Mohammad Ali Salehinejad ^1,2,3 • Vahid Nejati⁴ • Mohsen Mosayebi-Samani ^1,5 • Ali Mohammadi⁴ • Miles Wischnewski⁶ • Min-Fang Kuo¹ • Alessio Avenanti ^7,8 • Carmelo M. Vicario⁹ • Michael A. Nitsche ^1,10

¹ Department of Psychology and Neurosciences, Leibniz Research Centre for Working Environment and Human Factors, 44139 Dortmund, Germany

² International Graduate School of Neuroscience, RuhrUniversity Bochum, 44801 Bochum, Germany

³ Institute for Cognitive and Brain Sciences, Shahid Beheshti University, Tehran 1983963113, Iran

⁴ Department of Psychology, Shahid Beheshti University, Tehran 1983963113, Iran

⁵ Institute of Biomedical Engineering and Informatics, Ilmenau University of Technology, 98693 Ilmenau, Germany

⁶ Donders Institute for Brain, Cognition and Behaviour, Radboud University, 6525 HR Nijmegen, The Netherlands

⁷ Centro studi e ricerche in Neuroscienze Cognitive, Dipartimento di Psicologia, Alma Mater Studiorm, Universita` di Bologna, 47521 Cesena, Italy

⁸ Centro de Investigacio´n en Neuropsicologı´a y Neurociencias Cognitivas, Universidad Cato´lica del Maule, 3605 Talca, Chile

⁹ Dipartimento di Scienze Cognitive, Psicologiche, Pedagogiche e degli studi culturali, Universita` di Messina, 98121 Messina, Italy

¹⁰ Department of Neurology, University Medical Hospital Bergmannsheil, 44789 Bochum, Germany

Abstract

Transcranial direct current stimulation (tDCS) is a promising method for altering cortical excitability with clinical implications. It has been increasingly used in neurodevelopmental disorders, especially attention-deficit hyperactivity disorder (ADHD), but its efficacy (based on effect size calculations), safety, and stimulation parameters have not been systematically examined. In this systematic review, we aimed to (1) explore the effectiveness of tDCS on the clinical symptoms and neuropsychological deficits of ADHD patients, (2) evaluate the safety of tDCS application, especially in children with ADHD, (3) model the electrical field intensity in the target regions based on the commonly-applied and effective versus less-effective protocols, and (4) discuss and propose advanced tDCS parameters. Using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses approach, a literature search identified 14 empirical experiments investigating tDCS effects in ADHD. Partial improving effects of tDCS on cognitive deficits (response inhibition, working memory, attention, and cognitive flexibility) or clinical symptoms (e.g., impulsivity and inattention) are reported in 10 studies. No serious adverse effects are reported in 747 sessions of tDCS. The left and right dorsolateral prefrontal cortex are the regions most often targeted, and anodal tDCS the protocol most often applied. An intensity of 2 mA induced stronger electrical fields than 1 mA in adults with ADHD and was associated with significant behavioral changes. In ADHD children, however, the electrical field induced by 1 mA, which is likely larger than the electrical field induced by 1 mA in adults due to the smaller head size of children, was sufficient to result in significant behavioral change. Overall, tDCS seems to be a promising method for improving ADHD deficits. However, the clinical utility of tDCS in ADHD cannot yet be concluded and requires further systematic investigation in larger sample sizes. Cortical regions involved in ADHD pathophysiology, stimulation parameters (e.g. intensity, duration, polarity, and electrode size), and types of symptom/deficit are potential determinants of tDCS efficacy in ADHD. Developmental aspects of tDCS in childhood ADHD should be considered as well.

Keywords

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