Light and Myopia - An Incredible ConnectionIntroWhile walking outside, we sometimes get hit by a drastic increase of sunlight intensity, and then, whatever that was previously blurry, becomes crystal clear followed by a perceived increase in 3-dimensionality. What's going on?
Precursor: Outdoor ActivitiesOne of the breakthroughs in eye research (most notably in the last decades) is the myopia-inhibiting effect of outdoor activities. It all started with
epidemiological studies:
Study 1) Myopic children tends to engage in less outdoor activities. In fact, the amount outdoor activities seem to have more influence on myopia development than near work (there is no paradox here, we also know that near work can be done in ways that does not invite myopia development, this explains why in certain studies there are no significant association between near work and myopia)
Study 2) A longitudinal study also shows that children engaging in less sport and outdoor activities tend to become more myopic.
Study 3) Which one is it? Sport or outdoor activities? In 12-year-old children, the higher amount of outdoor activities is associated with less myopia, and
not the sport being practiced --- It's the
total amount of time outdoor that counts.
Study 4) Apparently, the environment plays the leading role in myopia development. When 6 and 7-year-old children of Chinese ethnicity of Sydney are compared with their Singapore counterpart, the Sydney group has substantially much less prevalence of myopia (3.3% vs. 29.1%). The total hours of outdoor activities per week (13.75h vs. 3.05h) was found to be the strongest factor associated with the difference in myopia prevalence (granted, the data are collected from the questionnaires, but the statistical and clinical significance is there).
These fairly-recent studies sparked a new momentum towards figuring out what makes outdoor activities so beneficial. One other study, showing that myopia progresses faster in winter than in summer, points to the possible link between light and myopia. Other researchers suspect Vitamin D, relaxed accommodation and sunlight-induced increase in retinal dopamine level.
Sequel: Lens-Induced / Form-deprived Myopia vs. Light intensityThe ensuing research split into different directions, occasionally leading to equivocal conclusions. But here are a few from the animal research:
Study 1) To test whether the intensity of lighting could have an effect on myopia. Rhesus monkeys are form-deprived under 2 conditions: normal lighting condition (up to 630 lux) and high ambient lighting condition (25000 lux). Both eyes of the ambient group become more hyperopic than those of the normal group.
Study 2) In chicks, exposure to either bright laboratory light or sunlight, even for just 15 minutes a day, substantially retards diffuser-induced form deprivation myopia. What’s more, the diffuser-wearing chicks who were exposed to only 15 minutes of sunlight a day are still more hyperopic than the
non-form-deprived chicks under low laboratory light setting.
Study 3) This is a study with an unusual twist. Chicks under constant light, day and night, exhibit more hyperopia. However, if the chicks wear eye cover or have a hood above them to cover the light, for 12 hours a day, then it would protect them against hyperopia. For chicks,
emmetropization is maintained by a regular diurnal light-dark rhythm.
Study 4) Young tree shrews who are form-deprived by living in darkness end up developing axial myopia. Refractive status is maintained partially through regular exposure to light.
Study 5 - Ashby and Schaeffel) This is the most interesting find so far. Chicks wearing -7D lenses were put into 2 conditions: low lighting (500 lux) and high lighting (15000 lux). The chicks under high lighting compensate fully for -7D at a much slower rate than their low lighting counterpart. What now? Other chicks wearing +7D lenses were also put into the same 2 categories, and the chicks from the high lighting group compensate fully for the +7D lenses at a much faster rate then their low lighting counterpart.
Light inhibits lens-induced myopia and accelerates lens-induced hyperopia.What's more, when chicks got injected with spiperone (a dopamine antagonist), exposing to high illumination no longer spare the chicks from form deprivation myopia. On the other hand, when the chicks got injected with a placebo solution, exposing to high illumination would show its form-deprivation-myopia-inhibiting effect again.
What to extrapolate from these?In light of these studies, we speculate that:
In terms of
image quality, the more intense the light, the smaller the pupil, which results in an increase of depth of focus. As the pupil constricts, the light rays that normally would scatter around the periphery (a.k.a., "optical noises") no longer make their way into your eyes. Consequently, aberration decreases and an increase in depth of focus and perceived clarity ensues (i.e., sharper image with increased contrast). Light provides clarity in a safe way, while reducing unnecessary staring tension at the same time - A brightly-lit image is clearer and more easily focusable.
Although the change in light intensity can be mistaken as clear flashes, distance gazing, under moderately bright sunlight, greatly promote double vision and clear flashes, possibly also in part through a significant reduction in
accommodation and
convergence.
There is a catch though. Sunlight alone is not the solution to myopia. The human folks in the studies only witness a slower myopia progression (i.e., not a improvement in dioptric terms), as they are still being overprescribed. Animal research suggests that light only
reduces the speed of compensation to minus lens, and does
not change the target refractive status the eye is compensating for.
Actually, we haven't even talk about
the key finding. The Ashby and Schaeffel study did provide us with some clues as to how sunlight inhibits myopia. In that study, a dopamine antagonist essentially disabled the protective effect of sunlight - As the retinal dopamine (a neuromodulator) level decreases, elongation also fails to stop.
This strongly suggests that the process of elongation is
in part modulated by....some chemicals in your retina! The biochemical component of axial length regulation could very well look like this:
Level of sunlight => Different levels of retinal dopamine/melatonin => Different rate of proteoglycan synthesis => Different choroidal thickness => Different level of resistance to axial stretching.
Oh. And here comes Dr. Alex:
Over the past 40 years between my father's practice and my own, regions with less sunlight, more rain, longer winters, etc, tend to bring us higher degree of myopia clients. Of course there are various other possible explanations we wouldn't discount ... however, the client with the desk job in the south of France almost always has a half diopter or even a full diopter less of a prescription than a very similar case in the north of the country.
[Note: On Dr. Alex's
Oct. 29 blogpost, the relationship between ambient lighting and refractive status is explored.]
[Update: On March 18, 2015, Nature publlished an article titled "
The Myopia Boom", essentially covering in less detail what you just read.]
