Corneal damage from exposure to IR radiation: Rabbit endothelial damage thresholds

Rabbit endothelial damage thresholds have been determined for exposure to CO₂ laser radiation (10.6 μm). The laser was operated in the TEM₀₀ mode (Gaussian irradiance profile), and the 1/e diameter of the beam was 2.0 mm. Endothelial damage was detected using a staining technique. Threshold damage is characterized by distorted cells and uneven staining of the cell borders; while at exposure times of up to 90% of the threshold, the stained endothelia were indistinguishable from those in control eyes. Three fixed peak irradiance levels (24.5, 10.0 and 3.6 W/cm²) were investigated and the exposure duration was varied until threshold damage occurred. In each instance the threshold exposure duration for endothelial damage (1.0, 5.2 and 240 sec, respectively) was about 10 or more times the accepted threshold duration for epithelial damage. These results suggest that protecting against epithelial damage will also ensure against endothelial damage for CO₂ laser systems. However, the calculated peak endothelial temperature increases at these threshold exposures are similar to those of the epithelium at its damage threshold. Thus, as discussed in the text, other IR laser systems could harm the endothelium at epithelial damage threshold exposure levels.