Transcranial Direct Current Stimulation for Treatment of ADHD: A Review of the Mechanisms of Action

Background and Objective: Attention deficit hyperactivity disorder (ADHD) is a common neuropsychiatric disorder. The current pharmaceutical treatments are associated with side effects and with low efficacy so that about one third of the adults do not respond to these treatments. Transcranial direct current stimulation (tDCS) is a non-invasive and safe brain modulation technique with promising therapeutic effects on ADHD symptoms. This paper aims to comprehensively review the clinical trials of tDCS conducted in humans for the treatment of ADHD. We aim to review the clinical efficacy and mechanisms of action of the technique in ADHD treatment. Method: The databases of PubMed (1990-2017), Web of Sciences (1990-2017), Google Scholar (1990-2017) and Scopus (1990-2017) were searched using the keywords “Attention deficit hyperactivity disorder” OR “ADHD” AND “Transcranial direct current stimulation” OR “tDCS” AND "treatment" AND "mechanism". The title and abstract of the papers were reviewed by at least two authors and the relevant papers were selected for in depth review. We selected the preclinical and clinical trials that studied the effects of tDCS on ADHD patients through measuring behavioral, hemodynamic, or neurophysiological features. Results: The current evidence supports the therapeutic efficacy of tDCS in the improvement of ADHD, but, the findings are controversial. Anodal tDCS seems to be more effective than cathodal in ADHD. In addition, the appropriate sites of stimulation are frontal cortex in particular left dorsolateral prefrontal cortex and right inferior frontal gyrus. The tDCS can improve inhibitory control and interference control in ADHD patients. Conclusion: tDCS has shown promising but limited clinical efficacy for ADHD treatment. However, the current evidence supports continuing the preclinical and clinical studies to determine physiological mechanisms and dose response of tDCS in ADHD to establish a clinical protocol.