The Effectiveness of Transcranial Direct Current Stimulation (tDCS) on Severity of Obsessive-Compulsive Symptoms, Executive Functions and Rumination in People with Obsessive-Compulsive Symptoms: A Randomized Clinical Trial

Obsessive-compulsive disorder (OCD) is a chronic mental illness characterized by repetitive behaviors (compulsions) and unwanted thoughts (obsessions). While repetitive actions are common in daily life, individuals with OCD experience these behaviors and thoughts more frequently and intensely, significantly impacting their lives almost constantly (1, 2). Obsessive-compulsive disorder is the fourth most common mental disorder globally (3). Its estimated lifetime prevalence is 2.3%, with a range of 1.1 - 3.3%, varying by factors such as location, age, and other demographic variables (1, 4). Individuals with OCD may display various symptom categories, including persistent unwanted or religious thoughts, frequent hoarding, symmetry, frequent checking, fixation with contamination and cleaning methods, and ordering. These symptoms and behaviors may fluctuate, become irregular, or change in manifestation over the course of the disorder (5, 6).

Research in the past few decades has significantly expanded our understanding of the phenomenology and treatment of OCD. Numerous mechanisms and factors contribute to the etiology of OCD, leading to its chronic nature. Recent years have seen increased attention on the neuropsychology of OCD. This disorder is linked to neuropsychological impairments, particularly in areas such as attention, memory, and executive functions (7, 8). Based on the neurological mechanisms implicated in OCD, it is believed that the disorder involves deficiencies in two primary inhibitory processes: (1) Cognitive inhibition process: Associated with obsessive symptoms; (2) behavioral inhibition process: Associated with compulsive symptoms (9).

Deficiencies in these two inhibitory systems explain not only the cognitive and behavioral symptoms of OCD but also many neuropsychological deficits observed in this disorder, such as impairments in attention, memory, planning, and decision-making (7, 9).

There is substantial evidence linking OCD to brain dysfunction and cognitive abnormalities (10). Neuropsychological studies consistently reveal that individuals with OCD perform worse compared to other populations and clinical samples (11).

Underlying factors often accelerate the progression of diseases, and in the case of OCD, rumination appears to be a significant contributing factor. Memory interference can lead to rumination, which is frequently observed in individuals with OCD and generalized anxiety disorder (12). Research confirms that rumination plays a role in exacerbating psychological disturbances (13). This style of thinking is also prevalent in several emotional disorders, including depression, OCD, generalized anxiety disorder, and post-traumatic stress disorder (14).

The clinical literature on OCD highlights cognitive behavioral therapy (CBT), particularly in the form of exposure and response prevention (ERP), as the first-line treatment for this disorder (15). However, despite significant advances in OCD treatment, some patients either do not respond to treatment or show minimal improvement (16). While certain patients experience partial recovery, residual symptoms persist, leading to significant functional impairment and a reduced quality of life. Additionally, some patients are unwilling or unable to tolerate the anxiety associated with ERP, while others abandon or refuse the treatment altogether (17). Consequently, the treatment of OCD remains a challenging area in psychological therapy.

The ongoing search for more effective treatment methods continues to drive extensive and varied research in this field. Among the emerging therapeutic techniques is brain stimulation, which has gained attention in recent years as a safe and effective non-invasive intervention. In particular, transcranial direct current stimulation (tDCS) has shown promising results (18).

Harika-Germaneau et al. (19) studied the efficacy of 10 sessions of tDCS in treating treatment-resistant OCD. In this study, 80 treatment-resistant outpatients with OCD were randomly assigned to receive either active or sham tDCS. Patients were assessed at baseline, at the end of the treatment, and during one- and three-month follow-ups. A significant interaction between time and treatment was observed. However, the primary endpoint — change in Yale-Brown Obsessive Compulsive Scale (Y-BOCS) scores after two weeks—was not achieved. In contrast, the secondary endpoint, which measured changes in Y-BOCS scores after three months, was successfully met (19). These findings underscore the importance of studying the placebo effect in tDCS interventions and the necessity of long-term follow-up to evaluate the optimal effects of the treatment. Similarly, Brunelin et al. (20) concluded that despite methodological limitations and variability in stimulation parameters, tDCS appears to be a promising tool for alleviating OCD symptoms.

Obsessive-compulsive disorder is a complex disorder, recognized as the tenth leading cause of disability among patients. It is also one of the most prevalent mental health conditions in Iran, with an estimated prevalence ranging from 11.7% to 43.2% (21). Given that conventional psychological and pharmacological treatments do not consistently result in recovery and that a significant proportion of patients are resistant to these interventions, researchers have increasingly focused on novel treatment approaches. These include neurobiological methods that offer the potential for neurological adjustments (22).

Considering the limited research on the efficacy of such interventions, this study was designed to evaluate the effectiveness of tDCS in improving executive functions, reducing the severity of obsessive-compulsive symptoms, and addressing rumination in individuals with OCD symptoms.

The current research falls under the category of applied research in terms of its goal and experimental designs regarding its data collection method. This study was a randomized clinical trial with a pre-and post-test design, consisting of two groups. The statistical population included all individuals with obsessive-compulsive symptoms who visited the psychiatric clinic of Golestan Hospital and the Counseling Center of Jundishapur University of Medical Sciences in Ahvaz during 2022 - 2023. Thirty participants meeting the inclusion and exclusion criteria were randomly assigned to two groups. In this double-blind, controlled, randomized study, 30 patients with obsessive-compulsive symptoms (determined by a score higher than 11 on the Maudsley Questionnaire) were assigned to receive 10 sessions (one session per day) of either active tDCS (2 mA) or sham treatment, each lasting 20 minutes. Of these, 15 patients received active tDCS, and 15 received sham treatment.

3.2. Exclusion Criteria

Individuals were excluded if they had a history of psychotic disorders, serious head injuries, substance abuse, acute suicidal ideation, or implanted metal or devices in the brain. Pregnant women were also excluded. Additionally, patients previously exposed to tDCS could not participate. Other exclusion criteria included unavailability during the research period, severe physical illnesses such as cancer, the use of a cardiac pacemaker, severe organic brain disorders based on patient and family history or psychiatric examination, the presence of evident bipolar or psychotic symptoms, and absence from tDCS sessions.

After coordinating and obtaining the code of ethics from Ahvaz University (IR.AJUMS.HGOLESTAN.REC.1400.152) and the clinical trial code (IRCT20220220054069N1), the researcher entered the research environment with permission. The research was conducted at the Golestan Hospital Psychiatry Clinic and the Counseling Center of Ahvaz University of Medical Sciences. Before the preliminary evaluations, all conditions for participating in the research were explained to the applicants, allowing them to proceed with the evaluation process if they wished. Full explanations about the purpose of the project were provided to participants before obtaining consent and completing the questionnaire. After explaining the research, its conditions, and other ethical matters, participants were asked to complete the informed consent form.

Regarding sample selection, it should be noted that in experimental methods, each subgroup should consist of at least 15 people to ensure the sample is a true representative of the community and that the research maintains high external validity (23, 24). A total of 38 individuals were evaluated for eligibility. Eight people did not participate, leaving a total of 30 participants who completed the study. Of these, 15 patients received active tDCS, and 15 patients received sham treatment. In this randomized double-blind controlled study, 30 patients with obsessive-compulsive symptoms were assigned to receive 10 sessions (one session per day) of active tDCS (2 mA) or sham treatment. Pre-test evaluations were conducted, and after the treatment concluded, re-evaluations were performed.

It should be noted that no costs were imposed on the participants in this study. At the beginning of the research and after the intervention, all participants were assessed using the MOCI, the Executive Functions Questionnaire, the Y-BOCS, and the Mistake Rumination Scale. As shown in CONSORT Figure 1, the sample size was based on previous research indicating a high effect size according to Cohen's criteria for the studied treatments. With a test power of 0.80 and a significance level of 0.95, 15 participants were allocated to each group.

Figure 1.

Flow chart of the 2010 CONCORT process

Regarding sample selection, it is important to reiterate that in experimental methods, each subgroup should consist of at least 15 participants to ensure the sample is representative of the community and the research has high external validity (23, 24). To analyze the findings, descriptive statistics, analysis of covariance, independent t-tests, and chi-square tests were employed. All analyses were performed using SPSS-23 software.

Obsessive-compulsive disorder is a chronic and treatment-resistant neuropsychological disease that often develops during childhood and adolescence, leading to significant long-term problems in a person's life. This disorder involves uncontrollable thoughts that compel individuals to repeat specific actions, causing disruptions in daily functioning (32). Despite advances in patient management, approximately 30 - 60% of patients show only a partial response to current medications or experience no improvement (33). Increasing evidence suggests that the neurobiological substrates of OCD include abnormal activity and connectivity in the orbitofronto-striato-pallido-thalamus network, with heightened activity in the OFC, cingulate gyrus, and caudate (34). Consequently, this research aimed to evaluate the effectiveness of tDCS on executive functions, the severity of obsessive-compulsive symptoms, and rumination in individuals with obsessive-compulsive symptoms.

The results of our study comparing the two groups demonstrated that the mean post-test scores, compared to the pre-test scores, in the active tDCS group showed a greater reduction in obsessive-compulsive symptoms and rumination. Additionally, there was a greater improvement in executive functions in the active tDCS group compared to the sham treatment group, with a significant difference between the two groups. Fineberg et al. (32) reported that tDCS treatment improves cognitive control in individuals with OCD.

Asadollahzadeh Shamkhal et al. (35) indicated that tDCS induces changes in brain complexity in individuals with contamination-related OCD by influencing neuronal interactions and balancing neuronal activity. Bation et al. (18) showed that, despite significant acute effects, tDCS was not effective in achieving long-term symptom reduction in patients with treatment-resistant OCD. In another study (36), tDCS, with the anode placed on the right cerebellum and the cathode placed on the left OFC, was found to be a safe, favorable, and appropriate approach to reducing OCD symptoms in treatment-resistant patients.

Brunelin et al. (20) conducted a systematic study investigating the effects of tDCS on OCD reviewing 12 studies involving a total of 77 patients (20). The findings indicated that, despite methodological limitations and heterogeneity in stimulation parameters, tDCS appears to be a promising tool for reducing obsessive-compulsive symptoms, as well as co-occurring depression and anxiety, in patients with treatment-resistant OCD.

Transcranial direct current stimulation enables simultaneous stimulation of various regions and modulation of different brain areas involved in cortico-subcortical loops (37). It has been suggested that tDCS may effectively reduce symptoms in patients with treatment-resistant OCD (38), although the optimal target sites and stimulation parameters remain a topic of debate.

One possible explanation for its effectiveness is that tDCS helps regulate brain activity, leading to a reduction in obsessive-compulsive symptoms. Previous studies have shown that tDCS can improve symptoms by influencing the interactions and dynamics of brain neurons. It is concluded here that tDCS, by modulating brain neuron activity and bringing it closer to a normal state, causes changes in the complexity of brain activity in individuals with obsessive-compulsive symptoms.

In explaining these findings, it can be posited that individuals with OCD experience disturbances in executive functions. By stimulating the frontal lobe, tDCS improves executive functions in these individuals, thereby enhancing their cognitive performance. Furthermore, tDCS's ability to modulate neuroplasticity, specifically by increasing cortical excitability in the left dorsolateral prefrontal cortex (DLPFC) and decreasing it in the right DLPFC, contributes to the improvement of obsessive-compulsive symptoms.

One of the limitations of the present research is the sample size. Although comparable to other studies in this field, it can be considered relatively small, particularly given the heterogeneity in OCD. Further studies should focus on factors that can enhance the clinical efficacy of tDCS. First, inducing activity during the stimulation of targeted neural networks is crucial for achieving neurobiological and clinical effects, as tDCS primarily acts by enhancing neural plasticity (39) and learning (40). Using a greater number of sessions over a longer period may be necessary to achieve clinically significant outcomes. The optimal placement of electrodes in OCD remains a challenging issue. Neuroimaging studies can only infer correlation, not causation, which limits the understanding of specific stimulation targets.

Another limitation is the lack of access to a sufficient sample size to examine and compare stimulation protocols with multiple current intensities. Due to time constraints, it was not possible to include a follow-up phase. Additionally, the generalizability of the results is limited. Therefore, caution should be exercised when applying these findings to non-experimental conditions and other clinical groups.

Future research should be conducted with larger sample sizes and include follow-up procedures. The effectiveness of stimulation protocols using multiple current intensities and different electrode sizes should also be investigated. Furthermore, the protocol used in this study should be tested on other clinical groups and common psychiatric disorders. This study did not account for the effect of medications, which should be considered in future research. Follow-up periods should also be included to assess the long-term stability of tDCS effects.