Comparative histopathological study of free-running mode and continuous wave Nd:YAG laser photocoagulation of monkey retina and choroid.

The effects of free-running mode (FRM) or continuous wave (CW) Nd:YAG laser application on the retina and choroid of 12 monkeys were examined periodically up to 24 months by light and electron microscopes. FRM Nd:YAG laser produced severe choroidal hemorrhage even in mild burns, because of the extensive coagulation of melanocytes at the center of the burned lesion. The rupture of Bruch's membrane did not depend on the power of the laser. On the other hand, CW Nd:YAG laser rarely produced subretinal hemorrhage through the rupture of Bruch's membrane in the medium-grade and severe burns. However, choroidal hemorrhage was seen histologically even in mild CW burned lesions. In the healing stage of mild FRM burned lesions the retinal pigment epithelium (RPE) and choroidal fibroblasts showed different repair processes. FRM medium-grade burned lesions showed the proliferation of Müller cells, while CW burns showed the regeneration and proliferation of RPE, Müller cells and fibroblasts. Because of the difference in the repair processes evoked by these two modes of laser application, these findings suggest that CW Nd:YAG laser is preferable for the treatment of subretinal and choroidal disorders.