Transcranial Direct Current Stimulation for the Treatment of Depressive Disorders: A Review of Clinical Applications

Background and Objective: Preclinical and clinical studies have demonstrated promising therapeutic effects of transcranial direct current stimulation (tDCS) for depressive disorders (DDs). Although food and drug administration has not approved tDCS as a clinical treatment option for DDs, clinical outcomes of the initial studies indicate significant therapeutic efficacy of tDCS which has encouraged further studies to develop this technique as a treatment option for some types of DDs. The present study aims to comprehensively review the therapeutic applications of tDCS in DDs and clinical efficacies of the technique. Method: The databases of PubMed (1980-2018), Web of Sciences (1980-2018), Scopus (1980- 2018), CINAHL (1990-2018), PsycINFO (1990-2018), and Google Scholar (1980-2018) were searched. The searching keywords were "transcranial direct current stimulation" OR "tDCS" AND "depressive disorder" OR "DD" OR "depression" AND "treatment". The clinical trials that assessed the tDCS effects through a cognitive or neurophysiological measure were included in the study. Results: The current evidence shows tDCS, particularly in bifrontal anodal design to be effective in reducing depressive symptoms. In addition to the potential clinical utility and minimal adverseeffects of tDCS, it could improve different behavioral and cognitive functions in DDs patients. Furthermore, the interesting clinical importance of tDCS is its therapeutic efficacy for both treatmentresistant and non-treatment-resistant patients with mild to moderate depression. Conclusion: The clinical trials and metal analyses showed that tDCS is effective as first-line antidepressants. Further studies are needed to reach the exact dose-response profile of tDCS for DDs as well as to identify the mechanisms of actions of antidepressant effects of tDCS.