This study employed a double-blind, randomized controlled trial design. A total of 38 chronic post-stroke patients (20 men, 18 women; mean age 64.34 years) were recruited and randomly assigned to one of two groups: an active dual-site tDCS group (“experi-mental1”) and an active M1/sham DLPFC tDCS control group (“experimental2”). Both groups received a total of five sessions of tDCS intervention. All participants had experienced a first ischemic stroke in the middle cerebral artery (MCA) territory more than 6 months prior to enrollment (mean onset duration 34.2 weeks). These participants were selected from a cohort of 430 individuals admitted to Pars Hospital with a diagnosis of stroke between June 20, 2022, and July 20, 2023. Stroke diagnosis was confirmed through clinical assessment and neuroimaging (CT or MRI).

Inclusion criteria were: A Modified Modified Ashworth Scale (MMAS) score of 1 or higher for wrist flexor spasticity, the ability to communicate verbally in Persian, and a Persian Mini-Mental State Examination (MMSE) score of 23 or higher. Exclusion criteria included severe cognitive or memory impairment, upper limb deformities that would interfere with motor assessment or tDCS electrode placement, a family history of chromic neurological disorders (e.g., epilepsy), and the use of medications known to affect cognition (e.g., benzodiazepines, anticholinergics).

Outcome measures were collected at baseline (before the first tDCS session) and 24 h after the fifth and final tDCS intervention session. The primary outcome measures were the Fugl-Meyer Assessment (FMA) for upper extremity motor function and the Modified Modified Ashworth Scale (MMAS) for spasticity. Passive range of motion (PROM) for the upper extremity was also measured. Written informed consent was obtained from each participant prior to enrollment. The clinical trial code is IRCT20211030052912N1.

Double-blind randomization was performed using the Randomization.com website. Participants were randomly allocated to one of the two experimental groups (1 and 2) using a computer-generated block randomization design. The allocation sequence was concealed from the researchers enrolling and assessing participants. Experiment 1 received five sessions of active anodal tDCS targeting M1 and DLPFC (a-tDCS M1-DLPFC). Experiment 2 received five sessions of active anodal tDCS targeting the M1 and sham stimulation targeting the DLPFC [a-tDCS M1-DLPFC (sham)]. The tDCS devices were programmed and operated by a researcher not involved in the participant recruitment or outcome assessment, ensuring blinding of both participants and assessors to the stimulation condition (Figure 1).

tDCS was delivered using two single-channel devices. Direct current was administered through saline-soaked electrodes. Electrode placement followed the international 10–20 EEG system. Active electrodes (4 × 4 cm2) were placed over the primary motor cortex (M1, C3/C4) and the dorsolateral prefrontal cortex (DLPFC, F3/F4) on the affected hemi-sphere. Reference electrodes (standard 5 × 7 cm2) were positioned on the contralateral supraorbital area (Figure 2) (15). A constant current of 1 mA was delivered for 20 min in both groups (10). However, in the “experimental2” group, DLPFC stimulation was turned off after 30 s to create the sham condition for this area (16) (Figure 2).

Outcome measurements were collected before the first tDCS intervention and 24 h after the fifth intervention. The primary outcomes were FMA for upper extremity motor function and MMAS for spasticity, which were assessed through questionnaire and therapist evaluation. PROM for the upper extremity was also assessed.

The Fugl-Meyer Assessment is a widely used test to evaluate motor function in stroke patients, encompassing five domains: Motor function, Sensory function, Balance, Joint function, and Joint pain (17, 18). In this study, only the motor function domain of the up-per extremity was assessed. This domain evaluates upper limb tasks without compensation and voluntary joint movement. Each item within this domain is scored between 0 (inability) to 2 (full performance). The patient’s upper extremity was positioned appropriately for each test item.

The Modified Modified Ashworth Scale (MMAS) is a clinical measure assessing muscle spasticity based on the resistance felt during passive movement (19). Spasticity is graded on a scale of 0 to 4, with higher scores indicating greater resistance. For this study, elbow flexor spasticity was measured. The patient was positioned supine with the head neutral and the arm at the side. The forearm was moved from maximum elbow flexion to full extension, and the level of resistance was recorded. One measurement was taken per session, with a maximum score of 4 representing severe spasticity and 0 indicating no spasticity (20). This assessment was conducted before the first treatment and after the fifth treatment to evaluate the intervention’s effect on spasticity.

Data analysis was performed using SPSS software (version 26, IBM SPSS Statistics for Windows, IBM Corp, Armonk, NY, USA). Continuous variables were presented as mean ± standard deviation. The Shapiro–Wilk test confirmed the normal distribution of the data. Between-group comparisons were conducted using the Mann–Whitney U test, and with-in-group comparisons were performed using the Wilcoxon signed-rank test. The level of statistical significance was set at p < 0.05. The sample size was determined using G*Power software (v3.1, Heinrich-Heine-University), based on the effect size (d = 2.0) from the Rayen study, a power of 0.90, and an alpha level (α) of 0.05. A 20% increase was added to account for potential attrition.

There were no significant differences in demographics, comorbidities, or spasticity levels between the two study groups (Table 1), indicating that the randomization process was effective in creating comparable groups at baseline. Several key outcomes were assessed in this study, including mean Ashworth scores for the elbow and wrist, FMA scores for UE, wrist, hand, and total performance and PROM scores for the UE. These were measured at baseline and after the intervention. Table 1 shows the demographic and clinical characteristics of the participants. Slightly more than half of the patients (approximately 50%) were female, with a mean age of 64.34 years (SD = 8.79) and a mean time since stroke onset of 32.82 weeks (SD = 6.99). As shown in Table 1, no significant differences in baseline characteristics were observed between the sham tDCS group and the active tDCS group (Table 1).

Figure 3 shows the Ashworth scores for the elbow and wrist before and after the intervention in both groups. At baseline, there were no significant differences between the groups in elbow spasticity (p = 0.999) or wrist spasticity (p = 0.999). Furthermore, no significant changes were observed within either group after the intervention for elbow or wrist spasticity. Between-group comparisons post-intervention also revealed no significant differences in elbow (p = 0.418) or wrist spasticity (p = 0.325). These results suggest that the a-tDCS intervention, regardless of the DLPFC stimulation condition, did not significantly impact elbow or wrist spasticity in this cohort (Figure 3).

The Fugl-Meyer Assessment of the upper extremity (Figure 4A) showed no statistically significant difference between the sham and active tDCS groups either before (p = 0.146) or after (p = 0.154) the intervention. In addition, FM score of wrist and hand did not show any significant difference between groups before the intervention (Wrist: p = 0.603, Hand: p = 0.296) or after the intervention (Wrist: p = 0.506, Hand: p = 0.296), and no significant changes were observed within groups (Wrist: Sham p = 0.317, Active p = 0.180; Hand: Sham p = 0.999, Active p = 0.999) (Figures 4B,C). Although the between group difference in the FMA total score was not significant at either time point (before: p = 0.111, after: p = 0.110), a statistically significant improvement within groups was observed in both the sham (p = 0.002) and active (p = 0.011) tDCS groups. This indicates that both the active and sham a-tDCS protocols were associated with improvements in overall upper extremity motor function, although there was no significant difference in the magnitude of improvement between the groups (Figure 4).

As shown in Figure 5, there were no statistically significant differences in the passive range of motion of the upper extremity between the sham and active tDCS groups before (p = 0.334) or after (p = 0.334) the intervention. This suggests that the a-tDCS intervention did not have a differential effect on upper extremity PROM between the two groups (Figure 5).

Changes in joint pain scores between the experimental groups were statistically in-significant at baseline (p = 0.277), post-intervention (p = 0.339), and for the change score (p = 0.708). However, statistically significant reductions in joint pain within both the sham (p = 0.001) and active (p = 0.001) tDCS groups were observed (Figure 6). This indicates that both the active and sham tDCS protocols were associated with a reduction in reported joint pain.

This study has several limitations that may affect the interpretation and generalizability of its findings. The small sample size and limited number of treatment sessions (five) may limit the robustness of the conclusions, particularly regarding insignificant findings or the effect of lesion sites on motor recovery. Future studies with larger sample sizes, control groups, longer treatment durations, and neurophysiological assessments are recommended to address these limitations and provide a more comprehensive understanding of time-dependent changes following a-tDCS intervention. Future research could explore the optimal parameters for dual site a-tDCS in chronic stroke, including different stimulation intensities, durations, and combinations with various rehabilitation therapies, with a clear emphasis on the integration of motor practice protocols. Investigating the influence of individual lesion characteristics on the response to different tDCS protocols would also be valuable.