Efficacy of Transcranial Direct Current Stimulation and Photobiomodulation in Improving Cognitive Abilities for Alzheimer's Disease: A Systematic Review.

Background: Due to the increasing global prevalence of Alzheimer's dementia (AD), neuromodulation techniques such as transcranial direct current stimulation (tDCS) and photobiomodulation (PBM) are considered potential complementary therapies. Objective: We assessed the efficacy and safety of tDCS and PBM and their potential to enhance cognitive functions in individuals with AD. Methods: This review primarily examined studies designed to evaluate the efficacy, followed by an assessment of the safety of tDCS and PBM for people with AD. The databases searched were PubMed, Scopus, and Web of Science Core Collection, resulting in 17 published randomized and controlled trials. References were screened over 5 years (2020-2024). The research design used PRISMA guidelines. Results: Fourteen studies were considered for tDCS, and the current literature supports efficacy and safety at an amperage of 2 mA, with electrodes placed on the dorsolateral prefrontal cortex (DLPFC). Three studies were included for PBM. The heterogeneity of these study measures made them unsuitable for combined efficacy analysis, and they did not provide a safety evaluation. Conclusions: Despite differences in efficacy assessments, tDCS and PBM improved cognitive abilities. There is an urgent need to standardize metrics for evaluating efficacy and safety, particularly for PBM. Future research is encouraged.