Could autonomic system assessment be helpful in disorders of consciousness diagnosis? A neurophysiological study

Although patients with chronic disorders of consciousness (DOC), including unresponsive wakefulness syndrome (UWS) and minimally conscious state (MCS), show a limited repertoire of awareness signs, owing to a large-scale cortico-thalamo-cortical functional disconnectivity, an activation of some cortical areas in response to relevant stimuli has been described by means of electrophysiological and functional neuroimaging approaches. In addition, cognitive processes associated with autonomic nervous system (ANS) responses elicited by nociceptive stimuli have been identified in some DOC patients. In an attempt to identify ANS functionality markers that could be useful in differentiating UWS and MCS individuals, we measured the amplitude, latency and γ-band power (γPOW) of ultra-late laser-evoked potentials (CLEPs) and skin reflex (SR), which both express some aspects of cognitive processes related to ANS functionality, besides other ANS parameters either during a 24(hh)-polygraphy or following a solid-state laser repetitive nociceptive stimulation. MCS showed physiological modification of vital signs (O2 saturation, hearth rate, hearth rate variability) throughout the night and a preservation of SR-γPOW, whereas UWS did not show significant variations. Following repetitive nociceptive stimulation, MCS patients had a significant increase in CLEP-γPOW, O2 saturation, hearth rate, and hearth rate variability, whereas UWS individuals did not show any significant change (but two patients, who reached high Coma Recovery Scale-Revised scores). Hence, our work suggests that a wide-spectrum electrophysiological evaluation of ANS functionality may support DOC differential diagnosis. Interestingly, the two above-mentioned UWS patients showed MCS-like vital sign modifications and electrophysiological pain responsiveness. It is therefore hypothesizable that our approach could be helpful in identifying residual aware autonomic system-related cognitive processes even in some UWS patients. Such issue draws the attention to either DOC clinical diagnosis or adequate pain treatment in DOC patients.