ULTRAVIOLET-B ENHANCES CORNEAL STROMAL RESPONSE TO 193-NM EXCIMER-LASER TREATMENT

Purpose: The purpose of the study was to evaluate the biomicroscopic, light microscopic, and electron microscopic effects of ultraviolet-B ( UV-B) exposure on the outcome of photorefractive keratectomy (PRK). Me thods.: A total of 24 pigmented rabbits were used in the study. One ey e of 16 rabbits received a 193-nm, 45-mu m deep (-5.0 diopter) excimer laser PRK. Twenty-one days after PRK, eight of the laser-treated eyes were exposed to 100 mJ/cm(2) UV-B (280-315 nm) UV radiation by placin g the rabbits in a standard clinically used dermatologic chamber for 7 minutes. Eight PRK-treated rabbits received no further treatment. The remaining eight non-PRK-treated rabbits received 100 mJ/cm(2) UV-B on ly to one eye. Subepithelial haze was assessed before and after UV irr adiation. Corneal morphology was assessed 4, 8, 12, and 16 weeks after UV-B exposure, using light microscopic and transmission electron micr oscopic (TEM) techniques. Results: Untreated eyes exposed to 100 mJ/cm (2) UV-B only exhibited photokeratitis for 2 days, but showed no haze and were normal histologically at all intervals. The PRK-treated UV-B irradiated eyes exhibited a significant increase of stromal haze compa red to eyes receiving PRK alone. Histologically, the main difference b etween the UV-B irradiated and nonirradiated post-PRK eyes was the pre sence of anterior stromal extracellular vacuolization in the UV-B-expo sed eyes. The vacuolated foci were confined to the PRK treatment area and showed increased keratocyte density and disorganization of normal collagen lamellae. TEM showed activated keratocytes containing abundan t rough endoplasmic reticulum, prominent Golgi zones, and extracellula r vacuoles filled with amorphous material. The haze and morphologic ch anges showed a tendency to incomplete resolution over the period of 16 weeks. Conclusions: The UV-B exposure during post-PRK stromal healing exacerbates and prolongs the stromal healing response and is manifest biomicroscopically by augmentation of subepithelial haze. The finding s suggest that excessive ocular UV-B exposure should be avoided during the period of post-PRK stromal repair and that UV-B may modulate the response of tissues to 193-nm excimer laser and perhaps other laser en ergy in general.