Effects of non-invasive brain stimulation over the supplementary motor area on motor function in Parkinson's disease: A systematic review and meta-analysis

Background: Motor dysfunction profoundly affects individuals with Parkinson's disease (PD). Non-invasive brain stimulation (NIBS) targeting the supplementary motor area (SMA), a critical region for movement-related processing, offers a promising approach to enhance motor function for PD. Objective: This systematic review and meta-analysis aims to evaluate the efficacy of NIBS over the SMA (SMANIBS) in alleviating motor symptoms in PD. Methods: We conducted literature searches in MEDLINE, EMBASE, Physiotherapy Evidence Database, Web of Science, the Chinese National Knowledge Infrastructure, and Scopus. The meta-analysis utilized an inverse variance method and a random-effects model. Subgroup analyses were performed based on stimulation types (e. g., TMS and tDCS), stimulation protocols (e.g., facilitatory and inhibitory stimulation), and medication status during stimulation. Results: Twenty randomized control trials involving 442 individuals with PD were included. Compared to sham stimulation, SMA-NIBS significantly improved motor function as measured by the motor section of Unified Parkinson's Disease Rating Scale (UPDRS-III) (mean differences [MD]: -3.45, 95 % confidence interval [CI]: -5.65 to -1.26). Subgroup analysis revealed that only TMS (MD: -3.62, 95%CI: -6.15 to -1.08), not tDCS (MD: -2.47, 95 % CI: -5.03 to 0.08), has significant effect on motor function. Both facilitatory (MD: -2.59, 95 % CI: -3.37 to -1.82) and inhibitory stimulation (MD: -4.98, 95 % CI: -9.29 to -0.66) significantly improved the UPDRS-III score. Effectiveness was observed only during ON medication. Statistically significant effects of SMANIBS were reported on Freezing of Gait Questionnaire, not timed up and go test and walking speed. Conclusion: SMA-NIBS is a promising approach to enhance motor function in PD.