Photochemical lesions in the primate retina under conditions of elevated blood oxygen

Under conditions of nonthermal radiant exposure to blue light (440 nm) the primate retina can suffer photic injury by a mechanism that must be photochemical in nature. We have examined the effects of elevated blood oxygen (pO2 of 270 mmHg) on the retinal photosensitivity to blue light in two macaque monkeys by histologic analysis of 12 lesions at 1 to 57 days after irradiation. The retinal image diameter from a xenon arc lamp source was 1 mm, the duration of exposure was 100 sec, and the radiant exposures ranged from 11 to 36 J/cm2. When blood oxygenation is not elevated experimentally, the threshold radiant exposure for a blue light lesion to be visible funduscopically at 2 days postexposure is about 30 J/cm2. At a high blood pO2 level, a radiant exposure of only 11 J/cm2 gave a funduscopically visible lesion at 1-day postexposure. This large increase in retinal sensitivity to blue light damage appears to be due to photodynamic action. The only direct effect of elevated blood pO2 on the retina observed histologically was the presence of numerous granules in the cells of the retinal pigment epithelium (RPE). However, there was no apparent histopathology associated with the elevation of blood pO2 alone. Analysis of the various photic lesions showed only moderate damage to the neural retina, but a strong response was seen in the RPE. This is the histopathologic pattern of a typical blue light lesion shown in previous studies but more severe. So the effect of elevated blood O2 is to increase retinal sensitivity to photic damage, to lower the damage threshold, and to increase the severity of damage at a given radiant exposure. The status of lesions at 23 and 57 days postexposure suggests that such injuries are repairable.