Therapeutic range of repetitive nanosecond laser exposures in selective RPE photocoagulation

Background: The aim of this study was to investigate whether selective damage the RPE while sparing the adjacent photoreceptors is possible with repetitive 200-ns pulses of Nd:YAG laser (532 nm) and what potential side effects can be expected with higher pulse energies. Methods: We irradiated the retinas of 19 eyes of 10 chinchilla rabbits with 500 pulses from a Nd:YAG laser, each 200 ns in duration, at a repetition rate of 500 Hz (158 μm, 0-120 μJ). Threshold curves for different effects were established. Representative lesions were investigated by light and transmission electron microscopy. Results: It was possible to produce lesions, which were only visible by fluorescein angiography. The ED₅₀ threshold energy per pulse for visibility by fluorescein angiography was 2.1 μJ per pulse, for visibility by ophthalmoscopy 8.6 μJ. Bubble formation, an uncommon phenomenon in retinal photocoagulation, occurred at energies of 15-25 μJ. Hemorrhage occurred at surprisingly high energy levels of more than 100 μJ. Histology performed on lesions visible only by angiography showed damage primarily to the RPE and outer segments, with very little damage to some inner segments dependent on the energy used. Conclusions: Selective RPE damage is possible with repetitive 200-ns laser pulses and appropriate energy; however, the collateral damage to the adjacent retina is more pronounced than with repetitive microsecond laser pulses. There is no risk of hemorrhage of retinal photocoagulation with the repetitive 200-ns laser pulses at low energy levels which would be used clinically.