Outcomes and Factors Affecting Transcranial Direct Current Stimulation in Alzheimer's Disease Patients: A Systematic Review

Introduction: Alzheimer's disease (AD) is the most common form of dementia. There have been various proposed pharmacologic and non-pharmacologic treatments proposed for preventing cognitive decline in AD patients. Transcranial Direct Current Stimulation (tDCS) is a neuromodulation technique used to enhance cognitive functions and motor skills of the brain. Our study aimed to assess the effects of tDCS in AD patients, including effects on general cognitive status, memory, attention, executive functions, language, IQ, and neuropsychological effects, along with the factors influencing the outcomes. Methods: Systematic searches were conducted for relevant evidence using PubMed, ScienceDirect, and Cochrane Library databases for (Transcranial Direct Current Stimulation) and (Alzheimer’s). Duplicates were removed, and the remaining articles were screened for double-blind, placebo-controlled, randomized clinical trials (Phase III), case studies, and case series on patients diagnosed with AD using tDCS. The articles were assessed for full text, and studies were selected and analyzed to include in the review. Results: Overall, 20 studies were reviewed. Cognitive status, executive function and working memory, recognition memory, and language function may improve following AtDCS depending on the stimulation polarity and area stimulated. No significant effects of tDCS were seen on attention, associative memory, recall memory, visuospatial ability, and neuropsychiatric symptoms. Discussion: Therapy outcomes and the factors that could affect them were analyzed, which included the number of sessions taken, current density, stimulation polarity (cathodal/anodal/dual), area stimulated, training(s) given, and study timeline. Conclusion: tDCS is a well-tolerated therapy that can be used for improving several cognitive domains in patients having Alzheimer’s disease. Its treatment outcomes are affected by polarity (cathodal/ anodal), site of stimulation, number of sessions taken, and any training(s) given during the study.