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Anne Bertolet, Debora L Nickla, Frances J Rucker, Fuensanta A Vera-Diaz; Ocular Biometry at Different Times of the Day in Human Subjects. Invest. Ophthalmol. Vis. Sci. 2016;57(12):6243.
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© ARVO (1962-2015); The Authors (2016-present)
Previous studies show that axial length (AL) and choroidal thickness (CT) fluctuate diurnally. Work in animal models suggests the time of day defocus is introduced has an effect on the diurnal changes seen in AL and CT (Nickla et al 2014). We investigated how CT, retinal thickness (RT) and AL change at two different times of the day and after blur.
AL, RT and CT were measured at the fovea using a Lenstar LS900 before and after a period of myopic defocus at two times of the day (around 12pm and around 6pm, randomized order, two visits). The sequence of tests for each visit was: 20min wash out period, baseline measures, 35min blur [+2.00D over best correction, movie @4m], post-blur measures.Subjects were 21-28yrs old with normal vision in each eye and were classified into myopes (-3.23±2.34D; n=24) or emmetropes (0.24±0.25D; n=18). Criteria for CT and RT followed the guidelines by Read et al (2010). AL, RT and CT were compared between baseline measures at noon and 6pm, and between the pre- and post-blur conditions.
As expected, baseline data showed that CT was significantly larger at 6pm (348.2±62.3μm) than at noon (333.9±60.5μm) in emmetropes (p=0.01), but not in myopes. There were no correlations with CT and the amount of myopia. RT was larger at noon (190.6±12.7μm) than at 6pm (188.2±11.9μm) for the overall group (p=0.03). In the myopic group, RT was significantly greater for higher amounts of myopia at noon (ρ -0.54, p=0.01), but not at 6pm. No correlation was found for emmetropes. As expected, there was a significant correlation for AL and the amount of myopia (ρ=-0.81, p<0.01). No differences in AL were found between the noon (24.07±0.34mm) and 6pm (24.11±0.32mm) measures (p=0.35) in either group.While exposure to positive blur caused a decrease in RT at noon that was significant for emmetropes (-1.8±10.1μm; p=0.03), not in myopes, these changes are very small. No significant effect of blur was found on CT for any condition, maybe due to higher variability of the measures. Exposure to blur significantly decreased AL at 6pm (Emmetropes: -5.2±33.0μm, p=0.01; Myopes: -6.9±55.3μm, p=0.02), but not at noon. This may be due to the effect of positive blur.
While CT and AL have been shown to follow diurnal rhythms, this is the first report of thicker retinas at noon compared to evening in emmetropes. While AL decreased post blur in the evening, further studies are needed to explore this relationship.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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