The Dependence of Cataract Formation Rate on Interval Between in vivo Exposures to UVR, for Recurrent Chronic Close to Threshold Dose

Abstract

Purpose:: To elucidate the impact of the interval between recurrent in vivo exposures to ultraviolet radiation (UVR) at close to threshold dose, on cataract formation rate.

Methods:: It was demonstrated with in vivo exposure of albino rats to UVR with the exposure fractionated into two equivalent doses that at close to threshold dose, 18 % of the damage is repairable while the remaining 82 % is not. The repair was shown to decrease exponentially declining with time, with a time constant (1/e) of 8 days. On the basis of this information, a model for accumulated damage after recurrent daily exposure to equivalent doses of UVR was developed. Then, groups of albino Sprague Dawley rats were unilaterally exposed to UVR in the 300 nm wavelength region. One of the eyes received either of 0.1, 0.4 or 0.5 kJ/m²/day for 40, 10 and 8 days respectively, to a total accumulated dose of 4 kJ/m². In all groups, the rats were kept 1 week after the last exposure and then sacrificed. The lenses were extracted and the intensity of forward light scattering was measured in the lenses. Thereafter, the impact of the interval between recurrent in vivo exposures to ultraviolet radiation (UVR) at close to threshold dose, on cataract formation rate was examined with the model.

Results:: The model predicted that 8 days of recurrent 0.5 kJ/m²/day exposures would induce above threshold cataract. This was supported by the animal experiment that showed development of significant light scattering after 8 days of 0.5 kJ/m²/day but not for the other groups. The model was found to predict that with increasing equivalent inter-exposure interval, the cataract formation rate decreases more than is expected if there is no repair.

Conclusions:: Repair modulates the effect of recurrent daily in vivo exposures of the lens to UVR. With increasing inter-exposure interval, the cataract formation rate is increasingly reduced, absolutely, and as compared to if there was no repair.

Keywords: cataract • radiation damage: light/UV • protective mechanisms