Don’t want your kids to need glasses? Send them outside!

Myopia, more commonly known as near-sightedness, is on the rise. The rise is rapid and alarming.

Writing for one of the top scientific magazines, Nature, in 2015, Elie Dolgin [1] mentions the following shocking statistic:

“Sixty years ago, 10-20% of the Chinese population was short-sighted. Today, up to 90% of teenagers and young adults are. In Seoul, a whopping 96.5% of 19-year old men are short-sighted.” (p.  276)

Dolgin goes on to say that there have also been staggering increases in rates of myopia in Europe and North America and that at the current rate of increase, by 2020 one in three people in the world might have myopia.

If you are like me, the obvious question that immediately comes to mind is: Why?

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The next set of perhaps equally obvious questions are: Can myopia be prevented? And, if so, how?

First the 'why'.

There is strong evidence suggesting that the development of myopia in children and in youth can be blamed at least in part on bad genetics that are passed from parents to their children.  Myopia tends to run in families [2] and it is more likely to co-occur in genetically similar twins [3].

However, recent changes in incidents rates seem to be occurring too quickly to match the timescale expected if weak genetics were the only contributor. Rapid increases in incidents of myopia in the Canadian north [2], and in China (as noted above), are good examples of changes that seem to outpace known heritability mechanisms [1].  In addition, data from Singapore suggest a strong link between the occurrence of myopia and the highest level of education reached, which can’t easily be accommodated by a genetic account. [3]

So, researchers began to focus on exploring environmental factors.  According to Hammond and colleagues [4], the “ . . .recent dramatic increases in the prevalence of myopia, particularly in the Far East,  have moved the focus of research toward environmental causes, particularly close work.” (p. 1232).

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'Close work,' refers to work that involves close viewing of objects like books, televisions, computers and hand held devices.  On the close-work account, the recent increase in myopia can be blamed on increases in time spent viewing objects up close [5].  The recent increase in close work might be partly due to increases in educational involvement by a wider segment of the population, and partly due to the well documented rapid increase in the use of technological devices that involve close viewing (e.g., TVs, computers) [6].

Yet, recent findings seem to point to a different explanation, and it has to do with the amount of time children spend outdoors.  It turns out that there is a strong relation between time spent outside and the incidence rate of myopia, such that the more time a child spends outside, the less likely they are to develop myopia.  In fact, a recent meta-analysis [7] of the available data supports the claim that there are “. . . 2% reduced odds of myopia per additional hour of time spent outdoors per week, after adjustment for covariates . . .” (p. 2141).  The relation between time spent outside and myopia occurs even when the amount of close work is accounted for.

While the exact mechanism linking time spent outdoors and the development of myopia remains to be determined, it might have something to do with the amount of light hitting the eye [1]. To understand why the amount of light matters, it helps to know that myopia results from a slightly misshapen (slightly elongated) eye ball which prevents the light from optimally focusing on the light receptors at the back of the eye.  It turns out that healthy eye shaping during development depends on the presence of chemicals in the eye that are released in the presence of light. The idea is that reduced exposure to light early in life might prevent proper eye shaping. The fact that the eye is exposed to more light when a child is outside than when she is inside, explains the link between time spent outdoors and the likelihood of developing myopia.

Another possible mechanism concerns the amount the eye has to accommodate objects located at different depths [1].  Outside environments include objects at a greater variety of depths than do inside environments, which means the eye has to work harder to accommodate objects outside than inside. As time spent indoors during development increases, the eye has to do less work accommodating objects at different depths, and so it doesn’t get the stimulation and exercise it needs to attain its proper shape during development.

This leads to the next two questions: Can myopia be prevented? And, if so, how?

The fact that there is an environmental contributor to the development of myopia suggests that it could be prevented. Of course, because genetic factors are at play, not all cases will likely be eliminated.  However, the evidence does clearly suggest that increasing a child’s time playing in the wild might have a protective effect. 

In fact, the authors of the meta-analysis discussed earlier [7] conclude the following:

“The overall findings indicate that increasing time spent outdoors may be a simple strategy by which to reduce the risk of developing myopia and its progression in children and adolescents.” (p. 2141)

So, the verdict is in.  Let’s get those kids out of the house and into the great outdoors!

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References:

1.     Dolgin, E. (2015). The myopia boom. Nature, 519, 276 - 278.

2.     Saw, S.M., Katz, J., Schein, O.D., Chew, S.J., Chan, T.K. (1996).  Epidemiology of Myopia. Epidemiologic Reviews, 18(2), 175-187.

3.     Tay, M. T., Au, E. K., Ng, C. Y., & Lim, M. K. (1992). Myopia and educational attainment in 421,116 young Singaporean males. Annals of the Academy of Medicine, Singapore, 21(6), 785-791.

4.     Hammond, C. J., Snieder, H., Gilbert, C. E., & Spector, T. D. (2001). Genes and environment in refractive error: the twin eye study. Investigative ophthalmology & visual science, 42(6), 1232-1236.

5.     Young, F. A. (1977). The nature and control of myopia. Journal of the American Optometric Association. [CLOSE WORK AS A REASON]

6.     Rideout, V. J., Foehr, U. G., & Roberts, D. F. (2010). Generations M2 media in the lives of 8- to 18-year-olds. Henry J. Kaiser Family Foundation.

7.     Sherwin, J. C., Reacher, M. H., Keogh, R. H., Khawaja, A. P., Mackey, D. A., & Foster, P. J. (2012). The association between time spent outdoors and myopia in children and adolescents: a systematic review and meta-analysis. Ophthalmology, 119(10), 2141-2151.