The efficacy of cooling on excimer laser photorefractive keratectomy in the rabbit eye.

We cooled the surface of the cornea to reduce the thermal damage by the excimer laser ablation and referred to this method as "cooling photorefractive keratectomy" (cooling PRK). We performed conventional PRK and cooling PRK on rabbits' eyes and measured the thermal change during laser ablation. We also examined the degree of subepithelial haze and the tissues with a light microscope and an electron microscope. Normal corneal temperature of live rabbit corneas was about 32 degrees C and it rose to 41 degrees C after 300 laser shots. However, when we cooled the rabbit corneas to 24 degrees C with continuous irrigation of chilled BSS PLUS solution before laser ablation, the temperature rose only to 34 degrees C after the same ablation. Slit-lamp evaluation showed that more severe corneal haze occurred with the conventional PRK procedure. At 2 weeks postoperatively, the mean haze score was 1.77 +/- 0.87 in the conventional PRK and 0.87 +/- 0.38 in the cooling PRK (P < 0.01). Light-microscopy examination showed epithelial hyperplasia and fibroblasts, type III collagen, and heat shock protein-70 in the subepithelium of corneas with conventional PRK procedure than with cooling PRK. Under the electron microscope, more disruption of the layers of fibroblasts and collagen fibrils was observed in the conventional PRK procedure than that of the cooling PRK. In conclusion, the cooling of the corneal surface in PRK effectively reduces tissue damage related to subepithelial haze and may enable us to reduce the degree of myopic regression.