Fronto-parietal coherence response to tDCS modulation in patients with disorders of consciousness

A transcranial direct current stimulation (tDCS) protocol (20 min, 2 mA, anodal electrode at the left dorsolateral prefrontal cortex and cathodal electrode at the right supraorbital area) was applied in patients with different degrees of disorders of consciousness (DoC). Although previous research indicates that it could improve patients' coma recovery scale-revised (CRS-R) scores, the brain's electrophysiological responses to tDCS are still unclear. Therefore, the present study was performed to explore the underlying brain responses of patients in a minimally conscious state (MCS) and an unresponsive wakefulness syndrome (UWS) to tDCS modulation.

Seventeen patients with DoC were recruited in a sham controlled crossover study receiving real and sham tDCS. EEG coherence was used to measure functional connectivity changes induced by the tDCS modulation.

After real tDCS modulation, the fronto-parietal coherence significantly increased in the theta band and decreased in the gamma band in the MCS group. No significant changes were found in the UWS group. The coherence responses significantly correlated with the patients' baseline CRS-R scores. No distinct alteration occurred in the sham session for either the MCS or UWS patients.

The coherence responses to the present tDCS protocol may be a tool for diagnosing MCS versus UWS, as they may be a crucial cause of the different clinical effects in the two states.