The effect of single and repeated tDCS sessions on motor symptoms in Parkinson's disease: a systematic review.

Objectives: to update understanding of the effectiveness of transcranial direct current stimulation (tDCS) on motor dysfunction in Parkinson's disease, since the last review was published in 2016.

Methods: in order to identify suitable publications for inclusion, an online search of the Pubmed, Scopus and Cochrane databases was carried out. Searches of relevant full-text articles were performed through specific keywords. The final database check was performed in July 2019. Papers were restricted to studies investigating motor rehabilitative effects of tDCS in adult patients with Parkinson's disease. Studies involving either single or repeated tDCS sessions with a sham or controlled trial type design (which incorporated outcomes on motor performance measures) were considered. As studies varied widely in terms of methodology, a qualitative analysis of the selected studies was performed using the Newcastle-Ottawa Quality Assessment Scale or the Delphi list (depending on the study design).

Results: twenty-nine studies were retained in this systematic review. Of the studies included, fifteen involved single tDCS session (patients = 256) and fourteen involved repeated tDCS sessions (patients = 294). Eight investigations of single tDCS and ten investigations of repeated tDCS demonstrated significant results. Studies involving multi- target stimulation demonstrated significant improvements on mobility (p=0.006), balance (by 50.9%), gait velocity (by 29%), fall reduction (p0.05) compared to mono-target stimulations.

Conclusions: despite increasing evidence that tDCS may improve motor symptoms, the results showed that fully optimized tDCS protocols are not yet established.