Date Published: April 12, 2019
Publisher: Public Library of Science
Author(s): Mohammad Ali Salehinejad, Miles Wischnewski, Vahid Nejati, Carmelo M. Vicario, Michael A. Nitsche, Alessio Avenanti.
Transcranial direct current stimulation (tDCS) is a promising method for altering cortical excitability with clinical implications in neuropsychiatric diseases. Its application in neurodevelopmental disorders especially attention-deficit hyperactivity disorder (ADHD), is in early stage and promising but its effectiveness has not been systematically examined yet. We conducted a meta-analysis on the effectiveness of tDCS on the most studied neuropsychological symptoms of ADHD, which is the first reported meta-analysis of tDCS studies on ADHD. Data from 10 randomized controlled studies (including 11 separate experiments) targeting inhibitory control, and/or working memory (WM) in ADHD were included. Results show that overall tDCS significantly improved inhibitory control. Sub-analyses further show that dorsolateral prefrontal cortex (dlPFC) (but not right inferior frontal gyrus) tDCS and anodal (but not cathodal) tDCS significantly improved inhibitory control with a small effect size. Anodal dlPFC-tDCS had the largest significant effect on inhibitory control with a small-to-medium effect size. Additionally, a significant improving effect of tDCS on inhibitory control accuracy (but not response time) and WM speed (but not accuracy) were found. Overall, this meta-analysis supports a beneficial effect of tDCS on inhibitory control and WM in ADHD with a small-to-medium effect size. TDCS seems to be a promising method for improving neuropsychological and cognitive deficits in ADHD. However, there might be a dissociation between neuropsychological deficits and clinical symptoms of ADHD and therefore, the significance of this meta-analysis for clinical purposes is limited. Future studies should systematically evaluate the role of inter-individual factors (i.e., ADHD subtype, types of the deficit) and stimulation parameters (i.e., site, polarity, intensity, duration, repetition rate) on tDCS efficacy in ADHD population and examine whether benefits are long-term.
Attention-deficit hyperactivity disorder (ADHD) is the most common neurodevelopmental disorder with an estimate of 11% prevalence in school-age children . ADHD is primarily characterized by symptoms of inattention, hyperactivity, impulsivity  and various cognitive dysfunctions  that often persist into adolescence and adulthood. Apart from symptoms of hyperactivity and inattentiveness, a wide range of cognitive deficits is observed in individuals with ADHD such as problems in attention, inhibitory control working memory (WM), planning, problem-solving and executive functions [4–7]. Executive dysfunctions, especially inhibitory control and working memory, are pervasive and influential in ADHD pathophysiology to the extent that ADHD was labeled a disorder of cognitive control [7, 8] but, cognitive deficits in ADHD are heterogeneous [9–11].
Our meta-analysis follows the guidelines of the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA)  which consists of a checklist intended to facilitate preparation and reporting review/meta-analysis studies by identifying, selecting, and critically appraising relevant research, and collecting and analyzing data from the included studies. A brief version of the PRISMA checklist is available as supporting information (See S1 Table). The following steps are based on the PRISMA protocol.
ADHD is a major neurodevelopmental disorder with remarkable heterogeneity in symptoms, etiologies, and treatment response. Recent studies have emphasized on executive dysfunctions and brain functional abnormalities underlying ADHD pathophysiology [14, 20, 54, 57] suggesting that modulation of cortical activity in the involved brain regions via tDCS might ameliorate neuropsychological symptoms . Exploring the impact of tDCS especially on ADHD neuropsychological deficits has significantly increased in the last few years. However, no meta-analysis about tDCS efficacy in ADHD is available. We conducted a meta-analysis of randomized, placebo-or-baseline-controlled trials of tDCS application in ADHD, which is the first meta-analysis for ADHD patients.