Effects of transcranial direct current stimulation alone and in combination with rehabilitation therapies on gait and balance among individuals with Parkinson's disease: a systematic review and meta-analysis

Abstract

Background: Parkinson's disease (PD) is a neurogenerative disorder implicated in dysfunctions of motor functions, particularly gait and balance. Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation offered as a potential adjuvant therapy for PD. This systematic review and meta-analysis were conducted to identify whether tDCS alone and combined with additional rehabilitation therapies improve gait and balance among individuals with PD.

Methods: We searched PubMed, Embase, Web of Science, and relevant databases for eligible studies from inception to December 2022. Studies with a comparative design investigating the effects of tDCS on motor functions, including gait and balance among individuals with PD, were included. A meta-analysis was performed for each outcome using a random effects model for subgroup analysis and pooling of overall effect sizes.

Results: A total of 23 studies were included in the meta-analysis. The pooled results revealed that tDCS has moderate overall effects on gait, measured by gait speed (standardized mean deviation [SMD] = 0.238; 95% confidence interval [CI] - 0.026 to 0.502); stride length (SMD = 0.318; 95% CI - 0.015 to 0.652); cadence (SMD = - 0.632; 95% CI - 0.932 to - 0.333); freezing of gait questionnaire scores (SMD = - 0.360; 95% CI - 0.692 to - 0.027); step length (SMD = 0.459; 95% CI - 0.031 to 0.949); walking time (SMD = - 0.253; 95% CI - 0.758 to 0.252); stride time (SMD = - 0.785; 95% CI: - 1.680 to 0.111); double support time (SMD = 1.139; 95% CI - 0.244 to 0.523); and balance, measured by timed up and go (TUG) test (SMD = - 0.294; 95% CI - 0.516 to - 0.073), Berg balance scale (BBS) scores (SMD = 0.406; 95% CI - 0.059 to 0.87), and dynamic gait index (SMD = 0.275; 95% CI - 0.349 to 0.898). For the subgroup analysis, gait and balance demonstrated moderate effect sizes. However, only cadence, stride time, and TUG indicated a significant difference between real and sham tDCS (P = 0.027, P = 0.002, and P = 0.023, respectively), whereas cadence and BBS (P < 0.01 and P = 0.045, respectively) significantly differed after real tDCS plus other therapies rather than after sham tDCS plus other therapies.

Conclusions: Our results indicated that tDCS is significantly associated with gait and balance improvements among individuals with PD. The findings of this study provide more proof supporting the effectiveness of tDCS, encouraging tDCS to be utilized alone or in combination with other therapies in clinical practice for PD rehabilitation.

Keywords: Balance; Gait; Parkinson’s disease; Rehabilitation therapies; Transcranial direct current stimulation.

Fig. 1

PRISMA flowchart. Literature search and study selection based on inclusion and exclusion criteria from the initiation of search. PRISMA: Preferred Reporting Items for Systematic Reviews and Meta-Analyses

Fig. 2

Forest plot of standardized mean difference (SMD) and their 95% CI for gait speed. Black squares represent the SMD in individual trials. Horizontal lines represent 95% confidence interval (CI). The blue rhombus at the bottom indicates an overall pooled effect. tDCS: Transcranial direct current stimulation. The subjects received real tDCS showing an improvement in gait speed. However, this improvement did not reveal statistical significance compared to sham treatment patients (P = 0.077)

Fig. 3

Forest plot of standardized mean difference (SMD) and their 95% CI for stride length. Black squares represent the SMD in individual trials. Horizontal lines represent 95% confidence interval (CI). The blue rhombus at the bottom indicates an overall pooled effect. tDCS: Transcranial direct current stimulation. Similarly, the subjects in real tDCS showed an improvement in stride length. However, this improvement did not reveal statistical significance compared to patients in the sham treatment group (P = 0.062)

Fig. 4

Forest plot of standardized mean difference (SMD) and their 95% CI for cadence. Black squares represent the SMD in individual trials. Horizontal lines represent 95% confidence interval (CI). The blue rhombus at the bottom indicates an overall pooled effect. tDCS: Transcranial direct current stimulation. Subjects who received either real tDCS alone or combined with additional therapies had distinctly reduced cadence during walking. This shows strong evidence that tDCS has a substantial beneficial effect on cadence parameters (P < 0.001)

Fig. 5

Forest plot of standardized mean difference (SMD) and their 95% CI for freezing of gait questionnaire. Black squares represent the SMD in individual trials. Horizontal lines represent 95% confidence interval (CI). The blue rhombus at the bottom indicates an overall pooled effect. tDCS: Transcranial direct current stimulation. The pooled results showed that tDCS indeed reduces the freezing during gait as measured by the freezing of gait questionnaire with a moderate effect size of 0.360 (P = 0.034)

Fig. 6

Forest plot of standardized mean difference (SMD) and their 95% CI for timed up and go test. Black squares represent the SMD in individual trials. Horizontal lines represent 95% confidence interval (CI). The blue rhombus at the bottom indicates an overall pooled effect. tDCS: Transcranial direct current stimulation. The results of this meta-analysis show robust evidence that tDCS significantly improved the balance of PD patients compared with controls, as indicated by a reduction in the time required to complete the TUG test (P = 0.009)

Fig. 7

Forest plot of standardized mean difference (SMD) and their 95% CI for Berg balance scale. Black squares represent the SMD in individual trials. Horizontal lines represent 95% confidence interval (CI). The blue rhombus at the bottom indicates an overall pooled effect. tDCS: Transcranial direct current stimulation. The overall meta-analysis result from studies which compared with patients in the sham group with patients who received either tDCS alone or tDCS combined with additional rehabilitation therapies did not show a significant improvement in balance measured by Berg balance scale (P = 0.087)