Experimental argon laser photocoagulation. II. Effects on the optic disc.

Eight monkeys and five humans were subjected to photocoagulation with single argon laser burns of varying intensity on the optic nerve head. Energies greater than 400 milliwatts, 0.2 second and 100 mu spot size consistently caused neural parenchymal damage, even in the absence of heat-generating pigment epithelium. Papillary burns in monkeys and humans created with powers less than 400 mW showed only small, focal submicroscopic areas of degeneration, and were therefore considered relatively safe except for the occasional involvement of critical foveal fibers in which scotomas of disproportionately greater severity than one would expect from the size of the lesion could result. The threshold for toxicity following peripapillary photocoagulation is lower, because the burned pigment epithelium can radiate heat into the adjacent optic nerve fibers. Normal capillaries within the optic nerve were seldom destroyed or occluded, even after high energy densities. In normal vessels, we have found it impossible to create a highly selective lesion without concurrently causing neural damage about the vessel.