Dose and temporal parameters in delaying injured optic nerve degeneration by low-energy laser irradiation

Low-energy laser irradiation has been reported to postpone the degenerative processes in crushed optic nerves of rats, which are part of the nonregenerable mammalian central nervous system. In the present study, we evaluated the optimal irradiation parameters for this purpose. Optic nerves of 141 rats were subjected to crush injury and then irradiated through the eye, starting at different points of time before or after the injury, for different durations and periods, using various intensities of either helium-neon laser or noncoherent infrared light (904 nm). The effect was evaluated by measurements of the compound action potentials of the nerve segments between the site of injury and the optic chiasm. The compound action potential amplitude of the crushed nonirradiated nerves, as measured 2 weeks after the injury, was found to be 0.51 +/- 0.30 mV, in contrast to 3.10 +/- 1.03 mV measured in 232 normal nerves. Irradiation with a 10.5 mW helium-neon laser for 2 and 3 min once a day for 14 consecutive days resulted in maximal preservation of action potentials (1.78 +/- 0.72 and 1.95 +/- 0.71 mV, respectively). Irradiations beginning immediately prior to the injury were as effective as irradiations beginning soon after it. Irradiations for longer than 3 min or twice a day aggravated the damage. Noncoherent infrared light was ineffective or adversely affected the injured nerves. Our experiments suggest that optimal delay of posttraumatic optic nerve degeneration in rats is attainable with 10.5 mW helium-neon laser irradiations for 2 or 3 min once a day for 14 consecutive days.