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Selective RPE Laser Damage in Dependence from Pulse Duration and Number of Pulses
1Framme C., 2Schüle G., 1Roider J., 3Birngruber R., 3Brinkmann R.,
1Universität Regensburg, Klinik und Poliklinik für Augenheilkunde (Regensburg)
2Medizinische Universität zu Lübeck, Medizinisches Laserzentrum (Lübeck)
3Medizinisches Laserzentrum Lübeck GmbH (Lübeck)
Purpose: The therapeutic effect of laser treatment for macular diseases is confined to the RPE damage and the secondary restoration of the blood-retina-barrier due to RPE proliferation leading to improved "pump-function" of the cells. It is possible to damage the RPE selectively and to spare the photoreceptors by using repetitive microsecond laser pulses. It was the aim of the study to investigate the influence of pulse duration and number of pulses on the angiographically and ophthalmoscopically visible damage thresholds and the therapeutic range.
Method: There were 755 laser lesions of various parameters applied to the retina in 11 eyes of 6 Chinchilla bastard rabbits using an experimental laser system (Nd:YLF at 527nm). Pulse duration (1.7µs and 200ns) and number of pulses (100 and 10 pulses, single pulses) were varied at a constant repetition rate of 100Hz. Damage thresholds were measured by ophthalmoscopic and angiographic visibility and the therapeutic window (TW) between both thresholds was calculated.
Results: Generally the irradiation for ED50 cell damage decreased with a decay of pulse duration and an increase of the number of pulses. TW for both pulse duration (1.7µs and 200ns) was wider treating with 100 pulses than with single pulses. Widest TW (angiographic ED50 versus ophthalmoscopic ED50) was found for 100 pulses at 200ns pulse duration (5.7fold above angiographic threshold), smallest TW with a factor of 1.6 was found for 1,7µs single pulses. TW for the remaining parameters was always wider than a factor of 2.5 (TW for 200ns pulses always wider than for 1.7µs pulses). Regarding the confidence intervals (angiographic ED85 versus ophthalmoscopic ED15) a reduction of the TW for all parameters was found and only irradiation with 100 pulses at 200ns pulse duration led to a TW wider a factor of 2.
Conclusions: There was a significant relationship of RPE laser damage thresholds with pulse duration and number of pulses using a pulsed laser system for selective RPE laser treatment. Regarding the therapeutic range, due to intra- and interindividual pigmentation varying by a factor of 2 in humans, a TW with a factor of at least 2 should be ensured to treat clinically. Thus beside using the microsecond pulses also a treatment with repetitive 200ns pulses seems to be conceivable. However, histologic examinations need to show if with shorter pulse duration thermomechanical damages occur on photoreceptors, which might not be visible ophthalmoscopically.