Articles

Why tDCS Is Safe

Transcranial direct current stimulation (tDCS) is a non-invasive way to stimulate nerve cells without making damage to the nerve tissue. Studies have shown that increase or decrease in stimulation of certain parts of the brain contributes to better management of wide range of neurological and psychiatric disorders (Benninger et al., 2010; Tortella et al., 2015). This technique appeared in 1960, and further development led to its practical application in clinical settings. Besides efficacy, one of the main advantages of tDCS is its safety. Here are some of the reasons why this technique beats other brain stimulation techniques in terms of safety issues.

Rare Adverse Effects
Patients treated with tDCS describe mild discomfort in the form of tingling, itching, and rarely burning sensations in the place of contact between electrodes and the skin. In some cases, redness of the skin can be noticed after the electrodes are removed. All these side effects are mild, and temporary. The most severe side effects noted were nausea, headache, and dizziness, but they were also temporary (Poreisz, Boros, Antal, & Paulus, 2007). They appeared only in cases of stimulation of specific regions close to vestibular organs (inner ear), which are responsible for balance and hearing (Fitzpatrick & Day, 2004).

Sophisticated Mechanism Of Action
Older techniques, including Transcranial Electrical Stimulation (TES) and Transcranial Magnetic Stimulation (TMS) act very aggressively on neurons by changing electrical charge of their membranes and inducing rapid depolarization (firing series of impulses). In contrast, tDCS delivers the current that is not strong enough to produce such effect, but it acts by modulating neural activity. This means that, while conventional methods need to be conducted under general anesthesia, tDCS does not put the patient through that risk.

Tissue Damage
Standard methods of brain stimulation with electrical current, such as TES produce damage to nerve tissue due to overstimulation of neurons(Agnew & McCreery, 1987). On the other hand, tDCS is unable to produce such effect as it delivers the current of much lower intensity, even within normal, physiologic range(Bindman, Lippold, & Redfearn, 1964).

Recommended Safety Limits for tDCS
Transcranial DCS is generally considered safe, so there are not many studies dealing with limits of its use. Existing evidences based on research which included over 2000 patients in different laboratories across the world have shown no serious side effects of tDCS. Time limit for tDCS is set to maximum 13 minutes. Studies have shown that during this short period of action electrodes are not heated(Nitsche & Paulus, 2000), markers of neural damage are not increased(Nitsche & Paulus, 2001), and there are no changes in EEG, MRI, and cognitive functions(Iyer et al., 2005). In other words, tDCS stimulation of up to 13 minutes is completely safe according to all parameters. However, even longer stimulation periods of up to 50 minutes resulted with no side effects in several studies.

Contraindications
Although it is considered safe, tDCS can potentially worsen some medical conditions. These cases include (eczema) acute irritation after placing the electrodes. No type of brain stimulation is recommended in patients with existing epilepsy.

Being a relatively new technique, tDCS is considered safe but applied with caution and its application is never pushed to the limits, despite its obvious advantage over standard techniques in terms of safety. In the future, with more research, the duration of stimulation and intensity of electrical current in tDCS are likely to gradually increase.

References
Agnew, W. F., & McCreery, D. B. (1987). Considerations for safety in the use of extracranial stimulation for motor evoked potentials. Neurosurgery, 20(1), 143-147.

Benninger, D. H., Lomarev, M., Lopez, G., Wassermann, E. M., Li, X., Considine, E., & Hallett, M. (2010). Transcranial direct current stimulation for the treatment of Parkinson’s disease. J Neurol Neurosurg Psychiatry, 81(10), 1105-1111. doi: 10.1136/jnnp.2009.202556

Bindman, L. J., Lippold, O. C., & Redfearn, J. W. (1964). The Action of Brief Polarizing Currents on the Cerebral Cortex of the Rat (1) during Current Flow and (2) in the Production of Long-Lasting after-Effects. J Physiol, 172, 369-382.

Fitzpatrick, R. C., & Day, B. L. (2004). Probing the human vestibular system with galvanic stimulation. J Appl Physiol (1985), 96(6), 2301-2316. doi: 10.1152/japplphysiol.00008.2004

Iyer, M. B., Mattu, U., Grafman, J., Lomarev, M., Sato, S., & Wassermann, E. M. (2005). Safety and cognitive effect of frontal DC brain polarization in healthy individuals. Neurology, 64(5), 872-875. doi: 10.1212/01.WNL.0000152986.07469.E9

Nitsche, M. A., & Paulus, W. (2000). Excitability changes induced in the human motor cortex by weak transcranial direct current stimulation. J Physiol, 527 Pt 3, 633-639.

Nitsche, M. A., & Paulus, W. (2001). Sustained excitability elevations induced by transcranial DC motor cortex stimulation in humans. Neurology, 57(10), 1899-1901.

Poreisz, C., Boros, K., Antal, A., & Paulus, W. (2007). Safety aspects of transcranial direct current stimulation concerning healthy subjects and patients. Brain Res Bull, 72(4-6), 208-214. doi: 10.1016/j.brainresbull.2007.01.004

Tortella, G., Casati, R., Aparicio, L. V., Mantovani, A., Senco, N., D’Urso, G., . . . Brunoni, A. R. (2015). Transcranial direct current stimulation in psychiatric disorders. World J Psychiatry, 5(1), 88-102. doi: 10.5498/wjp.v5.i1.88

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