Origin of retinal pigment epithelium cell damage by pulsed laser irradiance in the nanosecond to microsecond time regimen

Background and Objective: Selective photodamage of the retinal pigment epit helium (RPE) is a new technique to treat a variety of retinal diseases with out causing adverse effects to surrounding tissues such as the neural retin a including the photoreceptors and the choroid. In this study, the mechanis m of cell damage after laser irradiation was investigated. Study Design/Materials and Methods: Single porcine RPE-melanosomes and RPE cells were irradiated with a Nd:YLF laser (wavelength lambda = 527 nm, adju stable pulse duration tau = 250 nsec-3 mu sec) and a Nd:YAG laser (lambda = 532 nm, tau = 8 nsec). Fast flash photography was applied to observe vapor ization at melanosomes in suspension. A fluorescence viability assay was us ed to probe the cells vitality. Results: The threshold radiant exposures for vaporization around individual melanosomes and for ED50 cell damage are similar at 8-nsec pulse duration. Both thresholds increase with pulse duration; however, the ED50 cell damag e radiant exposure is 40% lower at 3 mu sec. Temperature calculations to mo del the onset of vaporization around the melanosomes are in good agreement with the experimental results when assuming a surface temperature of 150 de greesC to initiate vaporization and a homogeneous melanosome absorption coe fficient of 8,000 cm(-1). Increasing the number of pulses delivered to RPE cells at a repetition rate of 500 Hz, the ED50 value decreases for all puls e durations. However, the behavior does not obey scaling laws such as the N -1/4 equation. Conclusion: The origin of RPE cell damage for single pulse irradiation up t o pulse durations of 3 mu sec can be described by a damage mechanism in whi ch microbubbles around the melanosomes cause a rupture of the cell structur e. The threshold radiant exposure for RPE damage decreases with increasing number of pulses applied. Lasers Surg. Med. 27:451-464, 2000. (C) 2000 Wile y-Liss, Inc.