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C.A. Johnson, G. Lytle, D. Troilo, D.L. Nickla; Chick Eyes Show A Diurnal Rhythm In Refractive Error . Invest. Ophthalmol. Vis. Sci. 2004;45(13):4295.
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Purpose: In normal growing chick eyes, the diurnal rhythms in axial length and choroidal thickness are approximately in antiphase, with axial length being longest and the choroid thinnest during mid day. These rhythms might result in small daily oscillations in refractive error that could serve as an error signal that may play a role in the visual regulation of eye growth. We measured refractive errors in the morning and evening in normal birds and in birds responding to spectacle lenses. Methods: Refractive error was measured using a Hartinger’s refractometer at 9 am and 6 pm over various cycles. Untreated birds: Eyes of 3 birds (2 weeks old) were measured over 2 days; 4 birds were measured over 4 days. Lens wearing birds: Birds wore monocular lenses (either +10D or –10D) and eyes were measured over the first 24 hours of lens wear (6pm, 9am, 6pm; +10D, n=8, –10D, n=6) or from day 4–day 5 of lens wear (n=7 each) or day 4–day 7 (n=3 each). High frequency A–scan ultrasonography was used to measure ocular dimensions and infrared keratometry was used to measure corneal curvature in a subset of birds. Results: There is a diurnal rhythm in refractive error, with eyes being significantly more myopic in the morning than in the evening in all groups. In untreated birds (over all cycles) eyes were a mean –0.6 D more myopic in the morning than in the evening (ttest am vs pm, p=0.001). This diurnal difference was also seen on the first day of lens wear in eyes wearing positive lenses (mean am – pm difference = –2.1 D) and in fellow untreated eyes (–1.1D; p<0.05 for both). On the fourth day of lens wear, the difference became significant in eyes wearing negative lenses as well (mean am – pm differences for plus and minus lens–wearing eyes: –0.8D and –1.0D respectively; p<0.05). Preliminary data show no significant diurnal differences in corneal curvature. Conclusions: The diurnal rhythm in refractive error is the same regardless of whether the eye is responding to myopic defocus (in which it might be expected to shift in the more hyperopic direction at the end of the day) or to hyperopic defocus (in which it might be expected to shift in the more myopic direction at the end of the day). Ascertaining the anatomical correlate of this rhythm may elucidate an underlying mechanism in the visual feedback regulation of refractive state, as will determining whether the rhythm free–runs in constant darkness (i.e. is circadian).
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