Myopia: more environmental, less genetic

The science magazine/journal Nature has an interesting article this week on myopia, aka near-sightedness. Myopia is on the rise among children worldwide, and is reaching epidemic levels in some Asian regions: in 1955, 10-20% of the Chinese population was short-sighted, and today up to 90% of teenagers and young adults are. This has led to an increase in research as to what is driving it.  Many efforts have shown there is a genetic component to it, and nearly 100 different genetic markers have been found that correlate with myopia. But that can’t be the whole story; evolution doesn’t move fast enough to account for the rapid ascent of myopia rates. There must be an environmental factor, and a strong one at that.

Historically, people have blamed obvious things, like too much close-reading, or staring at computer monitors, or reading in low light.  Researchers have run studies on all of those things, and found no correlation with myopia rates.  But one thing has appeared as a culprit: not enough time outdoors.

Now the studies on this are a bit confusing, because they tend to confound a few things. They are generally done as surveys rather than direct observation, and they ask how much time children spend on “outdoor sports or activities.” That suggests that reading outside, or sunbathing, wouldn’t count in the survey. The studies that do this tend not to find any correlation. They also don’t find a correlation with time spent participating in indoor sports.

But those that look at total time spent outdoors — both active and passive — find a correlation at the very least with the onset of myopia.  So if you are a young child with good eyesight and you don’t spend enough time outside, you stand a much higher chance of becoming nearsighted. There isn’t clear research yet that shows a connection between limited time outdoors and further progression of myopia; then again, those studies have tended to confound the “outdoor activity” and “time outdoors” distinction I described above, so the  science isn’t very strong. There may still be a correlation that simply hasn’t been rigorously proven yet.

So that leads to the question “Why?”  And answers are beginning to emerge for that question as well. What we understand clearly is how myopia happens: the eyeball gets enlongated so that the lens at the front no longer focuses the image onto the retina but instead some distance in front of it. We see myopia develop in children because their eyes grow as the rest of their body grows, and if something interferes with that growth the eyeball comes out misshapen.

The first thing that’s being looked at is the neurotransmitter dopamine. It’s well documented that light stimulates the production of dopamine, and our retinas produce it during the day — in amounts relative to our exposure to daylight. The hypothesis is that dopamine prevents elongation of the eyeballs, and there have been experiments on chicks that seem to confirm that connection. Some researchers believe that having a regular cycle of dopamine-producing hours and non-dopamine-producing hours is also important to proper eye growth and development as well.

Other researchers are looking at Vitamin D, or lack thereof, as another contributing factor. It is also well known that sunlight causes our bodies to naturally produce Vitamin D, which is important to all sorts of processes in our bodies.

If true, this bodes poorly for Asian cities, where the culture drives children to spend excessive amounts of time studying. And doubly so because the air pollution makes it a bad idea to be outside for long periods of time.

So far, scientists have not identified a minimum threshold of time spent in daylight that will make a difference, so any additional time your kids spend outside, the better.  At the other end, some researchers suggest that kids should spend three hours a day outside to get the maximum effect. That’s hardly a consensus view, though it’s something to consider as you figure out your children’s daily schedule. There is also some evidence that using the high-intensity indoor lights prescribed for people who suffer from seasonal affective disorder might help.

Or you could invest in glasses companies — especially Asian ones. The market is booming.

 

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