Light and electron microscopic evaluation of canine corneal endothelium following CO₂ photokeratotomy

Objective To determine using light and scanning electron microscopy if treatment with CO₂ photokeratotomy alters the corneal endothelium in healthy dogs. Procedure Eight surgery laboratory dogs were determined to be free of ocular abnormalities. Under general anesthesia, the left eye of each dog was treated in a quadrant from 12 to 3 o'clock with the CO₂ laser in a defocused mode. The right eye served as a control. There were four treatment groups, each with 2 dogs: group 1 (2 W, 0.1 J/s, 0.8 mm tip), group 2 (3 W, 0.3 J/s, 0.8 mm tip), group 3 (2 W, 0.04 J/s NovaScan), group 4 (3 W, 0.06 J/s, NovaScan). The 0.8 mm tip delivered a power density of 382 W/cm² or 573 W/cm², at 2 or 3 watts respectively. The NovaScan handpiece delivered a power density of 30 W/cm² or 40 W/cm², at 2 or 3 Watts respectively. Following euthanasia, right and left corneas including a 2-mm scleral rim were harvested and fixed in commercial grade Karnovsky's fixative. One piece of cornea was processed routinely, embedded in Embed 812 resin, sectioned at 1 um, stained with toluidine blue and evaluated with the light microscope. A separate piece of each cornea was routinely processed and examined with a JEOL 6400 scanning electron microscope (SEM) at 20 KV. Results No changes in endothelial cell morphology were detected by light microscopy in the sections examined. SEM indicated normal endothelial cell morphology in control eyes with presumed artifactual changes at the corneal free margin (4/8 eyes). Multiple punctate to linear regions of endothelial cell destruction were observed in 6/8 laser-treated corneas. A significant increase in corneal thickness ranging from 1.90 to 37.28% was observed in all laser treated corneas. This increase in thickness correlated linearly with the degree of endothelial damage. Ultrastructural findings also correlated with postoperative clinical findings. Conclusion CO₂ laser photokeratotomy alters corneal endothelial cell morphology and thickness.