Myopia and Outdoor Exposures

Children are less likely to develop myopia when they are more often outside. Animal models show that bright laboratory lighting (15,000–25,000 lux) slows deprivation myopia (chick, tree shrew, monkey) and negative lens–induced myopia (chick, tree shrew). Long-term exposure for 3 months to bright light in the lab increases retinal dopamine production and release (chicks); and because a dopamine antagonist was found to cancel the effect of bright light on deprivation myopia (chicks), it was proposed that bright light might inhibit myopia development by stimulating dopamine release from the retina. But does bright light in the lab really reflect the natural conditions outside? Stone and colleagues¹ have now compared deprivation myopia development in chicks in animal facilities and in real outdoor settings, with variable weather and illuminances. They found disappointingly small and temporary effects on deprivation myopia, and the effects on retinal dopamine were inconsistent—no effect of outdoor exposure and no dopamine involvement after all? There are at least points to consider. (1) The temporary nature of the protective effect of bright light was described before (chicks); also the inherent variability of bright light effects on myopia is not new, even in the lab. That exposure to bright light for 3 months made chicks with normal vision more hyperopic and increased retinal dopamine release raises the question about the time kinetics of myopia inhibition, and it remains unclear how a few days of deprivation in chicks relates to years of myopia development in children. (2) The question as to whether dopamine plays a role might be more convincingly answered by measuring time kinetics rather than at a single time point at which the inhibitory effects on myopia are about to disappear. (3) In children, outdoor activity delays myopia onset (but where is the “memory” for earlier bright light exposure?), and it is less clear whether bright light also affects its progression (which was studied by Stone and colleagues¹ in the chick model). Generally, the list of studies in which inhibitory effects of bright light on myopia were found is just too long to be ignored. But the study by Stone and colleagues¹ clearly shows that a simplified view is insufficient—light doses, mechanisms, and time kinetics are not sufficiently explored.