Photokeratitis from subablative 193-nanometer excimer laser radiation

Photokeratitis is a side effect of UV light exposure whereby the corneal epithelium is photochemically injured in a time delayed fashion. UV light exposure in the far ultraviolet wavelength range has not previously been observed. This study addresses the concern of photokeratitis from subablative 193-nanometer excimer laser light.

Dutch belted rabbit corneas were irradiated with subablative 193-nanometer excimer laser light over a wide range of total energy exposures, and examined by slit lamp biomicroscope for signs of photokeratitis. Photokeratitis was identified by epithelial haze and stippling, and rose bengal and fluorescein staining at varied time intervals between 1/2 to 26 hours post exposure.

At threshold energy exposures of 1.0 to 1.5 J/cm2, an immediate superficial epithelial haze was seen which disappeared within several hours. At higher energy exposures of 10 J/cm2, a delayed photokeratitis with deep rose bengal and even fluorescein staining was seen. This latter delayed photokeratitis resembles that of longer UV wavelengths and is due to excimer laser fluorescence, whereas the former is a direct response of the 193-nanometer light. The percentage of excimer laser light undergoing fluorescence is calculated as less than 1%.

The potential side effects and hazards of scattered 193-nanometer radiation during excimer laser surgery are extremely limited because of the shorter penetration depth of direct excimer radiation and the minimal fluorescent emission of longer UV wavelengths for energy exposures within the realm of clinical use.