Recent advance in the treatment of patients with disorders of consciousness: a review of transcranial direct current stimulation efficacy

Severe brain injured patients with disorders of consciousness (DOC) represent a challenging population to treat. Non-invasive brain stimulation techniques, such as transcranial direct-current stimulation (tDCS) can transiently improve the level of consciousness of DOC patients, as measured with the Coma Recovery Scale-Revised (CRS-R). However, it is still unclear which proportion of patients can benefit from it, how long the effects can last and which brain region represent the best area to target. We here reviewed tDCS-based clinical trials on DOC to shed light on these issues. Eight articles met our criteria: three articles evaluated the effects of a single stimulation session and 5 articles assessed the effects of repeated stimulation sessions. The areas targeted by the stimulation in the articles included the following regions: the left dorsolateral prefrontal cortex (left DLPFC – F3, 5 studies), the posterior parietal cortex / precuneus (Pz, one study), the primary motor cortex (M1 - C3 or C4, one study) and the frontoparietal network bilaterally (F3, F4, CP5 and CP6, one study). Left DLPFC was the most used target, both for single and repeated session studies and has showed to effectively increase patient’s responsiveness in the CRS-R compared to other stimulated regions. Repeated session studies generally show larger effect size of tDCS treatment and higher number of responders as compared to single session. Furthermore, we found that the number of responders of repeated session studies (39 out of 110 in total) is significantly higher (p=0.0125) than the number of responders of single session studies (22 out of 111 in total). From this retrospective exploration of tDCS clinical trials, it emerged that the left DLPFC seems to be the most powerful and promising target to improve behavioural responsiveness of DOC patients whereas the strength and duration of tDCS aftereffects seems to increase with the number of sessions.