Review

Volume 15, Number 2, March 2025, pages 65-70

Transcranial Direct Current Stimulation in Motor Therapy for Patients With Parkinson’s Disease: A Systematic Review

Parkinson’s disease (PD) is characterized as a neurodegenerative disorder that progressively starts with the loss of dopamine-producing neurons in the brain. This leads to both motor and non-motor symptoms. These symptoms include tremors, bradykinesia, rigidity, and postural instability, and impair patient quality of life, diminishing their mobility and independence [1-4]. Therapies often focus on physical therapy, pharmacological interventions and, in some cases, surgical procedures such as deep brain stimulation can be applied. However, these approaches have limitations, such as lower efficacy over time, side effects, and invasiveness [5].

Transcranial direct current stimulation (tDCS), on the other hand, is a non-invasive technique with potential benefits for PD patients [6, 7]. This technique involves a low electrical current delivered to the scalp that can modulate neuronal activity. Some studies suggest that it can improve motor function in patients with PD, for instance, in outcomes such as cadence, stride length, the Timed Up and Go (TUG) test, gait capacity, balance, and the Unified Parkinson’s Disease Rating Scale (UPDRS) III [8-10]. Despite that, the evidence is limited and inconsistent, highlighting the need to understand tDCS’s efficacy and safety.

Therefore, while tDCS shows some improvements in motor symptoms in PD patients, the literature remains fragmented, and clinical evidence remains scarce. This systematic review aims to evaluate the efficacy of tDCS as an adjuvant therapy for motor symptoms of patients with PD. By collecting data from various studies, we want to determine the consistency of tDCS effects across different methods, identify gaps in the literature, and provide a better understanding of this therapy and a non-invasive option. Also, it is a goal to guide future research directions, emphasizing both the benefits and limitations of tDCS in motor symptoms of PD.

This systematic review was carried out per the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [11], and the registration of this research project was completed on the PROSPERO platform CRD4202457096.

The inclusion criteria for articles were: 1) eligible randomized controlled trials (RCTs); 2) studies that included any form of tDCS in the PD population; and 3) articles that had a control group. Furthermore, the exclusion criteria were: 1) articles that compared different types of tDCS; 2) articles that compared different intensities of stimulation; and 3) articles lacking a specific analysis of the subgroup of patients with PD. No restrictions were applied regarding age, gender, healthcare professional’s classification, or the date of service use.

A systematic search was conducted from inception to January 2025 across PubMed, Embase, and Cochrane Library databases, to identify key studies evaluating the impact of tDCS on motor therapy in individuals with PD. No filters were applied regarding language, target audience, or publication date. The specific search strategies employed for each database are detailed in Table 1. After the initial search, two authors independently removed the duplicates, screened titles and abstracts, and evaluated the articles in full for eligibility. A consensus-based discussion with a third author resolved discrepancies.

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Table 1. Search Strategy

The data from each article were distributed in a Google Spreadsheet table. The following information was included: article title, first author, year of publication, eligibility criteria, number of participants, type of randomization, intervention, control group, outcomes, and results.

Two independent authors assessed the risk of bias in each RCT using the Cochrane Collaboration Risk of Bias tool version 2 (RoB 2). To evaluate small study effects for the primary outcomes, comparison-adjusted funnel plots were utilized.

As seen in Figure 1, 429 studies were identified. After removing duplicates and applying eligibility criteria, five articles were included in the final analysis [10, 12-15].

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Figure 1. PRISMA flow diagram for the systematic review and meta-analysis. Diagram illustrates the identification, screening, and inclusion process of studies for this systematic review.

The five articles included were conducted in five countries: United States of America, Israel, Brazil, Italy, and Taiwan (Republic of China). In total, there were 176 participants, 122 men and 54 women. The patients had an average age of 67.13 years old. More information about the studies is provided in Table 2 [10, 12-15].

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Table 2. Characteristics of the Included Studies

This systematic review highlights several key findings from various studies about the effects of tDCS in improving motor outcomes for individuals with PD. One study reported that applying tDCS to the dorsolateral prefrontal cortex (DLPFC) (P < 0.001), the primary motor cortex (P = 0.048), and the cerebellum (P = 0.001) significantly improved dual-task gait speed performance when compared to baseline [10]. However, it is noteworthy that there were no significant enhancements in single walking or TUG performance compared to control groups.

Another article found a significant difference in gait cadence between the real tDCS and the sham groups (P = 0.014), indicating that tDCS may effectively modify gait patterns in PD patients [12]. There were no significant differences in other outcomes (such as gait speed and duration) between groups in his study. Furthermore, one study presented secondary analyses showing that tDCS, compared with sham, overall reduced self-reported severity of freezing of gait (FOG) (P = 0.05) and significantly increased daily living step counts (P = 0.04) [14]. Among individuals with mild-to-moderate FOG severity, tDCS in comparison with sham notably improved performance on FOG-provoking tests (P = 0.048) and self-reported experiences of freezing (P = 0.05).

A different study reported pronounced improvements in several motor function variables in the tDCS group compared to the sham group, including enhanced overall posture (P = 0.014), reduced lateral trunk inclination during standing (P = 0.013), increased total range of motion of the trunk (P = 0.012), and improved Functional Independence Measure (FIM) scores (P = 0.048) [13]. Lastly, another trial, also evaluating postural responses to tDCS, showed that the active group experienced a reduction in recovery time following postural perturbations (P < 0.001) and decreased reliance on prefrontal executive-attentional resources (P = 0.017), indicating enhanced movement automaticity. There was also a trend toward shorter medial gastrocnemius (MG) muscle onset latency in the active tDCS group post-intervention (P = 0.040), though this effect did not persist at follow-up (P = 0.056) [15].

Collectively, these studies underscore the potential of tDCS to enhance motor functions, including gait, posture, and overall mobility, in individuals with PD, highlighting its promise as a therapeutic intervention for managing motor symptoms associated with the condition.

The risk of bias evaluation for all five included articles revealed an overall low risk of bias among all studies. Figure 2 presents further details, including assessing each domain of the analyses for every article.

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Figure 2. Risk of Bias 2 Tool assessment. Risk of bias assessment for included studies using the Risk of Bias 2 Tool. The figure presents “traffic light” plots with domain-level judgments for each study and bar plots summarize the distribution of risk-of-bias judgments across domains. Green indicates low risk of bias, reflecting high methodological quality.

This systematic review investigated the role of tDCS as an adjunctive therapy for motor symptoms in PD, addressing an important gap regarding its efficacy and safety. Current treatments for PD are often limited by invasiveness, side effects, and reduced effectiveness over time, then tDCS offers a non-invasive alternative that can improve motor outcomes, including gait speed, posture, and balance. This review synthesized data from five RCTs, indicating that tDCS can enhance motor functions such as dual-task gait speed and trunk stability. However, outcome variability underscores the need to clarify further the most effective stimulation parameters and treatment protocols.

The observed motor improvements, such as those reported in dual-task gait speed and cadence, align with existing evidence that tDCS can modulate cortical excitability, which may enhance motor performance in PD patients [6, 16]. The studies included in this review suggest that stimulation of regions like the primary motor cortex and DLPFC might reinforce pathways affected by dopaminergic neuron loss, potentially offering patients improved motor stability [17, 18]. For example, one study demonstrated gains in gait speed with DLPFC stimulation [10]. Meanwhile, another trial reported enhanced cadence with motor cortex targeting, supporting the hypothesis that strategic electrode placement can produce specific motor benefits [12].

Additional findings, such as reductions in FOG and improvements in trunk posture and balance, indicate that tDCS might benefit more complex motor functions, which are typically challenging to treat with standard therapies [19, 20]. For instance, it was shown that tDCS could reduce FOG severity, particularly in patients with mild-to-moderate symptoms, which is significant given the limited efficacy of pharmacological interventions in addressing FOG [14]. Furthermore, two articles highlighted improvements in trunk stability and postural control, reduced recovery time after perturbations and decreased reliance on prefrontal executive-attentional resources, suggesting improved movement automaticity and postural stability in PD, showing that tDCS may support better balance and gait, both critical for maintaining mobility and reducing fall risk in PD [13, 15].

The variability in tDCS efficacy across studies reflects differences in stimulation protocols, highlighting the need to establish standardized parameters. Electrode placement, for example, greatly impacts which cortical areas are targeted [21]. Motor cortex stimulation has shown promise for basic motor functions, while DLPFC targeting affects dual-tasking abilities. Similarly, current intensity and duration may play a role in effectiveness, as some studies applied currents of 1 mA while others used 2 mA, with differing session lengths. Standardizing these variables across trials would likely improve comparability and help identify optimal protocols for PD.

The limited sample sizes and short-term follow-ups in many of the reviewed studies constrain the generalizability of the findings, as small cohorts and a lack of long-term data make it difficult to assess whether observed motor improvements are sustained or diminished post-stimulation. It is essential to determine whether repeated tDCS sessions are required for sustained benefits or if continuous application has a cumulative effect. Future studies should focus on longer follow-up periods and larger sample sizes to better understand the durability of tDCS benefits. Another critical limitation is the need for more reporting of adverse effects, making it challenging to evaluate the safety of tDCS for long-term use fully. Although tDCS is widely considered safe, thorough documentation of potential side effects is necessary to assess its feasibility as a regular treatment option.

This review suggests that tDCS complements existing treatment strategies for managing motor symptoms in PD, particularly for patients who do not respond well to traditional therapies. Policymakers and clinicians may consider integrating tDCS into clinical practice as an adjunctive treatment, especially in cases where invasive procedures are not viable. Moving forward, more extensive and standardized studies are needed to explore combination therapies and to assess the impact of tDCS on PD across different disease stages.

This systematic review provides preliminary evidence that tDCS may offer modest but meaningful improvements in motor symptoms for patients with PD, particularly in gait and postural control. However, the heterogeneity across protocols and sample limitations underscores the need for further standardized and large-scale trials. With continued investigation, tDCS holds promise as a valuable adjunctive therapy for PD, potentially improving patients’ mobility, stability, and quality of life.

Acknowledgments

None to declare.

Financial Disclosure

None to declare.

Conflict of Interest

None to declare.

Author Contributions

Conceptualization: Lucas B. Meira, Andrey T. Ferreira, Joao V.A. Fernandes and Andre de S.B. Oliveira. Project administration: Joao V.A. Fernandes and Andre de S.B. Oliveira. Formal analysis: Joao V.A. Fernandes. Writing original draft: Lucas B. Meira, Andrey T. Ferreira and Joao V.A. Fernandes. Writing review and editing: Lucas B. Meira, Andrey T. Ferreira and Joao V.A. Fernandes.

Data Availability

The authors declare that data supporting the findings of this study are available within the article.

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