The purpose of this study was to determine the threshold energy for retinal damage by a blue light-emitting diode (LED) as determined by clinical observation, physiologic measurements and histology. Seven adult male rhesus monkeys were anesthetized, paralyzed with pancuronium bromide to immobilize eye muscles and maintained on a respirator. A Topcon slit-lamp microscope was adapted to hold an LED which was used to expose a portion of the macula. Measurement at 460nm produced a power of 0.85 mW at the objective lens placed before the eye and was calculated to range from 0.612 joules (J) to a maximum of 4.59J on the retina. Time increments were tested from 12 to 90 min and changes monitored at 2 and 30 days post-exposure by fundoscopy, confocal scanning laser retinal tomography, S-cone macular electroretinography (ERG) and digital fluorescein angiography (FAG). The contralateral eye served as internal control. Two days after exposure, a reproducible, optically pale, subretinal lesion measuring about 1.8-2.0 mm in diameter was observed between 40 (2.04J) and 90 min (4.59J) of exposure which overlapped approximately 10-13% of the macula. In supra-threshold experiments, FAG showed leakage of dye and retinal tomography showed damaged outer retina within the lesion at all exposures >30 min, the S-cone threshold decreased at 2 days post exposure relative to L-cone signals and by 30 days, had returned to baseline. Morphological examination revealed a selective pathology related to the outer retinal layers. At maximum exposure the total retinal thickness was reduced by 28% i.e., 145 μm for LED vs. 200 μm (unaffected area). Changes were also seen adjacent to the lesion (175 μm) suggesting extension beyond the clinically observed margin. These changes were not regularly observed below 34 min of exposure. Based on the most sensitive clinical measure, an increase of the ERG at just above 30 min, may represent a damage threshold level. Prolonged exposure may pose a toxic hazard for the human retina. This is the first time a potential safety level for an incoherent blue LED light has been reported.
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