Effects of power densities, continuous and pulse frequencies, and number of sessions of low-level laser therapy on intact rat brain

Objective: The aim of the present study was to investigate the possible short- and long-term adverse neurological effects of low-level laser therapy (LLLT) given at different power densities, frequencies, and modalities on the intact rat brain. Background Data: LLLT has been shown to modulate biological processes depending on power density, wavelength, and frequency. To date, few well-controlled safety studies on LLLT are available. Methods: One hundred and eighteen rats were used in the study. Diode laser (808 nm, wavelength) was used to deliver power densities of 7.5, 75, and 750 mW/cm² transcranially to the brain cortex of mature rats, in either continuous wave (CW) or pulse (Pu) modes. Multiple doses of 7.5 mW/cm² were also applied. Standard neurological examination of the rate was performed during the follow-up periods after laser irradiation. Histology was performed at light and electron microscopy levels. Results: Both the scores from standard neurological tests and the histopathological examination indicated that there was no long-term difference between laser-treated and control groups up to 70 days post-treatment. The only rats showing an adverse neurological effect were those in the 750 mW/cm² (about 100-fold optimal dose), CW mode group. In Pu mode, there was much less heating, and no tissue damage was noted. Conclusion: Long-term safety tests lasting 30 and 70 days at optimal 10× and 100× doses, as well as at multiple doses at the same power densities, indicate that the tested laser energy doses are safe under this treatment regime. Neurological deficits and histopathological damage to 750 mW/cm ² CW laser irradiation are attributed to thermal damage and not due to tissue-photon interactions.