Transcranial Direct Current Stimulation on Parkinson's Disease: Systematic Review and Meta-Analysis
Paloma Cristina Alves de Oliveira, Thiago Anderson Brito de Araújo, Daniel Gomes da Silva Machado, Abner Cardoso Rodrigues, Marom Bikson, Suellen Marinho Andrade, Alexandre Hideki Okano, Hougelle Simplício Gomes Pereira, Rodrigo Pegado, Edgard Morya
Abstract
Background: Clinical impact of transcranial direct current stimulation (tDCS) alone for Parkinson's disease (PD) is still a challenge. Thus, there is a need to synthesize available results, analyze methodologically and statistically, and provide evidence to guide tDCS in PD. Objective: Investigate isolated tDCS effect in different brain areas and number of stimulated targets on PD motor symptoms. Methods: A systematic review was carried out up to February 2021, in databases: Cochrane Library, EMBASE, PubMed/MEDLINE, Scopus, and Web of science. Full text articles evaluating effect of active tDCS (anodic or cathodic) vs. sham or control on motor symptoms of PD were included. Results: Ten studies ( n = 236) were included in meta-analysis and 25 studies ( n = 405) in qualitative synthesis. The most frequently stimulated targets were dorsolateral prefrontal cortex and primary motor cortex. No significant effect was found among single targets on motor outcomes: Unified Parkinson's Disease Rating Scale (UPDRS) III – motor aspects (MD = −0.98%, 95% CI = −10.03 to 8.07, p = 0.83, I 2 = 0%), UPDRS IV – dyskinesias (MD = −0.89%, CI 95% = −3.82 to 2.03, p = 0.55, I 2 = 0%) and motor fluctuations (MD = −0.67%, CI 95% = −2.45 to 1.11, p = 0.46, I 2 = 0%), timed up and go – gait (MD = 0.14%, CI 95% = −0.72 to 0.99, p = 0.75, I 2 = 0%), Berg Balance Scale – balance (MD = 0.73%, CI 95% = −1.01 to 2.47, p = 0.41, I 2 = 0%). There was no significant effect of single vs. multiple targets in: UPDRS III – motor aspects (MD = 2.05%, CI 95% = −1.96 to 6.06, p = 0.32, I 2 = 0%) and gait (SMD = −0.05%, 95% CI = −0.28 to 0.17, p = 0.64, I 2 = 0%). Simple univariate meta-regression analysis between treatment dosage and effect size revealed that number of sessions (estimate = −1.7, SE = 1.51, z-score = −1.18, p = 0.2, IC = −4.75 to 1.17) and cumulative time (estimate = −0.07, SE = 0.07, z-score = −0.99, p = 0.31, IC = −0.21 to 0.07) had no significant association. Conclusion: There was no significant tDCS alone short-term effect on motor function, balance, gait, dyskinesias or motor fluctuations in Parkinson's disease, regardless of brain area or targets stimulated.