Concept: Minimally conscious state
In W v M, family members made an application to the Court of Protection for withdrawal of artificial nutrition and hydration from a minimally conscious patient. Subsequent scholarly discussion has centred around the ethical adequacy of the judge’s decision not to authorise withdrawal. This article brings a different perspective by drawing on interviews with 51 individuals with a relative who is (or was) in a vegetative or minimally conscious state (MCS). Most professional medical ethicists have treated the issue as one of life versus death; by contrast, families-including those who believed that their relative would not have wanted to be kept alive-focused on the manner of the proposed death and were often horrified at the idea of causing death by ‘starvation and dehydration’. The practical consequence of this can be that people in permanent vegetative state (PVS) and MCS are being administered life-prolonging treatments long after their families have come to believe that the patient would rather be dead. We suggest that medical ethicists concerned about the rights of people in PVS/MCS need to take this empirical data into account in seeking to apply ethical theories to medico-legal realities.
One challenging aspect of the clinical assessment of brain-injured, unresponsive patients is the lack of an objective measure of consciousness that is independent of the subject’s ability to interact with the external environment. Theoretical considerations suggest that consciousness depends on the brain’s ability to support complex activity patterns that are, at once, distributed among interacting cortical areas (integrated) and differentiated in space and time (information-rich). We introduce and test a theory-driven index of the level of consciousness called the perturbational complexity index (PCI). PCI is calculated by (i) perturbing the cortex with transcranial magnetic stimulation (TMS) to engage distributed interactions in the brain (integration) and (ii) compressing the spatiotemporal pattern of these electrocortical responses to measure their algorithmic complexity (information). We test PCI on a large data set of TMS-evoked potentials recorded in healthy subjects during wakefulness, dreaming, nonrapid eye movement sleep, and different levels of sedation induced by anesthetic agents (midazolam, xenon, and propofol), as well as in patients who had emerged from coma (vegetative state, minimally conscious state, and locked-in syndrome). PCI reliably discriminated the level of consciousness in single individuals during wakefulness, sleep, and anesthesia, as well as in patients who had emerged from coma and recovered a minimal level of consciousness. PCI can potentially be used for objective determination of the level of consciousness at the bedside.
Bedside clinical examinations can have high rates of misdiagnosis of unresponsive wakefulness syndrome (vegetative state) or minimally conscious state. The diagnostic and prognostic usefulness of neuroimaging-based approaches has not been established in a clinical setting. We did a validation study of two neuroimaging-based diagnostic methods: PET imaging and functional MRI (fMRI).
Durable impairments of consciousness are currently classified in three main neurological categories: comatose state, vegetative state (also recently coined unresponsive wakefulness syndrome) and minimally conscious state. While the introduction of minimally conscious state, in 2002, was a major progress to help clinicians recognize complex non-reflexive behaviours in the absence of functional communication, it raises several problems. The most important issue related to minimally conscious state lies in its criteria: while behavioural definition of minimally conscious state lacks any direct evidence of patient’s conscious content or conscious state, it includes the adjective ‘conscious’. I discuss this major problem in this review and propose a novel interpretation of minimally conscious state: its criteria do not inform us about the potential residual consciousness of patients, but they do inform us with certainty about the presence of a cortically mediated state. Based on this constructive criticism review, I suggest three proposals aiming at improving the way we describe the subjective and cognitive state of non-communicating patients. In particular, I present a tentative new classification of impairments of consciousness that combines behavioural evidence with functional brain imaging data, in order to probe directly and univocally residual conscious processes.
Despite advances in resting state functional magnetic resonance imaging investigations, clinicians remain with the challenge of how to implement this paradigm on an individualized basis. Here, we assessed the clinical relevance of resting state functional magnetic resonance imaging acquisitions in patients with disorders of consciousness by means of a systems-level approach. Three clinical centres collected data from 73 patients in minimally conscious state, vegetative state/unresponsive wakefulness syndrome and coma. The main analysis was performed on the data set coming from one centre (Liège) including 51 patients (26 minimally conscious state, 19 vegetative state/unresponsive wakefulness syndrome, six coma; 15 females; mean age 49 ± 18 years, range 11-87; 16 traumatic, 32 non-traumatic of which 13 anoxic, three mixed; 35 patients assessed >1 month post-insult) for whom the clinical diagnosis with the Coma Recovery Scale-Revised was congruent with positron emission tomography scanning. Group-level functional connectivity was investigated for the default mode, frontoparietal, salience, auditory, sensorimotor and visual networks using a multiple-seed correlation approach. Between-group inferential statistics and machine learning were used to identify each network’s capacity to discriminate between patients in minimally conscious state and vegetative state/unresponsive wakefulness syndrome. Data collected from 22 patients scanned in two other centres (Salzburg: 10 minimally conscious state, five vegetative state/unresponsive wakefulness syndrome; New York: five minimally conscious state, one vegetative state/unresponsive wakefulness syndrome, one emerged from minimally conscious state) were used to validate the classification with the selected features. Coma Recovery Scale-Revised total scores correlated with key regions of each network reflecting their involvement in consciousness-related processes. All networks had a high discriminative capacity (>80%) for separating patients in a minimally conscious state and vegetative state/unresponsive wakefulness syndrome. Among them, the auditory network was ranked the most highly. The regions of the auditory network which were more functionally connected in patients in minimally conscious state compared to vegetative state/unresponsive wakefulness syndrome encompassed bilateral auditory and visual cortices. Connectivity values in these three regions discriminated congruently 20 of 22 independently assessed patients. Our findings point to the significance of preserved abilities for multisensory integration and top-down processing in minimal consciousness seemingly supported by auditory-visual crossmodal connectivity, and promote the clinical utility of the resting paradigm for single-patient diagnostics.
IMPORTANCE Despite the apparent absence of external signs of consciousness, a significant small proportion of patients with disorders of consciousness can respond to commands by willfully modulating their brain activity, even respond to yes or no questions, by performing mental imagery tasks. However, little is known about the mental life of such responsive patients, for example, with regard to whether they can have coherent thoughts or selectively maintain attention to specific events in their environment. The ability to selectively pay attention would provide evidence of a patient’s preserved cognition and a method for brain-based communication, thus far untested with functional magnetic resonance imaging in this patient group. OBJECTIVE To test whether selective auditory attention can be used to detect conscious awareness and communicate with behaviorally nonresponsive patients. DESIGN, SETTING, AND PARTICIPANTS Case study performed in 3 patients with severe brain injury, 2 diagnosed as being in a minimally conscious state and 1 as being in a vegetative state. The patients constituted a convenience sample. MAIN OUTCOMES AND MEASURES Functional magnetic resonance imaging data were acquired as the patients were asked to selectively attend to auditory stimuli, thereby conveying their ability to follow commands and communicate. RESULTS All patients demonstrated command following according to instructions. Two patients (1 in a minimally conscious state and 1 in a vegetative state) were also able to guide their attention to repeatedly communicate correct answers to binary (yes or no) questions. CONCLUSIONS AND RELEVANCE To our knowledge, we show for the first time with functional magnetic resonance imaging that behaviorally nonresponsive patients can use selective auditory attention to convey their ability to follow commands and communicate. One patient in a minimally conscious state was able to use attention to establish functional communication in the scanner, despite his inability to produce any communication responses in repeated bedside examinations. More important, 1 patient, who had been in a vegetative state for 12 years before the scanning and subsequent to it, was able to use attention to correctly communicate answers to several binary questions. The technique may be useful in establishing basic communication with patients who appear unresponsive to bedside examinations and cannot respond with existing neuroimaging methods.
In recent years, numerous electrophysiological signatures of consciousness have been proposed. Here, we perform a systematic analysis of these electroencephalography markers by quantifying their efficiency in differentiating patients in a vegetative state from those in a minimally conscious or conscious state. Capitalizing on a review of previous experiments and current theories, we identify a series of measures that can be organized into four dimensions: (i) event-related potentials versus ongoing electroencephalography activity; (ii) local dynamics versus inter-electrode information exchange; (iii) spectral patterns versus information complexity; and (iv) average versus fluctuations over the recording session. We analysed a large set of 181 high-density electroencephalography recordings acquired in a 30 minutes protocol. We show that low-frequency power, electroencephalography complexity, and information exchange constitute the most reliable signatures of the conscious state. When combined, these measures synergize to allow.
Objective We here aimed at characterizing heart-brain interactions in patients with disorders of consciousness. We tested how this information impacts data-driven classification between unresponsive and minimally conscious patients. Methods A cohort of 127 patients in vegetative state/unresponsive wakefulness syndrome (VS/UWS, n=70) and minimally conscious state (MCS, n=57) were presented with the ‘Local-Global’ auditory oddball paradigm, which distinguishes two levels of processing: short-term deviation of local auditory regularities and global long-term rule violations. In addition to previously validated markers of consciousness extracted from electroencephalograms (EEG), we computed autonomic cardiac markers, such as heart rate and variability (HR, HRV), and cardiac cycle phase-shifts triggered by the processing of the auditory stimuli. Results HR and HRV were similar in patients across groups. The cardiac cycle was not sensitive to the processing of local regularities in either the VS/UWS or MCS patients. In contrast, global regularities induced a phase-shift of the cardiac cycle exclusively in the MCS group. The interval between the auditory stimulation and the following R-peak was significantly shortened in MCS when the auditory rule was violated. When the information of the cardiac cycle modulations and other consciousness-related EEG markers were combined, single-patient classification performance was enhanced compared to classification with solely EEG markers. Interpretation Our work shows a link between residual cognitive processing and the modulation of autonomic somatic markers. These results open a new window to evaluate patients with disorders of consciousness via the embodied paradigm, according to which body-brain functions contribute to a holistic approach to conscious processing. This article is protected by copyright. All rights reserved.
For patients with disorders of consciousness such as coma, a vegetative state or a minimally conscious state, one challenge is to detect and assess the residual cognitive functions in their brains. Number processing and mental calculation are important brain functions but are difficult to detect in patients with disorders of consciousness using motor response-based clinical assessment scales such as the Coma Recovery Scale-Revised due to the patients' motor impairments and inability to provide sufficient motor responses for number- and calculation-based communication.
Neuronal theories of conscious access tentatively relate conscious perception to the integration and global broadcasting of information across distant cortical and thalamic areas [1-6]. Experiments contrasting visible and invisible stimuli support this view and suggest that global neuronal communication may be detectable using scalp electroencephalography (EEG) [3, 5-11]. However, whether global information sharing across brain areas also provides a specific signature of conscious state in awake but noncommunicating patients remains an active topic of research [12-15]. We designed a novel measure termed “weighted symbolic mutual information” (wSMI) and applied it to 181 high-density EEG recordings of awake patients recovering from coma and diagnosed in various states of consciousness. The results demonstrate that this measure of information sharing systematically increases with consciousness state, particularly across distant sites. This effect sharply distinguishes patients in vegetative state (VS), minimally conscious state (MCS), and conscious state (CS) and is observed regardless of etiology and delay since insult. The present findings support distributed theories of conscious processing and open up the possibility of an automatic detection of conscious states, which may be particularly important for the diagnosis of awake but noncommunicating patients.