Intermittent episodes of bright light suppress myopia in the chicken more than continuous bright light

Abstract

Purpose: Bright light has been shown a powerful inhibitor of myopia development in animal models. We studied which temporal patterns of bright light are the most potent in suppressing deprivation myopia in chickens.

Methods: Eight-day-old chickens wore diffusers over one eye to induce deprivation myopia. A reference group (n = 8) was kept under office-like illuminance (500 lux) at a 10:14 light:dark cycle. Episodes of bright light (15 000 lux) were super-imposed on this background as follows. Paradigm I: exposure to constant bright light for either 1 hour (n = 5), 2 hours (n = 5), 5 hours (n = 4) or 10 hours (n = 4). Paradigm II: exposure to repeated cycles of bright light with 50% duty cycle and either 60 minutes (n = 7), 30 minutes (n = 8), 15 minutes (n = 6), 7 minutes (n = 7) or 1 minute (n = 7) periods, provided for 10 hours. Refraction and axial length were measured prior to and immediately after the 5-day experiment. Relative changes were analyzed by paired t-tests, and differences among groups were tested by one-way ANOVA.

Results: Compared with the reference group, exposure to continuous bright light for 1 or 2 hours every day had no significant protective effect against deprivation myopia. Inhibition of myopia became significant after 5 hours of bright light exposure but extending the duration to 10 hours did not offer an additional benefit. In comparison, repeated cycles of 1:1 or 7:7 minutes of bright light enhanced the protective effect against myopia and could fully suppress its development.

Conclusions: The protective effect of bright light depends on the exposure duration and, to the intermittent form, the frequency cycle. Compared to the saturation effect of continuous bright light, low frequency cycles of bright light (1:1 min) provided the strongest inhibition effect. However, our quantitative results probably might not be directly translated into humans, but rather need further amendments in clinical studies.

Figure 1. Myopia induced by diffusers over one eye when chickens were kept under constant bright light of 15 000 lux for 1, 2, 5, or 10 hours (“15k, time in hours”; filled gray bars) or under cycles of bright light, changing from 500 to 15 000 lux at different frequencies (“15k, half cycle duration; black bars).

Patterned gray bars show the amount of myopia that developed in two batches of chickens wearing monocular diffusers under regular laboratory illumination of 500 lux (“500,10 h”). Because there was no difference between both groups, their data were pooled and provided the reference for the bright light treatment groups. An inhibitory effect of constant bright light was observed only when the exposure lasted for 5 hours or more (P = 0.004 and P = 0.007 for 5 and 10 hours, respectively). No additional benefit was observed when the bright light exposure was extended to 10 hours, compared with those exposed to 5 hours (P = 0.796). When bright light was provided as a temporal square wave function, its protective effect against myopia was enhanced. Chickens kept under 7∶7 or 1∶1 minute cycles developed the least myopia, compared with other cycles (P = 0.033 for differences among groups reared under cycles of bright light; Post-hoc pairwise comparison: 7∶7 minutes vs 30∶30 minutes, P = 0.038; 1∶1 minutes vs 60∶60, 30∶30, 15∶15 minutes: P = 0.041, 0.006, 0.022, respectively). *<0.05, **<0.01.

Figure 2. Correlation between vitreous chamber depth and the amount of myopia in chickens under different light regimens.

Equations for the linear regression, and R² values are provided for each light regimen. Long dash line represents the data for intermittent bright light, dotted line for constant bright light and short dash line for standard illuminance, respectively. Note that one diopter of myopia was equivalent to about 0.1 mm of axial elongation across groups (data from one single animal were excluded from the plot because of apparent measurement error, data: −13.9D vs 0.25 mm).

Figure 3. Relative increase in vitreous chamber depth (VCD) in eyes with monocular diffusers (bars grey-scale coded as in Figure 1).

Although there was no significant difference among treatment groups for either paradigm (Paradigm I: F = 1.639, P = 0.204 and Paradigm II: F = 2.075, P = 0.109), the increase of VCD in chickens reared under constant bright light for 5 or 10 hours was signifcantly supressed compared with those under standard illuminance (P = 0.029 and 0.037, respectively). In comparison with constant bright light, this effect was further enhanced in chickens exposed to cycles of bright light at a frequency of 7∶7 or 1∶1 minutes (all P<0.05, except for the comparison between the 7∶7 minute cycle and the 5 h constant bright light exposure, P = 0.180 and a borderline significance between the 7∶7 minute cycle and the 1 h constant bright light exposure, P = 0.075).* <0.05, **<0.01. Abbreviations as in Figure 1.