Outdoor Exposure Does Not Inhibit Experimental Myopia

Abstract

Purpose: Rearing chicks and mammals under high-intensity laboratory lighting inhibits form-deprivation myopia, and it has been proposed that increasing outdoor exposure of children might favorably influence clinical myopia. We assessed the effects of "real-world" outdoor exposures on goggle-induced myopia in chicks.

Methods: Two cohorts of white Leghorn chicks, after placement of a uniocular translucent goggle at 5 or 9 days of age, were reared outdoors in a rural setting during daylight hours to the extent permitted by weather; they were not exposed to electric lighting. Two control cohorts were reared under incandescent lighting (500 lux) without outdoor lighting exposure, with light cycle durations matched to sunrise-sunset times of the outdoor chicks. Retinal dopamine/DOPAC (HPLC with electrochemical detection) and ocular refractions/dimensions (refractometry, ultrasound) were measured after 10 or 11 days. Analysis used paired t-test or one-way ANOVA.

Results: Non-goggled eyes had some 2.7 diopters (D) of hyperopia in all cohorts. The cohorts of goggled eyes averaged 12-23 D of myopia relative to control eyes, with no evidence that outdoor rearing lessened the myopic response. In fact, the myopic response was exaggerated in goggled eyes of the older outdoor vs. indoor cohort (-23 vs. -13 D; P<0.001). Similarly, the vitreous chambers and axial lengths were elongated in goggled eyes relative to controls, also with no evidence that outdoor rearing attenuated the myopic response (P>0.05, all between-cohort comparisons). In the younger cohort, retinal dopamine or DOPAC (pg/mg protein) in non-goggled eyes and the reductions in contralateral goggled eyes (dopamine ~15%; DOPAC ~48%) were similar between indoor and outdoor reared chicks (P>0.4, all between-cohort comparisons).

Conclusions: Contrary to current hypotheses, outdoor exposure does not reproduce the anti-myopia effects of increasing artificial lighting intensity in the lab, at least in chicks. The outdoor environment has much variability in richness of scene and chromaticity, along with major shifts in illumination from movement/density of clouds, individual movement into and out of shady areas, and direction of gaze. All these features were inherent to our study. While some element of outdoor and/or lighting exposure may favorably affect refractive development, our results point to a level of complexity that is not being adequately considered in laboratory or clinical investigations.

Keywords: 605 myopia • 677 refractive error development • 502 dopamine