Purpose: The aim of the present study was to characterize permissible exposure limits (MPE) for safety analysis, with an emphasis on the immediate retinal damage following SHG of Nd:YAG Q-Switched laser radiation and to test its correlation to physical parameters. Methods: Pigmented rabbits (n = 14) were exposed to single pulses of Nd:YAG laser radiation (532 nm, pulse duration:8-12 ns) in various energies ranging from 10 to 150 muJ. Exposures were conducted in retina tissue, very close to the optic nerve, with a total of 20 exposures per retina. Retinas were viewed during the first 15 min following exposure, using an on-line digital video camera. Thereafter, animals were sacrificed for histological evaluation. A quantitative analysis of the clinical findings, based on a severity score scale and a morphometric analysis of the extent of the lesions, was used to test the relationship with the laser energy. In addition, hemorrhage thresholds were computed using Probit Analysis. Results: Retinal damage, at various levels of severity, was observed immediately after exposure to energies above 26 muJ, characterized by edema and sub-retinal hemorrhages. The appearance and severity of the lesions varied among animals, between fellow eyes and even within the same retina. The ED50 for immediate pre-retinal hemorrhage was determined as 83 muJ and the lesions' diameter ranged from 141-640 mu. A significant correlation (R = 0.80, P < 0.0001) was found between the extent of the lesions and energy levels. The diameter of the lesions showed a linear (P < 0.008) increase with the laser energy. The histological observations indicated elevation of retinal layers and extensive damage in the outer segment of the photoreceptors and in the pigmented epithelial cells layer. Conclusions: A linear, laser-retinal tissue interaction was found immediately following exposure to single pulses of Nd:YAG laser radiation. It is suggested that unlike argon laser, which produces a thermal bum to the eye, Nd:YAG laser damage is a result of a combination of photo-mechanical and thermal mechanism.
