Morphological response to UV-B irradiation after excimer-laser photorefractive keratectomy

The purpose of this study was to evaluate the morphological effects of UV-B exposure on the outcome of photorefractive keratectomy (PRK). A total of 42 pigmented rabbits were used in the study. In all, 1 eye of 12 rabbits received a 193-nm and 45-micron-deep (-5.0-D) excimer-laser PRK, 1 eye of 12 rabbits received a 135-micron-deep (-15.0-D) excimer-laser PRK and 1 eye of 12 rabbits received a 270-micron-deep (-30.0-D) excimer-laser PRK. At 21 days after PRK, six of the laser-treated eyes from each group were exposed to 100 mJ/cm2 UV-B (280-320 nm) light by placement of the rabbits in a standard clinically used "dermatological chamber" for 7 min. One eye of six rabbits received only UV-B light, serving as a control. The other six rabbits from the PRK groups received no further treatment. Subepithelial "haze" was evaluated before and after UV irradiation. Corneal morphology was assessed at 4, 8 and 12 weeks after UV-B exposure by light microscope and transmission electron microscope techniques (TEM). Eyes exposed to 100 mJ/cm2 UV-B light exhibited only keratitis for 2 days but showed no haze and were histologically normal at all time points. The PRK-treated, UV-B-irradiated rabbit eyes exhibited a significant increase in stromal haze as compared with the eyes receiving PRK alone; this phenomenon correlated with the depth of photoablation. Histologically, the main difference observed between the UV-B-irradiated and nonirradiated post-PRK eyes was the presence of anterior stromal extracellular vacuolization in the UV-B-exposed eyes. The vacuolated foci were confined to the PRK treatment area, contained increased numbers of keratocytes and showed a disorganization of normal collagen lamellae. TEM revealed activated keratocytes containing abundant rough endoplasmic reticulum, prominent Golgi zones and extracellular vacuoles filled with amorphous material. The haze and morphological changes showed a tendency towards incomplete resolution over a period of 12 weeks. UV-B exposure during post-PRK stromal healing exacerbates and prolongs clinical symptoms and the stromal healing response in rabbits, which is manifested biomicroscopically by augmentation of subepithelial haze. The findings suggest that excessive ocular UV-B exposure should be avoided during the period of post-PRK stromal repair (at least 3 months) and that UV-B light may modulate the response of tissues to 193-nm excimer and, perhaps, other laser energy in general.