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C. Framme, G. Schuele, K. Kobuch, B. Flucke, R. Birngruber, R. Brinkmann; Evaluation of Selective Retina Treatment (SRT) Using 8ns Laser Pulses. Invest. Ophthalmol. Vis. Sci. 2007;48(13):4162.
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It has been shown that selective retina treatment (SRT) solely affecting the RPE while sparing of the photoreceptors is possible by using repetitive laser pulses in the microsecond-regime. It was our purpose to determine the angiographic and ophthalmoscopic damage thresholds and the histologic extent of tissue damage using much shorter laser pulses of 8ns in duration.
In total 130 laser lesions were applied onto the fundus of Chinchilla Bastard rabbits in vivo using a commercial laser slit lamp and irradiation from an arc-lamp pumped Nd:YAG laser at 532nm. The rabbits were irradiated with various energies with single 8ns pulses and a train of ten 8ns laser pulses at 100Hz. After treatment fundus photography and angiography were performed to determine the damage thresholds (ED50-probability of cell damage) and the therapeutic window (ophthalmoscopic ED50 versus angiographic ED50) as well as the safety range between both thresholds (angiographic ED86 versus ophthalmoscopic ED14). Histology was performed for single and repetitive pulse lesions.
Angiographic and ophthalmoscopic ED50-thresholds decreased with increasing number of pulses. No retinal hemorrhages or disruptions were observed for both sets of parameters. A factor of 3.1 for single pulse irradiation and 2.3 for 10 pulses of 8ns was obtained for the therapeutic window. Regarding the safety range factors of 0.8 (single pulses) and 1.7 (10 pulses) respectively were calculated. Histology for single and repetitive pulses at a radiant exposure of 292mJ/cm2 respectively 213mJ/cm2 above angiographic threshold revealed damaged RPE, intact Bruch's membrane and choriocapillaries. Photoreceptors were partly spared but also damaged to various extents.
Short laser pulses of 8ns pulse duration can lead to a selective damage of the RPE without retinal hemorrhage or disruption. However, the small safety range between angiographic and ophthalmoscopic threshold leads to various unintentional photoreceptor damage. So far, the use of the ns pulse-regimen for clinical application can not be suggested, since selective RPE effects cannot be ensured entirely.
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