Abstract
Purpose :
We investigated the effect of marginally induced astigmatism on refractive error development and eye growth in chicks.
Methods :
Four groups of chicks were fitted with custom-made two-zone spectacle lenses on one eye, the other serving as control, one week after hatching. Lenses for two treatment groups were Plano in the center and +15D cylinder at axis 90 in the periphery. The other two groups wore lenses that were Plano in the center and +15D sphere in the periphery. One sphere group and one cylinder group were fitted with lenses that had 5.5mm central zone diameter (CZD) and the other sphere and cylinder group, with 4.5mm CZD. Each group had 10 chicks. The White-Leghorn chicks were raised with normal diurnal lighting (12 hours off/on). All animal care and treatments conformed to the ARVO statement for the Use of Animals in Ophthalmic and Vision Research. Retinoscopy and A-scan ultrasound were performed before, 3, 6 &9 days after lens insertion. Refraction, Axial length and other biometric parameters were compared across the groups.
Results :
At 9 days of treatment, mean inter-ocular difference (IOD = Treatment eye – Control eye) in spherical equivalent refractive error was -7.53+/-1.81D, for the +15DS periphery 5.5mm CZD; -3.78+/-0.91D, for the +15DCX90 periphery 5.5mm CZD; -11.5+/-1.95D for the +15DS periphery 4.5mm CZD and -4.15+/-2.68D for the +15DCX90 periphery 4.5mm CZD. Optical axial length of treated eyes was shorter than control eyes in both spherical periphery groups (mean IOD of -0.371mm). The Vitreous Chamber Depths of treated eyes were shorter than the control eyes (mean IOD of -0.24mm), with the differences increasing with duration of lens wear. No statistically significant change was observed either in axial length or vitreous chamber depth for either of the cylindrical groups. Statistically significant choroidal thickening was observed in all four groups, although, the change was larger in the spherical group compared to cylindrical group.
Conclusions :
Whereas with induced spherical defocus, the changes in refraction continue to happen monotonically over the course of treatment, with induced cylindrical defocus there is an initial shift to hyperopic refraction, followed by a plateau. Nonetheless, when marginal light rays are astigmatically defocused, refractive development is biased towards the plane of best paraxial focus, not the most myopic plane of best focus for one meridian.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.