A scoping review of frontal cortex tDCS on neuropsychological functioning in older adults with mild cognitive impairment and Alzheimer’s Clinical Syndrome

Mild cognitive impairment (MCI) and Alzheimer's Clinical Syndrome (ACS) are prevalent, incurable, and are expected to increase in incidence over the next 30 years. Finding new treatments to address the cognitive and behavioral problems in MCI and ACS represent an urgent need. Brain circuitry disruption can cause cognitive dysfunction and neuropsychiatric symptoms (NPS) in both MCI and ACS. Therefore, one promising avenue of treatment is non-invasive brain stimulation through transcranial direct current stimulation (tDCS). This scoping review examined the current knowledge base for the potential neuropsychological and neuropsychiatric effects of frontal cortex tDCS in older adults with MCI and ACS. Of the 17 randomized controlled trials reviewed, treatment parameters such as session length, current intensity, number of treatments, and time between treatments varied widely across studies, which restricted identification of optimal tDCS treatment protocols. Mixed findings on neuropsychological outcomes were observed, though significant improvements were most commonly seen in studies measuring global cognition (10) followed by executive function (6). Only three studies yielded clinically significant cognitive improvement, and few studies assessed NPS outcomes. Additional rigorous research is indicated to enhance our understanding of tDCS as a treatment for cognitive and neuropsychiatric symptoms in older adults with MCI and ACS.