What's the latest research on the safety of neurostimulation?
The word “safety” is defined by and limited to the absence of any kind of incident or situation of any serious and adverse side effect. This post is made upon an evidence-based approach with repeated experience of using tDCS on humans.
Computational models were used to compare dose to brain exposure in humans and animals. For meaningful standards of safety, dose response curve and dose metrics (current, current density, current duration, charge, and charge density) were reviewed. Special consideration was given to children and the elderly with mood disorders, epilepsy, implants, stroke etc.
With regards to the word “safety”, application of tDCS with all the protocols used to this date are safe, as reported in the review by NItsche:
“Extensive animal and human evidence and theoretical knowledge indicate that the currently used tDCS protocols are safe. However, knowledge about the safe limits of duration and intensity of tDCS is still limited. Thus, if charge or current density is exceeded greatly beyond the currently tested protocols, which might be desirable, for example, for clinical purposes, we suggest concurrent safety measures.”
tDCS has been tested over thousand times on subjects varying world-wide, with no evidence of any bad/adverse results. With respect to thousands of experiments been carried out to check the adverse effects of tDCS, some experiments have especially been carried out to check its safety.
Based on the combined consequences gathered from all the research and experiments with tDCS, we have only found out that tDCS is only mildly associated with temporary headache and erythema (for duration of 40 minutes) in the stimulation side. Other side effects that are very less probable to occur include nausea, visual phosphine, vertigo and difficulty in concentration.
More than 100 experiments have been carried out in healthy controls and patients’ population using tDCS. If any side effects were observed; they were slight itching under the electrode, fatigue and nausea.
Other than that:
- No neuronal damage was seen as assessed by serum neuron-specific enolase.
- No pathological waveforms were seen on EEG.
- No worsening of neuropsychological measures was observed after frontal lobe stimulation.
- No heating occurred under the electrode.
The most severe side effects found in healthy volunteer were skin lesions on the area where electrodes were placed using 2mA current. These lesions were however very rare and most probably occurred due to insufficient skin-electrode contact. The problem could be avoided by using sodium chloride solution and regularly changing the sponge and carefully inspecting the condition of skin placed under the electrode, both, before and after the tDCS.
With no reports of serious side effects of tDCS, some proposed warnings are still to be very strictly and carefully abide by. Some of them are:
- Stimulation sessions that last more than 40 minutes are for research purpose only.
- Currents above 0.06mA are for advanced clinical or research purpose only.
- Before using, always check if electrodes and strastism components are undamaged and clean.
- If sponge electrodes are being used, it is recommended to use sodium chloride solution, regularly changing the sponge and carefully checking the condition of the skin before and after the tDCS session.
- Electrode positions above cranial foramina and fissures should be avoided because these could increase the effective current density beyond safety limits.
To this date the use of tDCS has not been reported to have produced any adverse side effects or irreversible injury in human trials of over 33,200 sessions. This is said on the basis of a wide variety of subjects, which even includes people from potentially vulnerable populations. However, there are many safety recommendations with regard to the application of tDCS. When tDCS is given combined with EEG, conductive fluids between the electrodes must be prevented so that short circuiting is avoided. So, electrode gel, or vybuds’ dry electrodes, are preferable to saline solution.
A technical guide to tDCS. (2016, feb). Retrieved from Science Direct: https://www.sciencedirect.com/science/article/pii/S1388245715010883
Low intensity transcranial electric stimulation: Safety, ethical, legal. (n.d.). Retrieved from Clinical neurophysiology : http://www.cognitiveneuroscience.it/wp-content/uploads/2018/01/antal-et-al.pdf
Safety of Transcranial Direct Current Stimulation. (2016, oct). Retrieved from National Centre for Biotechnology Information: https://www.ncbi.nlm.nih.gov/pubmed/27372845
tDCS clinical research – Safety of transcranial Current Stimulation. (2015, april 24). Retrieved from Neuro Electrics: https://www.neuroelectrics.com/wiki/images/6/6a/NEWP201501-Safety_tCS.pdf
Transcranial direct current stimulation. (n.d.). Retrieved from Brain Stimulation: https://www.aipass.org/files/TDCS_State%20of%20the%20art.pdf