Investigations on retinal pigment epithelial damage at laser irradiation in the lower microsecond time regime

PURPOSE. New lasers with a continuous wave power exceeding 15 W are currently investigated for retinal therapies, promising highly localized effects at and close to the Retinal Pigment Epithelium (RPE). The goal of this work is to evaluate mechanisms and thresholds for RPE cell damage by means of pulse durations up to 50 μs. METHODS. A diode laser with a wavelength of 514 nm, a power of 15 W, and adjustable pulse durations between 2 μs and 50 μs was used. Porcine RPE-choroidal explants (ex vivo) and chinchilla bastard rabbits (in vivo) were irradiated to determine threshold radiant exposures for RPE damage -HCell by calcein vitality staining and fluorescence angiography, respectively. Thresholds for microbubble formation (MBF) -HMBF were evaluated by time-resolved optoacoustics. Exemplary histologies support the findings. RESULTS. -HMBF is significantly higher than -HCell at pulse durations ≥ 5 μs (P < 0.05) ex vivo, while at 2 μs, no statistically significant difference was found. The ratios between -H MBF and -HCell increase with pulse duration from 1.07 to 1.48 ex vivo and 1.1 to 1.6 in vivo, for 5.2 and 50 μs. CONCLUSIONS. Cellular damage with and without MBF related disintegration are both present and very likely to play a role for pulse durations ≥ 5 μs. With the lower μs pulses, selective RPE disruption might be possible, while higher values allow achieving spatially limited thermal effects without MBF. However, both modi require a very accurate real-time dosing control in order to avoid extended retinal disintegration in this power range.