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 potent ial 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 mu m, 0-120 mu J). Threshold curves for different effects were established. Representative lesions were investigated by light and transmission electron microscopy. Results: It was possible t o produce lesions, which were only visible by fluorescein angiography. The ED50 threshold energy per pulse for visibility by fluorescein ang iography was 2.1 mu J per pulse, for visibility by ophthalmoscopy 8.6 mu J. Bubble formation, an uncommon phenomenon in retinal photocoagula tion, occurred at energies of 15-25 mu J. Hemorrhage occurred at surpr isingly high energy levels of more than 100 mu J. 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 po ssible with repetitive 200-ns laser pulses and appropriate energy; how ever, the collateral damage to the adjacent retina is more pronounced than with repetitive microsecond laser pulses. There is no risk of hem orrhage of retinal photocoagulation with the repetitive 200-ns laser p ulses at low energy levels which would be used clinically.