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 epithelium (RPE) is a new technique to treat a variety of retinal diseases without causing adverse effects to surrounding tissues such as the neural retina including the photoreceptors and the choroid. In this study, the mechanism 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 λ = 527 nm, adjustable pulse duration τ 250 nsec-3 μsec) and a Nd:YAG laser (λ = 532 nm, τ = 8 nsec). Fast flash photography was applied to observe vaporization at melanosomes in suspension. A fluorescence viability assay was used 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 damage radiant exposure is 40% lower at 3μsec. Temperature calculations to model the onset of vaporization around the melanosomes are in good agreement with the experimental results when assuming a surface temperature of 150°C to initiate vaporization and a homogeneous melanosome absorption coefficient 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 pulse 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 to pulse durations of 3 μsec can be described by a damage mechanism in which microbubbles around the melanosomes cause a rupture of the cell structure. The threshold radiant exposure for RPE damage decreases with increasing number of pulses applied. (C) 2000 Wiley-Liss, Inc.