Aug. 5, 2008 A growing number of the world's children are mildly to severely nearsighted (myopic), with rates especially high among urbanized East Asians. In addition to coping with poor distance vision, children with severe myopia are more prone to visual impairment and blindness later in life.
Although genetic inheritance plays a role, the rapid rise of myopia suggests that environmental factors are driving the trend. Myopia usually begins and progresses during children's school years, but research on the role of intensive reading or other "near work" has determined that this is a minor factor. A new study led by Kathryn A. Rose, MD, used data from the Sydney Myopia Study of more than 4,000 Australian school children to assess whether outdoor activity might be significant in controlling myopia.
Parents and their children, at age 6 or 12, reported on the children's daily activities, which were classified as indoor or outdoor, and as near, medium or distance. Myopic refractive error, if any, was diagnosed for parents and children, and the children's ethnicity was recorded. A key finding was that the lowest myopia rates in 12-year-olds were associated with high outdoor activity, independent of the level of near work activity.
In 12-year-old students myopia was most strongly associated with high levels of near work and low levels of outdoor activity. The findings suggest that it is the time spent outdoors rather than engagement in sports that is critical; the association between increased outdoor hours and lower myopia was found even if an outdoor sport was not included, while time spent on indoor sports, such as playing basketball in a gym, had no effect.
The researchers think the intensity of outdoor light may be an important factor. Myopic eyes are longer, measured front-to-back, than normal eyes; in response to intense light, the retina releases dopamine, a neurotransmitter that inhibits eye growth and may thus influence the development of myopia. Also, the pupils of the eyes constrict in intense outdoor light, which increases the visual depth of field, the distance at which objects can be clearly seen.
The researchers recommend further study to prove conclusively whether certain levels of time spent outdoors can control myopia and to define the mechanisms involved. The higher exposure to intense outdoor light may explain the lower prevalence of myopia in children in Australia, compared with ethnically matched peers in other countries, Dr. Rose says. She adds that "this protective effect suggests that a public health measure aimed at preventing development of myopia could be based on increasing the engagement of children in outdoor activity," including family and school activities and sports.
This research was published in the August issue of Ophthalmology.
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