Unravelling motor networks in patients with chronic disorders of consciousness: A promising minimally invasive approach

Behavioral responsiveness and awareness levels correlate with the degree of functional connectivity within cortical-thalamocortical networks, whose breakdown accounts for chronic disorders of consciousness (DOC). Our study was aimed at assessing the role of the primary motor area (M1) and premotor-M1 circuitry dysfunction in motor output deterioration in minimally conscious state (MCS) and unresponsive wakefulness syndrome (UWS) patients. As a control group, we included a healthy subject (HC) sample in the study. We evaluated the effects of different types of transcranial magnetic stimuli over M1 by recording post-stimulus time histogram (PSTH), which includes a series of peaks of unit firing activity that match with D and I-waves, characterizing the descending corticospinal volleys evoked by transcranial magnetic stimuli. As compared to HC, DOC patients showed a dysfunction of intra-M1 and premotor-M1 circuits, which correlated with the Coma Recovery Scale-Revised scorings. Nonetheless, one UWS patient showed a partially preserved premotor-M1 circuitry, paralleled by a severe intra-M1 circuitry dysfunction. Our data suggest that motor unresponsiveness in some DOC patients may be due to a pure motor output failure, as in the functional locked-in syndrome (fLIS), rather than to a premotor-motor connectivity impairment, which instead characterizes MCS and UWS.