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J. I. W. Morgan, J. J. Hunter, R. Wolfe, B. Masella, J. R. Sparrow, W. H. Merigan, D. R. Williams; Reciprocity of Light-Induced Reduction in Retinal Pigment Epithelial Autofluorescence. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4516.
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We have previously shown that long exposure to 568 nm at levels below the ANSI maximum permissible exposure (MPE) produces retinal damage preceded by a transient reduction in the autofluorescence (AF) of retinal pigment epithelial (RPE) cells in vivo. We sought to establish the effects of exposure power and duration in producing this AF reduction as well as the maximum exposure that does not produce a detectable reduction.
Macaque retinas were imaged using a fluorescence adaptive optics scanning laser ophthalmoscope to resolve individual RPE cells in vivo. The retina was exposed to 568 nm light over a square subtending Â½Â° with energies ranging from 1 J/cm2 to 788 J/cm2. Exposure power and duration were independently varied to establish their effects on AF reduction. Pre-, immediately post-, and long-term post-exposure AF images of the RPE cells were taken over a 2Â° field. The amount of AF reduction was quantified by the normalized ratio of AF intensity inside to outside the exposure.
In vivo exposures of 5 J/cm2 and higher caused an immediate decrease in AF. Subsequent imaging showed either full recovery of AF (exposures of 210 J/cm2 and lower) or permanent RPE cell damage at the site of the exposure (exposures of 247 J/cm2 and higher). Reciprocity of exposure power and duration held for exposures from 45 seconds to 900 seconds for all conditions. No significant AF reduction was observed for exposures of 1 J/cm2.
That reciprocity holds implies that the total energy delivered to the retina, rather than its distribution in time, determines the amount of AF reduction, consistent with a photochemical origin. The cause of reduced AF could be, among other things, A2E photoisomerization or photooxidation. If it is caused by photooxidation, reduced AF could be associated with retinal damage. We have observed AF reduction with exposures (5 J/cm2) 31 times less than the ANSI MPE (155 J/cm2, thermal limit). The implementation of safe methods, not only to image the RPE cell mosaic in vivo, but also to deliver high intensity light to the retina generally requires a better understanding of the cause and effects of AF reduction.
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