Abstract
Abstract: :
Purpose: If chicks wear spectacle lenses, within a few days they compensate for the defocus by changes in ocular length and choroidal thickness, even if the lens-wear is limited only to repeated brief episodes, separated by darkness. If negative and positive lenses are worn alternately, the effects of positive lenses dominate. Because even a single episode of lens-wear causes changes in choroidal thickness over hours, we studied the possible relationship of these immediate changes to the previously documented changes over days. Methods: Axial dimensions were measured by A-scan biometry immediately before each episode of monocular lens-wear, and at repeated intervals of 30 min to 2 hr thereafter while the birds were kept in darkness. Birds wore lenses of either (a) +10 D for 10 min, (b) +10 D for three 30-min episodes 4 hr apart, (c) –9 D for 1 or 2 hr or (d) –9 D for three 2-hr episodes 4 hr apart. Results: A single 10-min period of wearing positive lenses caused about 50 µm of choroidal expansion which peaked after 1-2 hr, and remained thicker than normal for 6 hr. Repeated episodes caused an accumulation of choroidal thickening. In contrast, negative lenses needed to be worn for 1-2 hr to cause choroidal thinning. Once the lens was removed, the choroids quickly returned to normal thickness. Repeated episodes of negative-lens-wear did not result in thinner choroids than after a single episode. Conclusions: Negative lenses require a longer time to have an effect, but this effect is transient. In contrast, positive lenses worn briefly affect choroidal thickness for many hours. These findings suggest that choroidal compensation for myopic and hyperopic defocus may employ different visual or effector mechanisms. These results may also contribute to an understanding of why brief, repeated wearing of positive lenses can dominate the effect of longer periods of wearing negative lenses.
Keywords: choroid • myopia • hyperopia