In vivo and in vitro selective targeting of the Retinal Pigment Epithelium using a Laser-Scanning Device

Background: Laser photocoagulation is a well-established treatment modality for a variety of retinal disorders, but is difficult to use near the fovea due to thermal retinal destruction. Certain diseases, such as drusen maculopathy, are thought to be caused by a dysfunction of the Retinal Pigment Epithelium. For those diseases selective targeting of the RPE, sparing the adjoining photoreceptors, might be the appropriate treatment to avoid laser scotoma, as it has been shown with application of a train of μs laser pulses by Birngruber and Roider. Material and methods: Our new approach is to use a conventional green cw laser and rapidly scan a small laser spot over the retina so as to produce μs-illumination at each RPE cell. Two scanning devices were developed using acousto-optic deflectors. Results: For the in vitro experiments the ED₅₀ value RPE cell damage was 170 mW with 100 exposures, scanning with a speed of 1 spot diameter/3 μs. In vivo experiments demonstrated an angiographic ED₅₀ threshold of 66 mW for 100 exposures while scanning with an effective illumination time of 5 μs. The ophthalmoscopic threshold was higher than a factor of 2 times the angiographic ED₅₀. Using separated scan lines we show selectivity in the form of surviving cells in between irradiated lines. Conclusion: Selective destruction of RPE cells is possible using laser-scanning devices.