One of the most common stereotypes about the young myope is that he or she is a reader. The Internet age may modify our picture somewhat, but parents and optometrists probably still imagine the young myope reading under the covers at night, although now with a tablet or smartphone instead of a book. But, is reading the problem? Can children read themselves into becoming myopic or needing a stronger prescription? Research funded by the National Eye Institute and conducted over the past 23 years suggests that the answer is no. Avoiding reading or the computer or other near work is not the way to maintain emmetropia or to slow progression.
The Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error (CLEERE) study, conducted by optometrists at 4 sites across the U.S. followed nearly 5,000 children over time to determine what factors were different between those who became myopic and those who remained emmetropic. Study investigators recently reported that myopic children do in fact fit the stereotype and engage in more near work than emmetropic children.1 The surprising finding was that near work did not cause the myopia. Children who became myopic did no more near work before the onset of myopia than children who remained emmetropic.2 Once myopic, children’s near work also had no effect on their rate of progression.3
If excessive near work does not cause myopia, is it all genetic? Is there any environmental influence on refractive error? CLEERE has shown that time outdoors, not near work, is the behavioral factor that affects the probability that an emmetropic child will develop myopia.
More time outdoors lowers the probability of onset, and potentially by a large amount. Emmetropic children with two myopic parents (the largest genetic risk) who spent the lowest amount of time outside (5 hours or less per week) had about a 60% chance of becoming myopic. However, for emmetropic children with two myopic parents who spent 14 hours per week or more outside, the probability of becoming myopic was reduced to 20%. Children with one or even no myopic parents also benefitted from more time outdoors.2
The great indoors
One might wonder whether more time outdoors is just less time reading, but this tradeoff behavior does not seem to be happening. The effect of time outdoors is independent of near work with no evidence of a negative correlation between them.2 Oddly, the evidence from CLEERE also shows that more time outdoors does not affect the rate of progression of myopia.3 The benefit seems to occur only in emmetropic children, suggesting mechanisms that lead to myopia onset may be different than those related to progression of myopia.
The next wave of research seeks to understand the mechanism by which time outdoors lowers the chances of becoming myopic. Some have suggested that increased physical activity while outside may be important, but careful measurement of activity levels by survey or by objective sensors in studies from Australia, Singapore, and England do not offer strong support for this idea.4-6
Seeing the light
A more widely accepted hypothesis is that the brighter light outdoors stimulates release of dopamine from the retina that inhibits the growth of the eye.4 However, not all myopia in animal experiments is inhibited by light. Myopia from form deprivation, when the eye is deprived of high-contrast vision, seems the most sensitive to the effects of light.7,8 Myopia induced by lenses, where the eye grows longer to compensate for hyperopic defocus imposed by minus lenses, is not inhibited by light in recent studies on research monkeys.9 This difference is important because lens-induced myopia is far more relevant to human myopia than form deprivation. The other problem is that bright light should have a general inhibitory effect on eye growth, should inhibit ocular elongation both before and after myopia onset, but that does not seem to happen. As mentioned earlier, time outdoors lowers the risk of onset, but does not seem to affect myopia progression.3