Abstract
The assessment of awareness in patients with chronic Disorders of Consciousness (DoC), including Unresponsive Wakefulness Syndrome (UWS) and Minimally Conscious State (MCS), is challenging. The level of awareness impairment may depend on the degree of deterioration of the large-scale cortical-thalamo-cortical networks induced by brain injury. Electrophysiological approaches may shed light on awareness presence in patients with DoC by estimating cortical functions related to the cortical-thalamo-cortical networks including, for example, the cortico-subcortical processes generating motor responses to the perturbation of the peri-personal space (PPS). We measured the amplitude, latency, and duration of the hand-blink reflex (HBR) responses by recording electromyography (EMG) signals from both the orbicularis oculi muscles while electrically stimulating the median nerve at the wrist. Such a BR is thought to be mediated by a neural circuit at the brainstem level. Despite its defensive-response nature, HBR can be modulated by the distance between the stimulated hand and the face. This suggests a functional top-down control of HBR as reflected by HBR features changes (latency, amplitude, and magnitude). We therefore estimated HBR responses in a sample of patients with DoC (8 MCS and 12 UWS, compared to 15 healthy controls −HC) while performing a motor task targeting the PPS. This consisted of passive movements in which the hand of the subject was positioned at different distances from the participant’s face. We aimed at demonstrating a residual top-down modulation of HBR properties, which could be useful to differentiate patients with DoC and, potentially, demonstrate awareness preservation. We found a decrease in latency, and an increase in duration and magnitude of HBR responses, which were all inversely related to the hand-to-face distance in HC and patients with MCS, but not in individuals with UWS. Our data suggest that only patients with MCS have preserved, residual, top-down modulation of the processes related to the PPS from higher-order cortical areas to sensory-motor integration network. Although the sample size was relatively small, being thus our data preliminary, HBR assessment seems a rapid, easy, and first-level tool to differentiate patients with MCS from those with UWS. We may also hypothesize that such a HBR modulation suggests awareness preservation.
Highlights
The severe impairment of the neural pathways subtending these processes accounts for different disorders of consciousness (DoC), including Unresponsive Wakefulness Syndrome (UWS) and Minimally Conscious State (MCS)
A possible reason of such misdiagnosis is that patients with UWS and MCS-minus may suffer from a cognitive-motor dissociation[13,14,15]
hand-blink reflex (HBR) responses are modulated by the hand-to-face distance in patients with MCS but not in those with UWS, in relation to their diagnosis based on the interpretation of the Coma Recovery Scale-Revised (CRS-R) scores
Summary
The severe impairment of the neural pathways subtending these processes accounts for different disorders of consciousness (DoC), including Unresponsive Wakefulness Syndrome (UWS) and Minimally Conscious State (MCS). A functional dissociation between these networks can be hypothesized in the DoC population This is in keeping with a possible preferential allocation of the post-injury available cognitive resources to control the subcortical areas that mediate the motor output, which are aimed at fostering defensive, reflexive behavioural responses[56]. In this way, a patient with DoC can be behaviourally unresponsive but covertly aware[57,58,59,60,61,62]. A patient with DoC may be unresponsive to the stimuli that fall within the extra-personal space when provided with the behavioural assessment (including the CRS-R), while being aware of the stimuli that trigger the PPS, despite a behavioural responsiveness that is limited to reflexive behaviours[63,64,65]
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