Protective effects of sunlight exposure against PRK-induced myopia in infant rhesus monkeys.

PURPOSE: Extensive clinical evidence suggests that time spent outdoors might reduce the risk of myopia. This study aimed to determine whether increasing sunlight exposure has a protective effect on hyperopic-defocus induced myopia in a non-human primate. METHODS: Twelve 2-month-old rhesus monkeys were treated monocularly with photorefractive keratectomy (PRK) (4.0 D) and divided randomly into two groups: artificial light (AL; n = 6) and natural light (NL; n = 6). Monkeys in the AL group were reared under artificial (indoor) lighting (08:00-20:00 h). Monkeys in the NL group were exposed to natural (outdoor) lighting for 4 h (09:00-11:00 and 15:00-17:00 h). Ocular refraction, corneal power and axial dimensions were measured before sunlight exposure and every 10 days after PRK. At day 180, retinal histology and apoptosis activity were evaluated by hematoxylin and eosin (H&E) staining and terminal deoxynucleotidyl transferase biotin (dUTP) nick end labelling (TUNEL) assay. RESULTS: Mean (±SD) PRK induced anisometropia was +3.11 (0.33) D. At the end of the experiment, both eyes of the NL monkeys exhibited significantly more hyperopia and shorter vitreous chamber depths (VCD), compared with AL monkeys (p < 0.05). The NL group exhibited a significantly slower rate of compensation to the induced anisometropia than the AL group (p < 0.05). The retinas of both groups exhibited normal histology and levels of apoptosis. CONCLUSIONS: Moderate sunlight exposure exerts protective effects against the myopic shift resulting from PRK-induced defocus in monkeys. These results are consistent with current clinical findings that increased outdoor exposure protects against myopia development. Sunlight exposure should serve as an independent positive factor in human myopia control.