Kathryn Rose of the University of Sydney also reported a link to outdoor exposure. Preteens who spent ample time outside — at leisure or sports — were less apt to develop myopia. Writing in Ophthalmology, the Australians noted that playing indoor sports didn’t seem to have the same effect. What’s more, the researchers didn’t find clear evidence of harm from near work. Simply being outside mattered. Subsequent studies in China, Singapore and the United States have bolstered these results.
The reports represent a departure in thinking from a few decades ago, when many in the field assumed myopia was mainly a hereditary problem. After all, having one nearsighted parent lends some risk of needing glasses, and having two imparts more.
But scientists searching for a genetic smoking gun haven’t found one. Reporting in Nature Genetics in 2010 and in Human Genetics last year, two international teams identified many variant forms of genes that showed up more often in nearsighted people. But, says Guggenheim, “These are subtle genetic effects that explain only a small proportion of myopia.” What’s more, work comparing Chinese children living in different environments and with different outdoor exposures suggests that genetic similarities linked to ethnicity can be trumped by daily behavior.
The new wave isn’t genetic, Morgan says. “The gene pool can’t change that much in a generation, not even in several,” he says. “We’ve now got a very convincing new factor, and that’s time spent outdoors.”
Into the light
While nearsightedness remains incompletely understood at the molecular level, what scientists have figured out supports the outdoor-light explanation. Laboratory work shows that bright light stimulates the release of the neurotransmitter dopamine in the retina, which limits unwanted eye growth.
Eye growth is a delicate balance influenced by dozens of natural chemicals, and light is part of it, says William Stell, a physician and neurobiologist at the University of Calgary in Canada. He likens the process to a car cresting a big hill, poised to zoom down unless the driver steps on the brakes. Dopamine and other stop signals on growth provide those brakes, and exposure to bright light turns on dopamine. A proper balance of homegrown chemicals in the eye, Stell says, “controls whether the car inches forward or plummets.”
Compared with daylight exposure, signals from indoor lighting may not slam on the brakes of growth quite as well. A sunny day delivers 28,000 to 130,000 lux, a measure of light intensity. The average house scores less than 1,000 lux.
Tests in animals strengthen the light link. Scientists can induce myopia in animals by strapping on goggles that blur or distort vision. Exposing chicks to sunlight or bright lights in the lab stopped such myopia development, biophysicist Frank Schaeffel of the University of Tübingen in Germany and colleagues reported in 2009. Working with goggled infant rhesus monkeys, researchers at the University of Houston reported similar results last year in Investigative Ophthalmology & Visual Science. Both findings lend credence to the outdoor-light theory.
But while the research community broadly agrees that good light and unimpeded vision regulate proper eye growth, questions remain. Animal work doesn’t precisely mimic the experience of children. While a distorted visual field can trigger myopia in chicks, simply living under indoor lighting doesn’t, Stell says. Millions of people, after all, develop myopia without wearing goggles. Though the outdoor-light theory stands as a strong finding at the population level, it lacks a full explanation. “We really haven’t identified the exact goodness of being outdoors,” Mutti says.
Apart from providing daylight, being outdoors presents a broad field of vision quite different from being indoors. Andy Fischer, a retinal neurobiologist at Ohio State, notes that the eye is in a more relaxed position in the outdoors. “It’s not working to bend the light,” he says. That relaxation may shut off growth signals that can distort the eye’s shape.
Outdoor exposure also provides a different set of peripheral images, those seen out of the corner of the eye, than does indoor viewing. Although images in the periphery are not at the center of vision, they can be in or out of focus. “If you are outdoors and look into the distance,” Schaeffel says, “the focus in the periphery is pretty homogenous.” That is, images entering via the periphery of the eye are broadly similar in distance and more apt to be in focus. But indoors, he says, there’s a mix of focused and unfocused images.
Animals wearing goggles that distort or block peripheral vision, but not objects at the center of vision, show the type of stimulation in eye growth associated with myopia.
“We used to think that only the center of the eye was, in major part, responsible for eye growth,” says Debora Nickla, a biologist at the New England College of Optometry in Boston. Now scientists realize that may not be the case.
Being outdoors also approximates prehistoric humans’ visual demands. Throughout evolution, Fischer says, humans spent almost all their time outdoors. New behavior patterns — living in an industrialized society and spending more time in classrooms — place unnatural demands on the eyes that may not mesh well with humans’ ancient programming, he says.
The other behavioral change that may not mesh well is near work. Human forebears didn’t read, and even those who chipped arrow points or did other fine work probably didn’t do it all day, every day. Frequent near work arrived with civilization; in many societies, it came about in the last century or two. A lot of myopia develops during childhood, and there may be some science behind the stereotypical bookworm with thick glasses. Myopia can also show up in adulthood, though, with textile workers and microscopists facing high rates of occupational myopia.
Recent work by Saw and others has linked less outdoor exposure and more near work to myopia. “Reading, writing and computer work contribute to myopia,” Saw says. “We found that children who regularly spend time on computers have a higher risk of myopia.”
Guggenheim has also found more nearsightedness among youths who read for pleasure, as did a team studying Danish medical students who were engrossed in lots of less pleasurable reading.
The near work explanation lost some adherents when the studies by Mutti and Morgan found little or no harm from it. But the vagaries in sorting out potential myopia triggers are obvious: Someone who reads a lot probably does it indoors; same with watching television or sitting at a computer.
The theories blurred further in a 2007 study of Turkish medical school students. Those with myopia had spent less time outdoors years earlier, up to age 7, than their 20/20 colleagues. And about one in seven of those with myopia had developed it after age 18. The researchers didn’t see a difference from near work — everyone had plenty of that — but the finding suggests that the consequences of less outdoor exposure can play out later in life.
Other studies hint at less-explored factors. Physical activity seems to protect 11-year-olds somewhat from myopia, a study found. That trend also showed up in the Danish medical students.
Mutti wonders whether the outdoor effect may also relate to vitamin D, since humans make the vitamin using ultra-violet rays (SN: 7/16/11, p. 22). His small study in teens and young adults who spent roughly equal time outdoors found that those with myopia indeed had about 20 percent lower vitamin D levels than those without myopia, even after adjusting for differences in age and diet.
As these theories get vetted, some scientists are already encouraging action. “We need to get the message out to parents,” Stell says. “Kick the kids outside.”
Morgan agrees, but says cultural expectations might block the way in some of the countries where myopia rates are already soaring. “It’s just stunning how strongly organized the life of a Chinese child is toward study,” he says. In school, children nap indoors for an hour or two at lunchtime, resting up to do hours of homework later. It would be hard to change this pattern, he says. “When I floated the idea of stopping naps at lunchtime in China, the response was almost like I was advocating child cruelty.” Western kids may work hard, Morgan says, “but you ain’t seen nothing until you’ve seen China.”
Even so, getting children outdoors might be easier than cutting back on near work, Saw says. Asian educational systems have reached a “saturation point” of intensity and competition, she says, “but it’s not a politically correct message to tell children to read or work less.”
In the United States, the clinical response to these new data has been uneven. While eye doctors are becoming aware of the outdoor-light theory, many don’t see a child until after the kid has failed an eye exam at school, says psychologist Jane Gwiazda of the New England College of Optometry. Gwiazda says pediatricians are the ones who might be able to encourage outdoor time before it is too late. “Once a child is nearsighted,” she says, “sending them outdoors to play may not stop the process.”
Short of getting out, maybe gazing out would help. It didn’t seem to hurt baseball player Hornsby, who was once asked what he did all winter during the off-season. “I’ll tell you what I do,” he snapped. “I stare out the window and wait for spring.”
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COMET study: [Go to]
The Guangzhou Outdoor Trial: [Go to]
More on myopia and other eye disorders from the National Eye Institute: [Go to]
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