Abstract

SA.19.09

Evaluation of selective retina therapy (SRT) in an animal model using ns laser pulses

Framme C.^1,2, Schüle G.², Kobuch K.¹, Flucke B.², Birngruber R.², Brinkmann R.^2
1Klinik und Poliklinik für Augenheilkunde, Universität Regensburg; ²Medizinisches Laserzentrum Lübeck

Objective: It has been shown that selective retina therapy (SRT) with repetitive 1.7µs pulse duration allows the treatment of the RPE while sparing the photoreceptors and thus avoiding laser scotoma. It was our purpose to determine the angiographic and ophthalmoscopic damage thresholds and the histologic extent of tissue damage using much shorter 8ns laser pulses.
Methods: A flashlamp-pumped frequency-doubled (532nm) Nd:YAG laser with 8ns pulse duration was used. In total 130 laser lesions were applied through a slit lamp onto the fundus of 4 eyes in Chinchilla Bastard rabbits. The rabbits were irradiated with increasing energies with single pulses and a train of ten laser pulses at 10Hz. After treatment fundus photography and angiography were performed to determine the damage thresholds (ED50-probability of RPE cell damage) and its ratio ( ophthalmoscopic ED50 versus angiographic ED50) as well as the therapeutic window between both thresholds (ratio of angiographic ED86 versus ophthalmoscopic ED14). Selected histology was taken for single and repetitive pulse lesions.
Results: Angiographic and ophthalmoscopic ED50-thresholds decreased with increasing number of pulses. For single pulse application an angiographic ED50 of 144mJ/cm² and an ophthalmoscopic one of 365mJ/cm² was found. Regarding ten pulse irradiation angiographic ED50 was 72mJ/cm² and ophthalmoscopic one 266mJ/cm². No retinal hemorrhages or disruptions were observed for both sets of parameters. The threshold ratio was 3.1 for single pulse irradiation and 2.3 for the train of pulses. The therapeutic window was 0.8 (single pulses) and 1.7 (10 pulses). Histology for single and repetitive pulses at radiant exposures within the therapeutic window of 292mJ/cm² respectively 213mJ/cm² revealed damaged RPE, intact Bruch's membrane and choriocapillaries. Photoreceptors were partly spared but also damaged to various extents.
Conclusions: 8ns laser pulses can selectively damage the RPE without retinal hemorrhage or disruption. However, due to the small therapeutic window unintentional photoreceptor damage cannot be excluded. Thus, the clinical use of short nanosecond laser pulses is not suggested.

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