Threshold Determinations for Selective RPE Laser Treatment With a Laser Scanner in Rabbits With Different Scan Times in the Microsecond Regime

Abstract

Abstract: : Purpose: To determine the angiographic and ophthalmoscopic thresholds and the therapeutic window between both thresholds for the selective RPE laser treatment in rabbits when using a conventional continuous wave laser beam, which is tightly focused and scanned across the RPE at different scan parameters. Methods: The laser beam from a diode-pumped cw Nd:YVO₂ laser (532nm) was delivered to a two-dimensional acusto-optic deflector (AOD) mounted onto a conventional slit lamp. Using a contact lens the software controlled AOD scans the laser spot (diameter 12.5µm) across the retina of dutch-belted rabbits. Different scan speeds (time for the laser beam to move one spot-diameter: 1.7µs, 5µs, 10µs, 20µs, 30µs, 50µs), repetitive number of scans (n=10 and 100) and scan patterns (6 separate lines versus 21 interlaced lines in a scanning field of 300 x 300µm size) were evaluated. Laser lesions were applied at different intensity levels determining ophthalmoscopic visibility (fundus photographs) and angiographic visibility (fluorescein angiography). Histology was obtained in selected cases. Results: For separated lines angiographic ED₅₀-thresholds were 252mJ/cm² (1.7µs, 10 scans), 312mJ/cm² (5µs, 100 scans), 411mJ/cm² (5µs, 10 scans), 519mJ/cm² (10µs, 100 scans), 642mJ/cm² (10µs, 10 scans), 756mJ/cm² (20µs, 10 scans), 1105mJ/cm² (30µs, 10 scans) and 1652mJ/cm² (50µs, 10 scans). For the interlaced line patterns 368mJ/cm² (5µs, 10 scans) and 480mJ/cm² (10µs, 100 scans) were achieved. In all but the last parameter (interlaced lines) and the 50µs scan speed (separated lines) ophthalmoscopic thresholds were always higher than a factor of 2 above angiographic ones (therapeutic window) indicating selectivity. Histology for 5µs and 10µs scan duration (10 scans) revealed damaged RPE while neurosensory tissue remained intact. Conclusions: Selective targeting of RPE using a cw-laser scanner appears to be possible with scan times up to 20µs – 50µs. It is suggested that due to the small spot size heat dissipation can take place within the RPE monolayer without causing denaturation of photoreceptors. This may lead to the selective effects even at scan times longer than the approximated thermal relaxation time of single RPE cells. However, further histologic examinations will have to show the extent of selectivity for the different sets of parameters and determine which parameters can be clinically useful.

Keywords: laser • retina • pump/barrier function