Now an analysis of children and adolescents suggests a possible reason: less time spent outdoors, researchers said at a recent Orlando meeting of the American Academy of Ophthalmology.
Each added hour spent outside per week appears to drop the chance of myopia by 2 percent, the analysis of eight separate studies with 10,400 young people found. Nearsighted children on average spent 3.7 fewer hours outdoors than children with normal or farsighted vision.
This suggested link — which needs to be tested more fully — is part of the effort to learn why the amount of nearsightedness has exploded. In the United States, for example, myopia in people ages 12 to 54 jumped from 25 percent in 1971-72 to 42 percent in 1999-2004, epidemiologists at the National Eye Institute reported in 2009. This was true for blacks, whose nearsightedness more than doubled from 13 percent to 34 percent, and whites, for whom it increased from 26 percent to 43 percent. It was also true across all degrees of nearsightedness, from mild to severe.
Myopia is believed to be caused by a combination of genes and the environment. But such a rapid increase in nearsightedness in just 30 years must mean that one or more environmental risk factors has increased, because genes don’t change that quickly, researchers say.
Could something as simple as spending more time outside reduce the amount of myopia? If that is true, it would not only be cost-effective but also would help improve general health, said Dr. Anthony Khawaja and Dr. Justin Sherwin, researchers at the University of Cambridge who authored the study.
More work is needed to pin down the actual factor or factors that could prevent myopia in growing children. Possibilities, they say, include the increased use of distance vision, reduced use of near vision, exposure to natural ultraviolet light outside, or physical activity.
In nearsightedness, light taken into the eye focuses at a spot short of the retina, the light-sensitive tissue on the back of the eye, leaving a blurry image. The explosion of nearsightedness is especially pronounced in parts of Asia, such as Singapore, where up to 80 percent of people have myopia.
Another study cited by Khawaja suggests that boosting outdoor time might also stop nearsightedness from getting worse. In that related work, 80 nearsighted Chinese children ages 7 to 11 were divided into two groups of 40 children each. One group was told to do less than 30 hours of “near work” — tasks that require focusing on close objects — a week and spend more than 14 hours outside. The other group had no special schedule.
At the end of two years, the first group, on average, was less nearsighted, Khawaja noted.
Childhood myopia tends to develop between 8 and 12 years, with another peak in the late teens and early 20s, said Dr. David Epley, an ophthalmologist at Children’s Eye Care in Kirkland, Wash.
“It the grand scheme of things, myopia in mild to moderate amounts can be useful, particularly in the 40s and beyond when presbyopia — the age-related progressive inability to focus on near objects — takes away the near vision of normally sighted individuals,” Epley said. “With modern glasses, contacts and laser surgery, myopia is easily correctable.”
But as National Eye Institute researchers noted in their 2009 study showing the increased prevalence of myopia in the United States over three decades, that increase costs an additional $1 billion a year for eye care — making it a concern to health planners and policy makers.
Epley said he doesn’t need to change what he tells parents in light of the new study linking outdoor time with less myopia.
“We are already recommending that children limit screen time to no more than 1 to 2 hours per day, and none for children less than 2 years old,” he said, citing American Academy of Ophthalmology guidelines. “In addition, outdoor play has been shown to foster better development of imagination, which can help with problem solving, attention and other issues in older childhood and adulthood.”
“Also,” he said, “outdoor play burns more calories and should be encouraged to help fight the epidemic of childhood obesity.”
Epley’s recommendations fit in with the new and groundbreaking Jefferson County Department of Health regulations for day care, which were passed in September and go into effect in January. These regulations will require:
• At least 90 minutes of active play — outdoors when possible — each day for children 3 and older, and at least 60 minutes for children 12 months to 3 years old. Active play is never to be withheld for misbehavior.
• A limit for preschool children 2 years and older of no more than 2½ hours a week of television, videos, video games and computer use. For infants and toddlers younger than 2, screen time is completely prohibited.