Tuesday, June 21, 2011

Nearsightedness: Ancient genes meet the modern world

In today's New York Times, Sandra and I have a piece on the modern increase in nearsightedness. We relied on research findings indicating that a likely cause is decreased exposure to bright, outdoor light. For a sampling, see the following papers on associations with outdoor activity, whether sports plays a role, lifestyle/schooling, and visual activity.. Also an article on the rising prevalence of myopia in the US.

There's a more general idea worth thinking about: our developmental program evolved to help us function well under ancient conditions, but those genes play differently in the modern world. This principle can help us think about problems such as dyslexia and ADHD - and also about the fact that IQ scores around the world are rising. It's a two-sided coin.

Also, a postscript - another possible explanation is the relative abundance of "near work" indoors, since this work involves less distance focus. However, this is hard to square with the epidemiology because there's no effect on myopia of playing indoor sports, which you'd expect to encourage distant focus without bright light exposure.


Daniel said...

Thank you!!! For your Op-Ed piece today. Ever since my kids were born 8 years ago, I have been asking optometrists and pediatricians what leads to nearsightedness, and hearing "genes". It never made sense to me, for the reasons you mention, and because its so much less prevlalent in less literate cultures. This provides a reasonable explanation, though I am not ready to write off "near work" completely either.

Now do you know anyone who can provide a similar explanation why orthodontic problems seem so much more prevalent in industrial cultures?

Hank Roberts said...

I'd think the wavelength rather than the "brightness" of the light may be the difference-- can you distinguish these by looking at the light sources used in the various populations at the time of studies?

Indoor light started off very red (fire, candlelight); moved toward yellow (gas mantle and low-watt incandescent); then more greenish (halogen)--all those are continuous spectrum emissions from hot surfaces, just with different peak 'color temperature' points.

Now we're using fluorescent and LED, both sources with narrow spikes of emission and a lot more blue light -- look at examples here:

The 400-500nm blue has a lot of different effects, e.g.

"For a given lens curvature, longer wavelengths have a longer focal length, i.e., red is the longest focal length and blue is the shortest. Since to have an image focused on the retina, the lens curvature must change with wavelength with red light requiring the greatest curvature and blue light the least curvature...."

Hank Roberts said...

PS, compare the solar spectrum -- lots more light in the blue range than the "bright" light we get from indoor sources: http://intiridesigns.com/images/lightsourcesChart.jpg

Compare "white" LEDs (which use a blue emitter and phosphors to change some of that to yellower light), e.g.

Just speculating, of course, I'm not a scientist -- but it looks to me like reading with artificial light makes the eye work much harder changing the shape of the lens to focus the more reddish indoor illumination. Isn't that likely to change the range over which the lens _can_focus, as a child grows up with indoor light sources instead of outdoor light?

If so, maybe, just maybe, going to LEDs indoors will reduce the amount of nearsightedness (while increasing problems with sleep and depression)

My guess is we'd do better to switch off the "white" LEDs and use amber/yellow light in the evenings to mimic the day/night cycle.

MWade said...

People interested in this topic may also be interested in this companion study by Dr. Rose, published in Ophthalmology journal (the journal of the American Academy of Ophthalmology): Outdoor Activity Reduces the Prevalence of Myopia in Children, Volume 115, Issue 8 , Pages 1279-1285, August 2008. For more information, please contact me: Mary Wade, Media Relations, AAO, mwade@aao.org

eyevoyager said...

Any thoughts on this as a mechanism?

Sam Wang said...

I think a fairly plausible mechanism has to do with visual input as a stop signal for eye growth. There's a whole literature that we didn't cite in that short essay, an example of which is this: http://www.ncbi.nlm.nih.gov/pubmed/19516016
In regard to the UV idea, I will have to read up on that. Thank you for pointing it out.