So far, the focus of therapeutic options for patients with DoCs has been to rewire consciousness-related networks. However, it is now admitted that a substantial proportion (15–20%) of patients with DoC may present covert awareness detected by neuroimaging or electrophysiological measures6. Several factors, such as motor disorders, language impairment, vigilance fluctuation, possible depression or akinetic mutism, could prevent patients from overtly expressing signs of consciousness. The outcome measures that are currently used in RCTs largely rely on overt behaviours, which might be suboptimal to detect infraclinical changes in patients with covert awareness.

tDCS ranks among the most commonly used NIBS techniques in the DoC literature, with the left dorsolateral prefrontal cortex (DLPFC) often being the target of choice. DLPFC-tDCS studies report reproducible efficacy in terms of behavioural improvements, as assessed with the Coma Recovery Scale — Revised1,7. Nonetheless, the median improvement in these scores might be considered trivial. Assessment of clinically meaningful difference in the field of DoC has recently been proposed8 but does not apply to patients with covert awareness.

In the current literature, most patients who responded to tDCS were minimally conscious, and tDCS failed to initiate signs of consciousness in unresponsive patients7. Knowing that a non-negligible proportion of unresponsive patients can have covert awareness, current tDCS protocols seem to be ineffective in terms of allowing patients to purposefully demonstrate behavioural signs of consciousness, highlighting the need to develop adapted and optimized protocols to increase behavioural output. In this context, rather than enhancing recovery of consciousness, tDCS might prove useful for refining a patient’s diagnosis.

“the use of neuromodulation to stimulate motor pathways could improve behavioural responsiveness”

A potential area of interest that seems to have been neglected in current therapeutic strategies for people with DoC is the motor system — only a few studies with small sample sizes have attempted to target this system. Theoretically, the use of neuromodulation to stimulate motor pathways could improve behavioural responsiveness, thereby enabling patients to demonstrate signs of consciousness rather than increasing their levels of consciousness per se.

To complement tCS, central approaches, such as low-intensity focused ultrasound pulsation (LIFUP) could be used to stimulate subcortical regions in a non-invasive manner9. The combination of cortical approaches (such as tCS) and central approaches (such as LIFUP) could provide a holistic strategy to promote recovery of both motor and consciousness-related networks.