Transcranial direct current stimulation (tDCS) is a method of non-invasive brain stimulation that involves the application of low-intensity electrical current (1-2 mA) to the head, via surface electrodes placed on the scalp in an area of cortical interest (James et al. 2018). In contrast to transcutaneous magnetic stimulation (TMS), tDCS modulates the resting membrane potentials of neurons rather than inducing action potentials to increase cortical excitability (James et al. 2018). To further enhance the electrical activity of neurons and promote activity-dependent neuroplasticity, tDCS may be paired with motor training (Siraman et al. 2014). In healthy individuals, tDCS is considered safe and efficacious as it is associated with bimanual coordination (Gomes-Osman et al. 2013). Moreover, its affordability and clinical accessibility make it an ideal treatment option for patients with SCI. Despite this, few studies have investigated the application of tDCS in SCI patients. The methodological details and results of these studies are presented in Table 17.
Numerous studies have investigated the effects of tDCS on cortical excitability in healthy subjects. However, the relationship of physiological changes due to tDCS stimulation in individuals with SCI remains unclear. As such, two studies recently investigated the effects of tDCS in chronic SCI patients for rehabilitation of upper extremity motor function.
In one RCT, Cortes and colleagues investigated the effects of one session of 1 mA, 2 mA, and sham anodal tDCS on upper extremity motor performance (hand grasp and release) in patients with chronic cervical SCI. Although clinical assessment of hand function using the box and blocks test showed no difference between groups, a significant improvement in hand grasp was observed in the 2 mA group. This suggests that a single session of 2 mA tDCS may improve hand motor function, although future studies are necessary to determine whether tDCS may be an effective long-term rehabilitation strategy. Correspondingly, Potter-Baker et al. investigated the effects of pairing tDCS with massed practice rehabilitation training for several sessions over two weeks. Significant improvements in muscle strength were observed in weak proximal, wrist, and hand muscles; however, no difference was observed between groups in clinical assessments. The lack of statistical significance may be due to the relatively small sample size. Despite this, the relative ease of integrating tDCS into routine clinical training for upper extremity rehabilitation in SCI patients, and the associated improvements in hand grasp/muscle strength, suggest further research is warranted. Future clinical trials should evaluate the efficacy of multiple tDCS sessions, as well as robotic-assisted training combined with tDCS.
There is level 1b evidence (from one RCT: Cortes et al. 2017) that a single session of tDCS significantly improves hand grasp in patients with chronic SCI, however, larger clinical trials are necessary to determine the effectiveness of tDCS as a long-term rehabilitation strategy.
There is level 2 evidence (from one cohort study: Potter-Baker et al. 2018) that tDCS paired with massed practice training may provide some advantage in improving the strength of proximal/hand muscles, however, larger clinical trials are necessary.