Refractive hazards of intraoperative retinal photocoagulation.

The refractions of a laser beam during transpupillary and endo-ocular intraoperative retinal photocoagulation as it passes through vitreous substitutes, with varying refractive indexes, were investigated with the use of mathematical algorithms and ray tracers. The following modifications and potential hazards were noted: 1) deviations of the laser beam (unlikely, however, to cause target error when surgery is properly conducted); 2) variations in laser-beam diameter and power density (up to 60%), easily leading to retinal underexposure or overexposure; and 3) asymmetry in beam distribution at the target site, leading to non-uniform retinal photocoagulation. Proper surgical techniques must be adopted before and during intraoperative photocoagulation to minimize retinal hazards. The relatively short distance between the endolaser probe and the retina means that fewer undesirable refractions occur with endo-ocular photocoagulation than with transpupillary indirect intraoperative photocoagulation.