Electrical stimulation techniques have gained an important place in the treatment of various medical conditions related to the central and peripheral nervous system. They use different approaches but are all based on delivering electrical current to specific parts of the brain in order to either increase or decrease the activity of local neural cells. During the past decades, highly invasive methods, such as Electroconvulsive therapy (ECT) were used (Hoffman, 1984). Modern medicine has a tendency towards less invasive and highly efficient techniques, such as Transcranial Direct Current Stimulation (tDCS).
What is tDCS?
Unlike ECT, which uses high intensity electrical currents and requires general anesthesia, tDCS is a technique of using low-intensity electrical currents applied to the particular regions of the brain (Medeiros et al., 2012). The current is applied through the surface of the scalp using different types of electrodes. This technique has very few side effects and the anesthesia is not required due to low currents which are not causing convulsions or any other unpleasant sensations, except in some cases mild itching in the place of the electrodes. Transcranial DCS has shown to be powerful in the treatment of depression, Parkinson’s disease, chronic pain, tinnitus, and other psychical and neurological disorders (Berlim, Van den Eynde, & Daskalakis, 2013; O’Neill, Sacco, & Nurmikko, 2015). Furthermore, it improved motor performance and cognitive functions in patients with stroke and cognitive impairment.
Different Types of tDCS Montages
In order to deliver the current to the brain tissue, through the scalp, different types of electrodes are used. They are optimized to provide good contact with the skin. Additional materials are used with the aim to increase conductivity and decrease irritation of the skin. Metal and rubber electrodes are most commonly used. They are usually surrounded by sponge pockets, and the sponges are saturated with electrolyte-rich fluid which is very important for reaching optimal conductivity (DaSilva, Volz, Bikson, & Fregni, 2011). Alternative way is to use rubber electrodes without sponge, but then conductive gel must be applied to the skin in order to increase conductivity.
Sponges with Saline vs. Rubber Electrodes
Electrodes incorporated in sponge pockets are more frequently used than rubber electrodes with gel and self-adhesive electrodes (Minhas, Datta, & Bikson, 2011). There are several reasons for promoting the use of this type of montage with sponges. Firstly, tDCS is a relatively novel technique, and much more research has been done using sponge with saline than other methods. As a result of more studies, there are precise recommendations regarding the appropriate shape of the sponges, amount of saline, and concentration of electrolytes for the treatment of specific conditions (Minhas et al., 2011).
Hair is one of the obstacles for current conductivity. It has been shown that saline solution provides much better contact between electrodes and the skin than gel. The reason for this is high viscosity of the gel, which does not allow optimal contact, especially on regions covered with hair (Horvath, Carter, & Forte, 2014).
Another property of the skin which is important for its conductivity is hydration. While saline-soaked sponges provide adequate hydration, conductive gel and adhesive electrodes put the skin at risk of becoming dry (DaSilva et al., 2011). Dry skin has significantly lower conductivity than wet skin (Grimnes, 1983). In addition, side effects, such as redness of the skin, itching, and even pain in the place of contact of poorly hydrated skin and the electrodes are very common.
In summary, electrodes incorporated into sponge pockets with saline represent well established approach with high effectiveness and low incidence of side effects. Rubber electrodes with conductive gel and self-adhesive electrodes are considered alternative approaches used mostly to reduce cost, but they increase the risk of side effects. Also, the efficacy of these alternative methods has not yet been well documented.
Berlim, M. T., Van den Eynde, F., & Daskalakis, Z. J. (2013). Clinical utility of transcranial direct current stimulation (tDCS) for treating major depression: a systematic review and meta-analysis of randomized, double-blind and sham-controlled trials. J Psychiatr Res, 47(1), 1-7. doi: 10.1016/j.jpsychires.2012.09.025
DaSilva, A. F., Volz, M. S., Bikson, M., & Fregni, F. (2011). Electrode positioning and montage in transcranial direct current stimulation. J Vis Exp(51). doi: 10.3791/2744
Grimnes, S. (1983). Skin impedance and electro-osmosis in the human epidermis. Med Biol Eng Comput, 21(6), 739-749.
Hoffman, B. F. (1984). Electroconvulsive therapy–a current view. Can Med Assoc J, 130(9), 1123-1124.
Horvath, J. C., Carter, O., & Forte, J. D. (2014). Transcranial direct current stimulation: five important issues we aren’t discussing (but probably should be). Front Syst Neurosci, 8, 2. doi: 10.3389/fnsys.2014.00002
Medeiros, L. F., de Souza, I. C., Vidor, L. P., de Souza, A., Deitos, A., Volz, M. S., . . . Torres, I. L. (2012). Neurobiological effects of transcranial direct current stimulation: a review. Front Psychiatry, 3, 110. doi: 10.3389/fpsyt.2012.00110
Minhas, P., Datta, A., & Bikson, M. (2011). Cutaneous perception during tDCS: role of electrode shape and sponge salinity. Clin Neurophysiol, 122(4), 637-638. doi: 10.1016/j.clinph.2010.09.023
O’Neill, F., Sacco, P., & Nurmikko, T. (2015). Evaluation of a home-based transcranial direct current stimulation (tDCS) treatment device for chronic pain: study protocol for a randomised controlled trial. Trials, 16(1), 186. doi: 10.1186/s13063-015-0710-5
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