Review articleA systematic review of the clinical efficacy of transcranial direct current stimulation (tDCS) in psychiatric disorders
Introduction
Mental disorders constitute a major public health issue, directly accounting for 7.4% of disease burden worldwide (Murray et al., 2012) and 17.8% in the European Union (Wittchen et al., 2011). They are the leading cause of years lived with disability globally (Whiteford et al., 2013), impacting personal well-being, social relationships and work productivity, and are associated with substantial loss of quality of life (Alonso et al., 2004). Despite an increase in the rate of treatment, psychiatric morbidity has remained relatively stable over the past two decades (Kessler et al., 2005, Wittchen et al., 2011), thus there is a need to develop novel therapeutic strategies to improve clinical outcomes.
Recent advances in functional neuroimaging have facilitated an improved understanding of the disturbances in neural circuitry that underlie mental disorders (Frangou, 2014, Price and Drevets, 2013). Consequently, there has been increased interest in neuromodulation methods which can be used to selectively disrupt patterns of neural activity that are associated with symptoms of illness, with the objective of improving behavioural outcomes whilst generating information about disease mechanisms. These emerging brain-directed interventions adhere to an experimental therapeutics approach, which is now widely regarded as the gold-standard strategy for treatment-focused psychiatric research (Insel, 2014, Insel and Gogtay, 2014, Medical Research Council, 2010).
Transcranial direct current stimulation (tDCS) is a non-invasive neuromodulation technique which delivers low-amplitude direct currents to the brain via two surface sponge electrodes (anode and cathode) attached to distinct areas of the scalp with a rubber headband (Wagner et al., 2007). The current penetrates the skull and enters the brain from the anode, travels through the tissue, and exits via the cathode (George and Aston-Jones, 2010). tDCS presents several practical advantages over alternative neuromodulation modalities – it has a favourable safety-feasibility profile, offers a convincing placebo, can be applied bilaterally, and is portable and inexpensive.
During the past decade, tDCS has been implemented in numerous trials across a range of patient populations and psychiatric conditions, with a rapidly increasing number of studies being published each year (Fig. 1). This systematic review critically evaluates the clinical efficacy of tDCS in people with mental illness, and is warranted given the limited efficacy of existing therapies, the evidence that psychiatric disorders are neural circuit-based disorders that could benefit from brain-directed interventions, and the appealing characteristics of tDCS in comparison to other forms of neuromodulation. Although several reviews and meta-analyses have previously addressed this topic, the majority have either studied major depression (Berlim et al., 2013, Brunoni et al., 2012a, Kalu et al., 2012, Meron et al., 2015, Shiozawa et al., 2014b, Shiozawa et al., 2014d) or schizophrenia alone (Mondino et al., 2015a, Mondino et al., 2015b), or used unsystematic search procedures (Brunoni et al., 2012b, Kuo et al., 2014, Tortella et al., 2015) which promote a number of biases (Schmidt and Gotzsche, 2005). To our knowledge, one prior publication has systematically reviewed the therapeutic effects of tDCS across all psychiatric disorders (Mondino et al., 2014). Given the high growth rate of publication in the field, we have provided an up-to-date and comprehensive synthesis of the full evidence base, which is inclusive of all psychiatric conditions and study designs, and which uses a standardised quality assessment.
Section snippets
Selection criteria
Studies in English of any design that investigated the clinical efficacy of tDCS in individuals with psychiatric disorders were eligible for inclusion. Studies of participants with neurological conditions were excluded, as were those that did not report any symptom outcome variables. Publications were not restricted based on whether details of a Diagnostic and Statistical Manual of Mental Disorders/International Classification of Diseases diagnosis were given, and those involving
Characteristics of included studies
We identified 66 studies (reported in 67 publications, including data from 1021 participants) that met the inclusion criteria for this review (Fig. 2). The majority (30 studies) evaluated the efficacy of tDCS for the treatment of major depression in patients with major depressive disorder (MDD) or bipolar disorder (BP). The remaining studies were of patients with schizophrenia (23 studies), substance use disorders (SUDs; 7 studies), obsessive compulsive disorder (OCD; 4 studies), generalised
Clinical efficacy
This review provides evidence that tDCS has the potential to ameliorate symptoms associated with several major psychiatric disorders. Most notably, data from a number of RCTs suggest that tDCS interventions comprised of multiple sessions can induce enduring therapeutic effects in patients with depressive disorders and schizophrenia. Further indication of clinical utility in these conditions has come from numerous open-label trials and case reports, often involving patients who have experienced
Conclusions and future directions
Research into the clinical efficacy of tDCS in psychiatric disorders has grown exponentially over the past decade. We have systematically reviewed the literature and have provided an objective and analytical account of its current state. Overall, data from studies appraised in this review suggest that tDCS has the potential to induce clinically relevant behavioural changes in often difficult-to-treat patient populations, and could thus represent a valuable tool for intervention in a range of
Contributors
The authors were the only individuals who contributed to this publication.
Role of the funding source
This paper represents independent research part funded by the National Institute for Health Research (NIHR) Biomedical Research Centre at South London and Maudsley NHS Foundation Trust and King's College London. The views expressed are those of the authors and not necessarily those of the NHS, the NIHR or the Department of Health. Maria Kekic is supported by an Institute of Psychiatry, Psychology and Neuroscience/Medical Research Council Excellence Studentship.
Acknowledgements
None to declare.
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Present address: Department of Psychosomatic Medicine and Psychotherapy, Medical University Hospital Tübingen, Osianderstr. 5, 72076 Tübingen, Germany.