Action spectrums of short wavelength light–induced changes in the neuroretina and retinal pigment epithelium of the albino rat.

Abstract

Abstract: : Purpose:To determine if there are any differences in the morphology of short wavelength light–induced retinal damage, within the 400nm to 480nm spectrum of wavelengths. Methods: Narrow–bandwidth (10nm) light, of constant irradiance, from a 500W Xenon short–arc lamp with a light delivery system between 400nm and 480nm was focused via a plano–concave contact lens to a 1mm spot on the retina of albino rats. One lesion was applied to each quadrant of the retina, two disc diameters from the optic disc. The location of the light spot was monitored and recorded before and after exposures via an operating microscope and beam splitter to ensure no movement. Retinal doses of light exposure were varied between 0.4 and 604 Jcm^–2 depending on the wavelength. There were no visible fundus changes, so Indian Ink perfusion of the vasculature assisted in locating the exact site of prior irradiation. Retinal damage was evaluated by at day 7 post–exposure histopathologically, and compared with adjacent retinal tissue that had not been irradiated. The percentage of exposures that resulted in a statistically significant alteration in the thickness of the neuroretina or retinal pigment epithelium (RPE) was determined. This was plotted against retinal dose and a best–fit curve was applied using non–linear regression. The ED50, being the dose that resulted in an alteration in the thickness of a retinal layer 50% of the time, was calculated. This was plotted against wavelength to determine the action spectrum for each layer.Results: There was a 2.7 log unit increase in the ED50 for retinal damage between 400nm and 480nm light (p<0.001) with intervening data following an exponential curve (R²=0.99). RPE damage became more frequent with increasing wavelength (OR 2.2, p=0.001). The ED50 for RPE damage was 11.5 times greater than that for neuroretinal damage at 400nm (6.9 Jcm^–2 cf 0.6 Jcm^–2, p<0.001) but was not statistically different at 480nm (256 Jcm^–2 cf 311 Jcm^–2, p=0.518). Conclusions: There is an exponential increase in the retinal dose of light required for threshold retinal damage between 400nm and 480nm. The RPE and neuroretina are equally susceptible to light damage at 480nm but the RPE is significantly less susceptible at 400nm. The differences in action spectrums, between these two layers, could result from chromophores of different absorption spectrums initiating tissue damage.

Keywords: radiation damage: light/UV • retina: distal (photoreceptors, horizontal cells, bipolar cells) • lesion study