CLINICAL AND MORPHOLOGICAL RESPONSE TO UV-B IRRADIATION AFTER EXCIMER-LASER PHOTOREFRACTIVE KERATECTOMY

This paper represents an update on a study that has been reported else where (Nagy ZZ et al: Ophthalmology 104:375-380, 1997). The aim of the study was to evaluate the clinical and light- and electron-microscopi c effects of ultraviolet-B (UV-B) exposure on the outcome of photorefr active keratectomy (PRK). A total of 42 pigmented rabbits were used in the study. One eye from each of 12 rabbits received a 193 nm 45-mu m deep (-5.0 diopters [D]) excimer laser PRK, one eye from each of 12 ra bbits received a 135-mu m deep (-15.0 D) excimer laser PRK, and one ey e from each of 12 rabbits received a 270 mu m deep (-30.0 D) excimer l aser PRK. Twenty-one days after PRK, six of the laser-treated eyes fro m each group were exposed to 100 mJ/cm(2) UV-B (280-320 nm). The other six rabbits from the PRK groups received no further treatment. One ey e from each of six rabbits received only UV-B irradiation, serving as control. Subepithelial haze was evaluated before and after UV-B irradi ation. Clinical changes were followed by laser tyndallometry, confocal corneal biomicroscopy, ultrasound biomicroscopy, and endothelial spec ular microscopy. Corneal morphology was assessed 4, 8, and 12 weeks af ter UV-B exposure, employing light microscopic and transmission electr on-microscopic techniques (TEM). Eyes only exposed to 100 m]/cm(2) UV- B exhibited keratitis for 2 days, but showed no haze and were histolog ically normal at all time intervals. The PRK-UV-B-irradiated rabbit ey es exhibited a significant increase of stromal haze compared to the ey es receiving PRK alone; this phenomenon correlated with the depth of p hotoablation. The severity of clinical findings also correlated with t he previously attempted photoablation depth; in PRK-UV-B-irradiated ey es the symptoms were much more serious than in eyes treated with PRK a lone. Histologically, the main difference between the UV-B-irradiated and nonirradiated-post-PRK eyes was the presence of anterior stromal e xtracellular vacuolization in the UV-B-exposed eyes. The vacuolated fo ci were confined to the PRK treatment area, contained increased number s of keratocytes and showed a disorganization of normal collagen lamel lae. Transmission electron microscopy revealed activated keratocytes c ontaining abundant rough endoplasmic reticulum, prominent Golgi zones, and extracellular vacuoles filled with amorphous material. The haze a nd morphological changes showed a tendency to incomplete resolution ov er a period of 12 weeks. Ultraviolet-B exposure during post-PRK stroma l healing exacerbates and prolongs clinical symptoms and the stromal h ealing response, which is manifest bimicroscopically by augmentation o f subepithelial haze. The findings suggest that excessive ocular UV-B exposure should be avoided during the period of post-PRK stromal repai r and that UV-B may modulate the response of tissues to excimer 193 nm , and perhaps other laser energy in general. (C) 1997 by Elsevier Scie nce Inc. All rights reserved.