Contribution to the study of the links between consciousness and swallowing

Following severe brain injuries (e.g., traumatic or anoxic brain injury, stroke), a small proportion of patients will remain in an altered state of consciousness. Patients with prolonged (> 28 days post-insult) disorders of consciousness (DOC) can open their eyes (sometimes showing electrophysiological sleep/wake cycles) and the majority no longer need invasive ventilation. However, most of them receive artificial feeding, suggesting that consciousness affects swallowing capacities.
The aim of this thesis is to contribute to the study of the links between consciousness and swallowing. The hypotheses are that swallowing capacities are linked to level of consciousness, and that the presence of some components of swallowing constitutes a possible sign of consciousness.
Based on a literature review, we show that the sequencing of the components of swallowing falls on a continuum of voluntary to reflex behaviors. Components of the oral phase may be considered as voluntary behaviors because they are controllable and suppressible (although largely automated), components of the pharyngeal phase as somatic reflexes, and components of the esophageal phase as autonomic reflexes. The triggering of the swallowing reflex inhabits the border region between voluntary behaviors and somatic reflexes, while the opening of the upper esophageal sphincter divides somatic from autonomic reflexes. If voluntary behaviors are considered possible signs of consciousness, the presence of components of the oral phase of swallowing should be considered as revealing conscious behaviors. Moreover, we show that a range of cortical areas (mainly the primary sensorimotor cortex, premotor cortex and supplementary motor area, anterior part of the cingulate cortex, insula and cerebellum) are involved in both volitional and non-volitional swallowing tasks.
In two retrospective studies analyzing swallowing in patients with DOC diagnosed by means of repeated behavioral assessment and neuroimaging, we demonstrate that almost all such patients present at least one dysfunction in the oral and/or pharyngeal phase. Patients who do not show behavioral signs of consciousness (unresponsive wakefulness syndrome – UWS) do not present an efficient oral phase of swallowing allowing oral feeding with solid food. Consequently, the preservation of components of the oral phase of swallowing should be considered as a sign of consciousness and be one of the diagnostic criteria for consciousness. The absence of an efficient oral phase of swallowing in patients with UWS and its presence in only a small proportion of minimally conscious (MCS) patients explain why no “typical” patients with UWS (i.e., with behavioral and neuroimaging assessments pointing in the same direction) can be fed entirely orally while no patients with MCS can eat ordinary textured food.
Furthermore, in the studied group, level of consciousness is linked to components of the pharyngeal phase (reflected by the absence of a tracheostomy, pharyngo-laryngeal secretions or saliva aspiration) and to the cough reflex. Indeed, more patients with MCS than UWS present efficient spontaneous saliva management and a cough reflex, although these components are present in some patients with UWS. For that reason, these components seem to represent cortically mediated behavior but do not constitute signs of consciousness as such.
Finally, this work highlights the lack of appropriate tools to assess and treat swallowing for patients with DOC. A protocol study for the validation of a swallowing assessment tool for patients with DOC is therefore proposed.