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Debora Nickla, Kristen Totonelly; The efficacy of brief periods of “vision” in chick eyes wearing negative lenses is dependent on time of day. Invest. Ophthalmol. Vis. Sci. 2013;54(15):5175.
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The influence of light cycles on eye growth has long been of interest in the field of myopia, but the crucial parameters, and the ocular mechanisms on which they impact, are as yet unknown. To elucidate effects of phase, we exposed eyes to brief myopic defocus at different times of day, and examined the effects on eye growth and on the rhythms in axial length and choroidal thickness.
Chicks, aged 12-16 d, underwent one of the following conditions for 5 -7 d. The light cycle was 12L/12D. Negative lenses: Birds wore monocular -10 D lenses that were removed for 2 hrs daily at 12 pm (n=6), 12 am (n=6), 5:30 am (“dawn”; n=5) or 7:30 pm (“dusk”; n=6). Positive lenses: Birds wore +10 D lenses for 2 hrs at 12 pm (n=11) or 12 am (n=12). Normal eyes: Lights went on for 2 hrs at 12:00 am; controls were in normal 12L/12D. Ultrasonography was done at the start and end for all. In some groups, eyes were measured every 6 hrs over the course of 24 hrs on the last day.
Negative lenses: 2 hrs of lens-removal at dusk was more effective at growth inhibition than 2 hrs at dawn (830 vs 988 µm/7d; p<0.05). There were no differences between the “mid day” and “mid night” groups in growth rates, but the night exposure (12 am) had an acute effect immediately following lens removal: growth rate increased from 12 am to 6 am (X night vs controls and X day: 127 vs 0 and 56 µm; p<0.05 for both). The choroidal rhythm was phase-delayed (5 am vs 9:30 pm). Positive lenses: There was no difference in eye growth between day vs nighttime myopic defocus (246 vs 251 µm/5d); both grew slower than fellow controls (p<0.001 for both). Night defocus had similar effects on the rhythms in axial length and choroid thickness as in the night “negative lens removal” group. Normal eyes: 2 hrs of light at night inhibited ocular growth (615 vs 780 µm/7d; p=0.0001) and disrupted the choroidal rhythm; there were no changes in thickness at any interval except 6 am-12 pm, when choroids showed normal thinning.
Myopic defocus has a time-of-day-dependent efficacy in inhibiting eye growth, and when it occurs at mid-night it alters the rhythms in axial length and choroid thickness. These results have implications for the current controversy over what attributes are important in the myopia-inhibiting effects in children of time spent outdoors.
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