- No serious adverse effects have been reported in experiments using either tDCS or tACS.
- Persistent adverse effects of tDCS are mainly skin problems; for tACS, none have been reported.
- Further safety investigations are needed.
Transcranial direct current stimulation (tDCS) and transcranial alternating current stimulation (tACS) have been applied to many research issues because these stimulation techniques can modulate neural activity in the human brain painlessly and non-invasively with weak electrical currents. However, there are no formal safety guidelines for the selection of stimulus parameters in either tDCS or tACS. As a means of gathering the information that is needed to produce safety guidelines, in this article, we summarize the adverse events of tDCS and tACS. In both stimulation techniques, most adverse effects are mild and disappear soon after stimulation. Nevertheless, several papers have reported that, in tDCS, some adverse events persist even after stimulation. The persistent events consist of skin lesions similar to burns, which can arise even in healthy subjects, and mania or hypomania in patients with depression. Recently, one paper reported a pediatric patient presenting with seizure after tDCS, although the causal relationship between stimulation and seizure is not clear. As this seizure is the only serious adverse events yet reported in connection with tDCS, tDCS is considered safe. In tACS, meanwhile, no persistent adverse events have been reported, but considerably fewer reports are available on the safety of tACS than on the safety of tDCS. Therefore, to establish the safety of tDCS and tACS, we need to scan the literature continuously for information on the adverse events of both stimulation techniques. Further safety investigations are also required.
Since the first reports of transcranial direct current stimulation (tDCS) by Priori et al. (1998)) and Nitsche and Paulus, 2000, Nitsche and Paulus, 2001), tDCS has been applied to many research issues because it can modulate the neural networks in the human brain painlessly and non-invasively (Priori et al., 1998, Nitsche and Paulus, 2000, Nitsche and Paulus, 2001). In other words, tDCS can induce neural plasticity (Ugawa, 2012). Most of its adverse effects are mild and disappear soon after stimulation, but several papers have reported that some adverse effects, most commonly skin problems, can persist even after stimulation. Recently, since the invention of transcranial alternating current stimulation (tACS) by Antal et al. (2008)), tACS has also been applied in research for the modulation of neural activity through the entrainment on brain oscillations (Antal et al., 2008, Antal and Herrmann, 2016). As in tDCS, the adverse effects of tACS are mild and disappear just after stimulation. Yet there have been far fewer papers on safety issues or adverse events of tACS as compared to tDCS. To date, there are no formal safety guidelines for the selection of stimulus parameters in either tDCS or tACS (Fertonani et al., 2015). Therefore, we aim to summarize the adverse events of tDCS and tACS in this review. At present, the safety and ethical issues of both stimulation techniques should be considered by each institution due to the lack of certainty about their risks. This review may provide some useful information for these considerations. In addition, this review is expected to be useful for the establishment of safety guidelines in the near future.