The effect of continuous light on refractive error and the ocular components of the rat

Phototoxic induced degeneration of the rat retina is a well-documented phenomenon resulting in losses of photoreceptors and their cell bodies, and an overall retinal thinning. This process may serve as a test of the hypothesis that the retinoscopic reflex originates from the inner limiting membrane of the retina. Retinal thinning should produce myopia in the absence of any other ocular component changes in a stable, mature eye if the inner limiting membrane model is correct. Phototoxic retinal degeneration was induced in 10 albino rats by exposure to 19 days of continuous light (1,800 cd m-2). Another 10 albino rats exposed to 12-hr on/12-hr off cycled light served as controls. Before and after the exposure to constant light, measures were made of refractive state by cycloplegic retinoscopy, corneal curvature and lens curvature by Purkinje image photography, and axial length of the globe by A-scan ultrasonography. Comparing pre- to post-exposure values, phototoxic degeneration resulted in a mean (+/- S.D.) myopic shift of -5.10 +/- 2.12 D (P < 0.002). The corneal curvature also steepened significantly (0.17 +/- 0.11 mm, equivalent to -6.0 D; P < 0.004), while the posterior curvature of the crystalline lens flattened by 0.21 +/- 0.22 equivalent mm (P < 0.027), and the axial length shortened by 0.11 +/- 0.11 equivalent mm (all tests Wilcoxon signed-rank; P < 0.025). Phototoxic rats underwent a mean retinal thinning of 49.6 mu compared to controls (Kruskal-Wallis test; P < 0.0008). No refractive or ocular parameters changed significantly in the controls. Phototoxic degeneration in the rat has optical consequences beyond simple retinal thinning. The size of the eye and the curvature of refractive surfaces can be altered in a mature eye well after the completion of development. The multiple changes which occur prevent phototoxic retinal thinning from serving as a test of the inner limiting membrane model for retinoscopic reflections.