Transcranial direct current stimulation (tDCS): a promising technique to enhance behavioral training?

Abstract

Transcranial direct current stimulation (tDCS) has gained increased attention in recent years due to its core characteristics: a simple and effective method to non-invasively modulate cortical excitability. This technique uses a weak direct current to influence the level of excitability in the human brain. Its effects on neuronal spontaneous activity associated with longlasting neuroplastic effects of some protocols make tDCS an attractive technique to be used with physical therapy interventions to enhance functional gains. The recent article published by Schabrun in this issue of Physical Therapy Reviews provides an overview of functional applications of tDCS with respect to different diseases. In this review, the author explores the use of tDCS to prime brain activity for a subsequent behavioral intervention (i.e. motor training) and also the use of tDCS alone in stroke, Parkinson’s disease and chronic pain. Schabrun discusses how traditional techniques can be combined with tDCS to enhance plastic changes. An important issue for combining tDCS with interventions used in physical therapy is to assess the safety of tDCS. The main risk of tDCS is tissue burn due to the physical property of the electrical current that leads to an increase in temperature caused by both Joule heat and metabolic changes under the electrodes during stimulation. Although tissue heating is observed in any type of stimulation, it might be worse in direct current stimulation as there is a reduced amount of time for the tissue to buffer a temperature increase. Heating can be a significant problem when there is poor contact between the scalp and electrode or the existence of previous skin lesions. It is also important to consider the correct type of electrode. The electrodes for tDCS need to be made of rubber and the sponges need to be soaked in saline solution to reduce potential toxic products and heating effects. Although there is a risk associated with the use of tDCS, a recent elegant animal study has shown that the current used in human studies is at least 100 times smaller than the current capable of induce heating lesions. In fact, the current density used with conventional electrodes does not increase scalp temperature as demonstrated by recent modeling studies. The investigator should carefully address current density when planning a tDCS study, especially if different electrode montages are used. Further discussion is needed for the combination of tDCS to enhance motor recovery in stroke and traumatic brain injury when there is a presence of skull defects. We recently showed that skull defects or skull plates can change the peak of current induced in the brain tissue, increasing the risk of heating, or even change the current flow and activate different areas than expected. Induced current can increase three times especially if the skull defect is small and under the area of electrode. Schabrun concludes that ‘promising findings suggest that tDCS may be a useful stand alone treatment, or a useful complement to other physical therapy treatments’. One important concern is that tDCS uses a very simple and inexpensive machine; thus, the easy accessibility of this technique can lead to incorrect (or self-) administration of protocols. It is important therefore that research personnel are appropriately trained and patients are educated regarding potential risks of this technique as to discourage potential unsupervised home use. An important issue is that although tDCS is not a technique with a high level of focality (targeting), as the current is dispersed in lower resistance tissues the researcher should not undervalue the importance of the electrode montage as significant differences are observed only by changing, for instance, the reference electrode in a bilateral cranial montage. Modeling studies, using computerized head models, attempt to overcome uncertainties behind the applicability of tDCS. These studies can comprise aspects of optimal electrode montage, size, and current density to achieve an expected result. However, results need to be clinically confirmed. Correspondence to: Felipe Fregni, Laboratory of Neuromodulation, Spaulding Rehabilitation Hospital, 125 Nashua Street #727, Boston, MA 02114, USA. Email: ffregni@bidmc.harvard.edu Commentaries