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Melanie C. W. Campbell, Kaitlin Bunghardt, Marsha L. Kisilak, Elizabeth L. Irving; Diurnal rhythms of refractive error components in normal chick. Journal of Vision 2008;8(17):48. doi: 10.1167/8.17.48.
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Purpose: Diurnal variations have been measured in intraocular pressure (IOP), axial length, and anterior biometrics in human. Human diurnal fluctuations in corneal topography and optical aberrations may be associated with eyelid pressure. In chick and marmoset, diurnal rhythms in IOP, axial length and choroidal thickness have been demonstrated. Furthermore, in chicks, normal ocular growth requires a diurnal light cycle. We wished to determine whether normal chicks show diurnal rhythms in refractive error.
Methods: Nine chicks were allowed to develop naturally with a 14 h light/10 h dark cycle. Hartmann Shack measurements were taken on both eyes on day 7 at 08:30, 12:30, 16:30, 20:30 and on day 8 at 00:30, 04:30, 08:30 and 16:30. Axial length measurements were taken using A-scan ultrasound.
Results: Nine eyes showed significant diurnal variation in axial length. In eight eyes, axial length variation had periods close to 12 hours while one eye had a much longer period. Astigmatic components showed a diurnal rhythm, with a period not significantly different from 24 hours; in 8 eyes of the 18 measured, for each of JCC45 and JCC0. For left eyes, average spherical refraction showed a significant diurnal variation with an 11 hour period, while 3 individual eyes with significant diurnal variations showed either 14 or 24 hour periods.
Conclusions: We confirmed the diurnal variation in axial length in chick previously found by others. However, in our case, an ultradian (12 hour) rhythm dominated. In chick, spherical refractive error and astigmatism appear to follow differing diurnal rhythms.
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