Abstract
Purpose: :
In chick and marmoset, animal models of refractive error development, diurnal rhythms in IOP, axial length and choroidal thickness have been demonstrated. In chicks, normal ocular growth requires a diurnal light cycle. We investigated whether normal chicks showed diurnal rhythms in refractive error components and their corresponding angular blurs on the retina.
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 at 8 time points on days 7 and 8 over a 32 hour period. Axial length measurements were taken using A-scan ultrasound from the cornea to the presumed retinal surface. Components of refractive error were calculated for a constant pupil size and angular equivalent blur on the retina was calculated from refractive error components and varying pupil size. Spherical refraction (MOR) and the Jackson cross cylinder components (JCC0 and JCC45) were calculated. After linear fits were subtracted from the time dependent data of individual birds, residual data were fit with a sinusoidal function.
Results: :
Pupil size and axial length showed significant sinusoidal diurnal variations in 13 and 14 of 18 eyes, respectively. The average period for pupil variation was not significantly different from 24 hours but the average period for axial length (14 hours) was significantly shorter. Ten eyes had significant diurnal sinusoidal variations in MOR, while a partially overlapping set of 10 had variations in the blur due to this defocus. For JCC0 data, sinusoids were significant in 7 eyes, and in 7 eyes for the corresponding blurs. In 4 eyes, sinusoids were significantly fit to JCC45 data and in 6 eyes to the corresponding blurs. The average periods of variation of all refractive error components and their blurs were not significantly different from 24 hours. Average amplitudes of oscillation of MOR and JCC0 were 0.8D and 0.27 D, respectively. MOR correlated with axial length while JCC0 correlated with pupil size.
Conclusions: :
We confirmed the diurnal variation in axial length in chick, previously found by others and also found a variation in pupil size, spherical refractive error and the components of astigmatism. The angular blurs due to these refractive components also showed significant diurnal variations.
Keywords: aberrations • emmetropization • astigmatism