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Debora L Nickla, Rinita Zanzerkia, Pearl Thai, Kristen Totonelly; Phase-dependent effects of brief periods of myopic defocus on the rhythms in axial length and choroid thickness in chicks. Invest. Ophthalmol. Vis. Sci. 2014;55(13):3607.
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© ARVO (1962-2015); The Authors (2016-present)
We have shown that the eye growth inhibition caused by 2-hr periods of myopic defocus is more effective when given during the mid-day than during the mid-night (Nickla & Totonelly, ARVO 2013). In this study we examined the effects of myopic defocus given at 4 times of day on the diurnal rhythms in eye length and choroid thickness.
12-d-old chicks wore monocular +10 D lenses for 2 hr periods at 5:30 am (“dawn”; n=11), 12:00 pm (“day”; n=8), 7:30 pm (“dusk”; n=11) or 12:00 am (“night”; n=6) for 5 d. Lights were on from 7:30 am-7:30 pm. Eyes were measured using ultrasound at the start, and at 6-hr intervals over 24 hrs on the last day (12pm, 6pm, 12am, 6am, 12pm). Refractions (RE) were measured on a Hartinger’s refractometer. To determine phase and amplitude, data from each eye was fit with a sine wave. Circular statistics were used for comparisons of phase. ANOVAs and Bonferroni post-hoc tests were used to determine between-group significance for growth changes.
Defocus during the day was more effective at inhibiting eye growth than at dawn or night (change/5d, X-C: ANOVA p=0.014; -174 µm vs -84 µm and -82 µm; p=0.042, p=0.075 respectively; RE: 3.8 D vs 1.1 and 2.8 D; p<0.05). This was associated with a phase-delay in the rhythm in axial length compared to normal (6:30 pm vs 2:45 pm; p<0.05; 1-tailed t-test). By contrast, defocus at night caused a phase-delay in the rhythm in choroid thickness (compared to dusk and day: 3:15 am vs 12:45 am (p=0.05) and 1:00 am (p<0.05, 1-tailed t-test)). It also abolished the diurnal rhythm in axial length due to an “acute” growth stimulation from 12 am to 6 am (night vs day, dawn and dusk, respectively: 119 µm vs -18 µm, -15 µm, 10 µm; ANOVA p<0.001; p<0.01). This effect did not occur in eyes receiving “normal” vision at night (27 vs 119 µm; p=0.005), in which growth was inhibited (629 vs 774 µm/7d; p<0.0001). In exp. eyes, the amplitude of the choroid rhythm was larger than controls in 3 groups (day: 110 vs 51 µm; night: 91 vs 46 µm; dawn: 131 vs 69 µm; p<0.05).
The greater efficacy of myopic defocus at mid-day is associated with a phase shift in the axial length rhythm, moving it towards the choroid rhythm. The lesser efficacy of defocus at night results from an acute stimulation of eye growth. These results have implications for future behavioral therapies involving light exposure to prevent myopia.
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