Evaluation of near-infrared light therapy for the treatment of neurodegenerative diseases: Limited penetration depth into the brain likely hinders efficacy

BackgroundNear-infrared (NIR) light therapy is used to treat various musculoskeletal disorders. It has been proposed that transcranial NIR light treatment may also be beneficial for Alzheimers disease (AD). However, the ability of NIR light to penetrate the scalp and skull efficiently and induce cytoprotective responses in the brain parenchyma has not been sufficiently examined so far. This study aimed to evaluate whether the amount of NIR light that can penetrate through the human skull can cause a biological effect. MethodsThree commercially available devices (a medical laser emitting light at a wavelength of 905 nm and two LED helmets operating at wavelengths of 810 nm and 1070 nm, respectively) were used to measure the NIR light transmittance through human post-mortem skulls with a thermal power sensor. Furthermore, the biological effects of the fraction of light power that passed through the skull were investigated in a human neuronal cell line and in C. elegans. ResultsThe 905 nm laser achieved transmittances of up to 0.31% (173 {micro}W/cm2) of its input power, and the LED helmets 0.71% (41 {micro}W/cm2; 810 nm) and 0.45% (19 {micro}W/cm2; 1070 nm) of their respective input powers. NIR light exposure at a power density of 134 mW/cm2 was sufficient to activate mitochondrial metabolism in cultured human neurons and C. elegans, as demonstrated by increased cytochrome c oxidase activity and induction of mitochondrial chaperones. However, this stimulatory effect was no longer observed when the applied power density was reduced to 2.5 mW/cm2. ConclusionsMore than 99% of the NIR light emitted by the investigated devices was either absorbed or scattered by the human skull. The residual NIR light that would reach underlying brain structures was too weak to elicit biological effects. In conclusion, NIR light treatment is unlikely to be effective to treat brain diseases such as AD due to the low penetrability of the skull.