Temporal parameters of low energy laser irradiation for optimal delay of post-traumatic degeneration of rat optic nerve

Compression injury of a central nerve results in its degeneration with irreversible loss of function due to the inability of the mammalian central nervous system (CNS) to regenerate. In contrast, the CNS of lower vertebrates has a high capacity to regenerate. Recently, low energy laser irradiation was shown to attenuate degeneration in injured CNS nerves. The optic nerves of rats were subjected to moderate crush, calibrated so that some electrophysiological activity was preserved. The nerves were then subjected to low energy laser irradiation (10.5 mW, 2 min daily) for various periods. The electrical activity of the nerves, distal to the site of injury, was determined by measuring the compound action potential at the termination of the experiment. Two weeks of irradiation begun immediately after injury and continued daily thereafter, resulted in a compound action potential which was significantly higher (mean ± S.E.M. 1856 ± 535 μV) than that of non-irradiated injured nerves (351 ± 120 μV). The effect was temporary and subsided within a week. This two-week irradiation was slightly more effective than a treatment lasting one week (1406 ± 225 μV) and was significantly more effective than 4 days of irradiation (960 ± 133 μV). The number of treatments is therefore important. The time at which the treatment commences relative to the injury is also critical. Irradiation initiated two hours after the crush was about half as effective as immediate irradiation (810 ± 42 μV). No apparent effect was evident when the laser was applied for the first time 5 h, or longer, after the crush. Pretreatment with a laser irradiation immediately before the crush was an effective (1430 ± 281 μV) as irradiation soon after the injury. It is important to note that the laser effect is demonstrable only in moderately injured nerves. Severely injured nerves are not affected by treatment even after prolonged (3 and 4 weeks) irradiation. Our results further suggest that low energy laser irradiation only delays degenerative processes but does not prevent them. Optimal results are obtained when nerve crush is moderate, and the irradiation is initiated immediately before or soon after the injury and continued for at least one week.