Long-Term Safety of Single and Multiple Infrared Transcranial Laser Treatments in Sprague–Dawley Rats

Background and Objective: Growing interest exists in the use of near-infrared laser therapies for the treatment of numerous neurologic conditions, including acute ischemic stroke, traumatic brain injury, Parkinson's disease, and Alzheimer's disease. In consideration of these trends, the objective of this study was to evaluate the long-term safety of transcranial laser therapy with continuous-wave (CW) near-infrared laser light (wavelength, 808 ± 10 nm, 2-mm diameter) with a nominal radiant power of 70 mW; power density, 2,230 mW/cm 2 , and energy density, 268 J/cm 2 at the scalp (10 mW/cm 2 and 1.2 J/cm 2 at the cerebral cortical surface) in healthy Sprague–Dawley rats. Materials and Methods: In this study, 120 anesthetized rats received sequential transcranial laser treatments to the right and left parietal areas of the head on the same day (minimum of 5 min between irradiation of each side), on either Day 1 or on each of Days 1, 3, and 5. Sixty anesthetized rats served as sham controls. Rats were evaluated 1 year after treatment for abnormalities in clinical hematology and brain and pituitary gland histopathology. Results: No toxicologically important differences were found in the clinical hematology results between sham-control and laser-treated rats for any hematologic parameters examined. All values fell within historic control reference ranges for aged Sprague–Dawley rats. Similarly, brain and pituitary gland histopathology showed no treatment-related abnormalities or induced neoplasia. Conclusions: Single and multiple applications of transcranial laser therapy with 808-nm CW laser light at a nominal power density of 10 mW/cm 2 at the surface of the cerebral cortex appears to be safe in Sprague–Dawley rats 1 year after treatment.