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 phen omenon resulting in losses of photoreceptors and their cell bodies, and an overall retinal thinning. This process may serve as a test of the hypothesi s 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 limiti ng membrane model is correct. Phototoxic retinal degeneration was induced i n 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 wer e made of refractive state by cycloplegic retinoscopy, corneal curvature an d lens curvature by Purkinje image photography, and axial length of the glo be 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 cornea l 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 f lattened by 0.21 +/- 0.22 equivalent mm (P < 0.027), and the axial length s hortened 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 comp ared 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 siz e of the eye and the curvature of refractive surfaces can be altered in a m ature eye well after the completion of development. The multiple changes wh ich occur prevent phototoxic retinal thinning from serving as a test of the inner limiting membrane model for retinoscopic reflections. (C) 1998 Acade mic Press.