Evaluation of the safety of recent surgical microscopes equipped with xenon light sources

Although recent surgical microscopes for neurosurgery are equipped with xenon light sources to obtain bright fields of vision, the safety of a xenon beam, which has strong energy intensity in a long ultraviolet light, for cortical neurons has not been evaluated. Cranial windows were made in the parietal bones of gerbils. The skull of each gerbil was covered with warmed saline (0.5 mm in depth) to maintain the brain temperature. Ultraviolet irradiation (365-nm) was performed for 30 minutes at energy levels of 9.6, 4.4, 1.3, and 0.3 mwatts/cm(2), and neuronal damage was observed in 90 +/- 4%, 42 +/- 23%, 9 +/- 6%, and 0 +/- 0% of pyramidal cells in the parietal cortex 24 hours later. With the use of a logistic regression curve, the energy level causing 50% of neuronal damage was estimated to be 5.4 mwatts/cm(2). By increasing the thickness of the saline layer over the skull surface (1 mm and 2 mm), neuronal damages were significantly attenuated (21 +/- 18% and 10 +/- 8%, respectively, 4.4 mwatts/cm(2). Because the highest energy levels of 365-nm ultraviolet rays emitted from surgical microscopes measured in the present study (0.379 mwatts/cm (2)) were much closer to the dose causing 0% damage than to the dose causing 9% damage, the risk of neuronal injury occurring during microsurgery could be negligible. However, care should be taken in patients who take medicine classified as photosensitizing agents, such as diphenylhydantoin, which are thought to concentrate ultraviolet energy. The use of saline over the cortical surface may be beneficial for reducing the detrimental effects of 365-nm ultraviolet light.