tDCS has been used in experimental and clinical neuroscience for the study of brain functions and treatment in a range of disorders of the central nervous system. Of particular interest to this thematic series, a growing body of evidence suggest that tDCS has potential to become a clinical tool for motor rehabilitation.
The existing tDCS protocols using well-defined montages, stimulus durations and intensities are safe and well tolerated by both healthy individuals and clinical populations. There are no reported indications of any serious adverse effects, such as damage of brain tissue or seizure induction, with the use of 1–2 mA protocols [5, 6, 7]. The most commonly reported adverse effects included redness, tingling and itching sensations under the electrodes, as well as headache [6, 8]. Moreover, the overall adverse effect rates are similar between active and sham tDCS , which suggests that the mild adverse effects are related to electrode positioning on the skin and not the stimulation itself.
As tDCS is portable, devices can easily be transported, which circumvents accessibility barriers to health care (i.e. tDCS can easily be moved into clinics or wards). It can be implemented in combination with other kinds of interventions, such as cognitive or physical training or exercise, with this pairing possibly leading to synergistic benefit . Although accumulating evidence highlights potential benefits offered by tDCS for motor rehabilitation, further research is required for tDCS to become an approved clinical tool. The majority of existing clinical trials has involved a limited number of participants, which may imply underpowered analysis. Thus, large-scale studies are needed to overcome this major flaw.
Due to the potential for self- or caregiver-application, remotely supervised protocols have been developed and recently found feasible for those with motor impairment . However, these studies employ highly structured protocols and rigorous criteria with real time supervision via teleconference, and do not support a “do-it-yourself” tDCS practice. Instead, the remotely supervised protocols can be used to facilitate the clinical trial designs that are necessary in order to advance tDCS towards therapeutic use.
Data on optimal protocols and predictors of response to tDCS are currently lacking in the literature. Future studies in this field should focus on determining the optimal stimulation parameters and predictors of response to tDCS in different clinical populations. It seems that one size does not fit all in tDCS. However, previous studies may be limited, as standard clinical assessments may miss subtle motor improvements. Future outcomes for determining the effectiveness of tDCS for motor rehabilitation need to be robust. Therefore, combining tDCS protocols with other validated mobile technologies to monitor motor performance, such as wearable inertial sensors or innovative Internet of Things devices, may provide important insight into effectiveness within clinic and beyond.
Despite the positive progression of research to clinical practice, there are still questions to be answered before tDCS can be extensively recommended for motor rehabilitation.
• What is the ideal intensity and duration of the session?
• How many sessions are required?
• What is the ideal interval between sessions?
• What about patients’ characteristics?
• Who will benefit from tDCS?
• Do specific demographic characteristics lead to greater benefits?