Influence of periodic vs continuous daily bright light exposure on development of experimental myopia in the chick

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In children, time spent outdoors has a protective effect against myopia development. In animal models, bright light reduces the development of experimental myopia. This study investigates how an increase in daily light exposure, presented either continuously during the day or periodically at different times of day, influences the development of experimental myopia in the chick. Myopia was induced in Cobb Chicks (Gallus domesticus) by monocular deprivation (MD) of form vision with a translucent diffuser for 3days (from 4days of age) under a 12:12 light: dark cycle. MD control chicks were exposed to constant 300lux (n=11) during the light period. MD treatment groups received either constant 2000lux (n=11) during the light period or 300lux for 10h with a 2h period of bright light (10000lux), either in the morning (n=10), midday (n=10) or evening (n=10), giving the same total daily light exposure as the 2000lux group. After 3days of MD, refractive status, corneal curvature and axial eye dimensions were measured for all eyes under anaesthesia. Myopia in the constant 2000lux group (-4.94±1.21D) was significantly less than in the 300lux control group (-9.73±0.96D; p=0.022). However, compared to the 300lux control group, 2h periods of 10000lux did not produce significant effects on refraction when presented either in the morning (-9.98±0.85; p=1.00), midday (-8.00±1.26; p=0.80), or evening (-13.14±1.16D; p=0.20), although significantly less myopia was induced in the midday group compared to the evening group (p=0.018). Orthogonal regression showed that myopia development was matched by changes in vitreous chamber depth (R(2) =0.69; p<0.0001). In chicks, an increase in daily light exposure continuously during the day is more effective at inhibiting myopia than adding an equivalent dose within a 2h period of bright light. A weak time-of-day effect also appears to be present in the response to bright light exposure. Our results suggest that future light-based myopia therapies in humans may be more effective if light levels are increased over the whole day, rather than through short periods of bright light exposure.