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-mu m-deep (-5.0-D) excimer-laser PRK, 1 eye of 12 rabbits received a 135-mu m-deep (-15.0-D) excimer-la ser PRK and 1 eye of 12 rabbits received a 270-mu m-deep (-30.0-D) exc imer-laser PRK. At 21 days after PRK, six of the laser-treated eyes fr om each group were exposed to 100 mJ/cm(2) UV-B (280-320 nm) light by placement of the rabbits in,2 standard clinically used ''dermatologica l 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 rece ived 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 ele ctron microscope techniques (TEM). Eyes exposed to 100 mJ/cm(2) UV-B l ight exhibited only keratitis for 2 days but showed no haze and were h istologically normal at all time points. The PRK-treated, UV-B-irradia ted rabbit eyes exhibited a significant increase in stromal haze as co mpared with the eyes receiving PRK alone; this phenomenon correlated w ith the depth of photoablation. Histologically, the main difference ob served between the UV-B-irradiated and nonirradiated post-PRK eyes was the presence of anterior stromal extracellular vacuolization in the U V-B-exposed eyes. The vacuolated foci were confined to the PRK treatme nt area, contained increased numbers of keratocytes and showed a disor ganization of normal collagen lamellae, TEM revealed activated keratoc ytes containing abundant rough endoplasmic reticulum, prominent Golgi zones and extracellular vacuoles filled with amorphous material, The h aze and morphological changes showed a tendency towards incomplete res olution over a period of 12 weeks. UV-B exposure during post-PRK strom al healing exacerbates and prolongs clinical symptoms and the stromal healing response in rabbits, which is manifested biomicroscopically by augumentation of subepithelial haze. The findings suggest that excess ive ocular UV-B exposure should be avoided during the period of post-P RK 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 e nergy in general.