Transcranial direct current stimulation (tDCS) is a low-intensity, noninvasive form of brain stimulation delivered by a small battery-powered portable machine. Conventionally, 2 disposable electrodes are positioned on the head, and a small current is passed between these electrodes to stimulate the brain “transcranially.” A typical session uses a low-intensity current of 1 to 2 mA (ECT by comparison is 800 mA), which is given continuously for about 30 minutes. Also in contrast to ECT, the tDCS current is continuous (not pulsed) and flows in one direction from the anode electrode to the cathode electrode (“direct current”).
Only a fraction of the current reaches the brain —it passes through the skin, skull, and cerebrospinal fluid —but the excitative qualities of the brain and its plasticity make it sensitive to even a low level of direct current stimulation. The injected electric current does not generate action potentials per se, but modulates ongoing brain activity and influences synaptic transmission, with effects on brain function that outlast the stimulation session. The potential to produce lasting changes in brain function with tDCS, especially after repeated sessions of stimulation, has encouraged clinical trials for the treatment of psychiatric and neurological disorders.
Initial tDCS protocols suggest that the anode and cathode electrodes can induce excitatory or inhibitory effects, respectively, in the underlying cortex. More recently, it was found that the direction of the effects also depends on other parameters, such as current intensity, session duration, brain state, and any concurrent pharmacology.1 Other distinct forms of low-intensity transcranial electric stimulation also exist, such as alternating, pulsed, or random noise (“tRNS”) electric stimulation. However, clinical research is currently most advanced with direct current.
The regulatory status of tDCS in the US is as an investigational device. In the European Union, Canada, Brazil, Australia, and Singapore, specific tDCS products have been approved for the treatment of various neuropsychiatric disorders.
Mild adverse effects during the stimulation session, such as skin redness, itching, and burning sensation are common, although these effects are generally well tolerated and limited to the duration of the sessions. There are case reports of tDCS-induced induction of manic and hypomanic episodes in patients with mood disorders. A recent meta-analysis found that the risk of manic switch is not statistically higher in patients receiving active (3.5%) compared with sham (0.5%) stimulation, although this analysis is limited by the low number of reports.
Currently, more than 50,000 sessions of tDCS have been used to treat more than 5000 participants in clinical studies. There have been no reports of serious adverse events (ie, hospitalization, seizures, cardiac arrest, death) associated with tDCS.1 Animal safety studies have shown that the clinical dose is over an order of magnitude lower than the minimum dose required to induce brain injury. Studies in humans showed that tDCS does not increase blood enolase (a marker of brain injury) levels. Several recent meta-analyses have concluded tDCS is well tolerated.2 The balance of evidence, therefore, shows that tDCS is a safe technique, within the parameters currently used.
Dr. Bikson is Professor and Ms. Unal is a graduate research assistant, Department of Biomedical Engineering, The City College of New York, NY. Dr. Brunoni is Professor of Psychiatry, Service of Interdisciplinary Neuromodulation, Laboratory of Neurosciences (LIM-27), Department and Institute of Psychiatry, University of São Paulo, São Paulo, Brazil. Dr. Loo is Professor of Psychiatry and Professorial Fellow, Black Dog Institute, University of New South Wales, Sydney, Australia.
Dr. Bikson reports that Soterix Medical Inc produces tDCS and high definition tDCS devices and he is the founder and has shares in the company. Ms. Unal and Dr. Brunoni report no conflicts of interest concerning the subject matter of this article. Dr. Loo reports that she has received equipment support from Soterix Medical Inc.
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