Method for quantifying arousal and consciousness in healthy states and severe brain injury via EEG-based measures of corticothalamic physiology

BACKGROUND

Characterization of normal arousal states has been achieved by fitting predictions of corticothalamic neural field theory (NFT) to electroencephalographic (EEG) spectra to yield relevant physiological parameters.

NEW METHOD

A prior fitting method is extended to distinguish conscious and unconscious states in healthy and brain injured subjects by identifying additional parameters and clusters in parameter space.

RESULTS

Fits of NFT predictions to EEG spectra are used to estimate neurophysiological parameters in healthy and brain injured subjects. Spectra are used from healthy subjects in wake and sleep and from patients with unresponsive wakefulness syndrome, in a minimally conscious state (MCS), and emerged from MCS. Subjects cluster into three groups in parameter space: conscious healthy (wake and REM), sleep, and brain injured. These are distinguished by the difference X-Y between corticocortical (X) and corticothalamic (Y) feedbacks, and by mean neural response rates α and β to incoming spikes. X-Y tracks consciousness in healthy individuals, with smaller values in wake/REM than sleep, but cannot distinguish between brain injuries. Parameters α and β differentiate deep sleep from wake/REM and brain injury.

COMPARISON WITH EXISTING METHODS

Other methods typically rely on laborious clinical assessment, manual EEG scoring, or evaluation of measures like Φ from integrated information theory, for which no efficient method exists. In contrast, the present method can be automated on a personal computer.

CONCLUSION

The method provides a means to quantify consciousness and arousal in healthy and brain injured subjects, but does not distinguish subtypes of brain injury.

European Academy of Neurology guideline on the diagnosis of coma and other disorders of consciousness

BACKGROUND AND PURPOSE

Patients with acquired brain injury and acute or prolonged disorders of consciousness (DoC) are challenging. Evidence to support diagnostic decisions on coma and other DoC is limited but accumulating. This guideline provides the state-of-the-art evidence regarding the diagnosis of DoC, summarizing data from bedside examination techniques, functional neuroimaging and electroencephalography (EEG).

METHODS

Sixteen members of the European Academy of Neurology (EAN) Scientific Panel on Coma and Chronic Disorders of Consciousness, representing 10 European countries, reviewed the scientific evidence for the evaluation of coma and other DoC using standard bibliographic measures. Recommendations followed the Grading of Recommendations Assessment, Development and Evaluation (GRADE) system. The guideline was endorsed by the EAN.

RESULTS

Besides a comprehensive neurological examination, the following suggestions are made: probe for voluntary eye movements using a mirror; repeat clinical assessments in the subacute and chronic setting, using the Coma Recovery Scale - Revised; use the Full Outline of Unresponsiveness score instead of the Glasgow Coma Scale in the acute setting; obtain clinical standard EEG; search for sleep patterns on EEG, particularly rapid eye movement sleep and slow-wave sleep; and, whenever feasible, consider positron emission tomography, resting state functional magnetic resonance imaging (fMRI), active fMRI or EEG paradigms and quantitative analysis of high-density EEG to complement behavioral assessment in patients without command following at the bedside.

CONCLUSIONS

Standardized clinical evaluation, EEG-based techniques and functional neuroimaging should be integrated for multimodal evaluation of patients with DoC. The state of consciousness should be classified according to the highest level revealed by any of these three approaches.

Sleep-like cortical OFF-periods disrupt causality and complexity in the brain of unresponsive wakefulness syndrome patients

Unresponsive wakefulness syndrome (UWS) patients may retain intact portions of the thalamocortical system that are spontaneously active and reactive to sensory stimuli but fail to engage in complex causal interactions, resulting in loss of consciousness. Here, we show that loss of brain complexity after severe injuries is due to a pathological tendency of cortical circuits to fall into silence (OFF-period) upon receiving an input, a behavior typically observed during sleep. Spectral and phase domain analysis of EEG responses to transcranial magnetic stimulation reveals the occurrence of OFF-periods in the cortex of UWS patients (N = 16); these events never occur in healthy awake individuals (N = 20) but are similar to those detected in healthy sleeping subjects (N = 8). Crucially, OFF-periods impair local causal interactions, and prevent the build-up of global complexity in UWS. Our findings link potentially reversible local events to global brain dynamics that are relevant for pathological loss and recovery of consciousness.

Many brain-injured patients retain large cortical islands that are intact, active and reactive but blocked in a state of low complexity, leading to unconsciousness. Here, the authors show that this loss of complexity is due to the pathological engagement of sleep-like neuronal mechanisms.

Propofol-induced unresponsiveness is associated with impaired feedforward connectivity in cortical hierarchy

BACKGROUND

Impaired consciousness has been associated with impaired cortical signal propagation after transcranial magnetic stimulation (TMS). We hypothesised that the reduced current propagation under propofol-induced unresponsiveness is associated with changes in both feedforward and feedback connectivity across the cortical hierarchy.

METHODS

Eight subjects underwent left occipital TMS coupled with high-density EEG recordings during wakefulness and propofol-induced unconsciousness. Spectral analysis was applied to responses recorded from sensors overlying six hierarchical cortical sources involved in visual processing. Dynamic causal modelling (DCM) of induced time-frequency responses and evoked response potentials were used to investigate propofol's effects on connectivity between regions.

RESULTS

Sensor space analysis demonstrated that propofol reduced both induced and evoked power after TMS in occipital, parietal, and frontal electrodes. Bayesian model selection supported a DCM with hierarchical feedforward and feedback connections. DCM of induced EEG responses revealed that the primary effect of propofol was impaired feedforward responses in cross-frequency theta/alpha-gamma coupling and within frequency theta coupling (F contrast, family-wise error corrected P<0.05). An exploratory analysis (thresholded at uncorrected P<0.001) also suggested that propofol impaired feedforward and feedback beta band coupling. Post hoc analyses showed impairments in all feedforward connections and one feedback connection from parietal to occipital cortex. DCM of the evoked response potential showed impaired feedforward connectivity between left-sided occipital and parietal cortex (T contrast P=0.004, Bonferroni corrected).

CONCLUSIONS

Propofol-induced loss of consciousness is associated with impaired hierarchical feedforward connectivity assessed by EEG after occipital TMS.

Correction to: Actigraphy assessments of circadian sleep-wake cycles in the Vegetative and Minimally Conscious States

The original article [1] contains an error affecting the actigraphy time-stamps throughout the article, particularly in Table 1.

Spasticity in disorders of consciousness: a behavioral study

BACKGROUND

Spasticity is a frequent complication after severe brain injury, which may impede the rehabilitation process and diminish the patients' quality of life.

AIM

We here investigate the presence of spasticity in a population of non-communicative patients with disorders of consciousness. We also evaluate the correlation between spasticity and potential factors of co-morbidity, frequency of physical therapy, time since insult, presence of pain, presence of tendon retraction, etiology and diagnosis.

DESIGN

Cross-sectional study.

SETTING

University Hospital of Liège, Belgium.

POPULATION

Sixty-five patients with chronic (>3 months post insult) disorders of consciousness were included (22 women; mean age: 44±14 y; 40 with traumatic etiology; 40 in a minimally conscious state; time since insult: 39±37 months).

METHODS

Spasticity was measured with the Modified Ashworth Scale (MAS) and pain was assessed using the Nociception Coma Scale-Revised (NCS-R).

RESULTS

Out of 65 patients, 58 demonstrated signs of spasticity (89%; MAS≥1), including 40 who showed severe spasticity (61.5%; MAS≥3). Patients with spasticity receiving anti-spastic medication were more spastic than unmedicated patients. A negative correlation was observed between the severity of spasticity and the frequency of physical therapy. MAS scores correlated positively with time since injury and NCS-R scores. We did not observe a difference of spasticity between the diagnoses.

CONCLUSION

A large proportion of patients with disorders of consciousness develop severe spasticity, possibly affecting their functional recovery and their quality of life. The observed correlation between degrees of spasticity and pain scores highlights the importance of pain management in these patients with altered states of consciousness. Finally, the relationship between spasticity and treatment (i.e., pharmacological and physical therapy) should be further investigated in order to improve clinical care.

CLINICAL REHABILITATION IMPACT

Managing spasticity at first signs could improve rehabilitation of patients with disorders of consciousness and maximize their chances of recovery. In addition, decreasing this trouble could allow a better quality of life for these non-communicative patients.

Electrophysiological investigations of brain function in coma, vegetative and minimally conscious patients

Electroencephalographic activity in the context of disorders of consciousness is a swiss knife like tool that can evaluate different aspects of cognitive residual function, detect consciousness and provide a mean to communicate with the outside world without using muscular channels. Standard recordings in the neurological department offer a first global view of the electrogenesis of a patient and can spot abnormal epileptiform activity and therefore guide treatment. Although visual patterns have a prognosis value, they are not sufficient to provide a diagnosis between vegetative state/unresponsive wakefulness syndrome (VS/UWS) and minimally conscious state (MCS) patients. Quantitative electroencephalography (qEEG) processes the data and retrieves features, not visible on the raw traces, which can then be classified. Current results using qEEG show that MCS can be differentiated from VS/UWS patients at the group level. Event Related Potentials (ERP) are triggered by varying stimuli and reflect the time course of information processing related to the stimuli from low-level peripheral receptive structures to high-order associative cortices. It is hence possible to assess auditory, visual, or emotive pathways. Different stimuli elicit positive or negative components with different time signatures. The presence of these components when observed in passive paradigms is usually a sign of good prognosis but it cannot differentiate VS/UWS and MCS patients. Recently, researchers have developed active paradigms showing that the amplitude of the component is modulated when the subject's attention is focused on a task during stimulus presentation. Hence significant differences between ERPs of a patient in a passive compared to an active paradigm can be a proof of consciousness. An EEG-based brain-computer interface (BCI) can then be tested to provide the patient with a communication tool. BCIs have considerably improved the past two decades. However they are not easily adaptable to comatose patients as they can have visual or auditory impairments or different lesions affecting their EEG signal. Future progress will require large databases of resting state-EEG and ERPs experiment of patients of different etiologies. This will allow the identification of specific patterns related to the diagnostic of consciousness. Standardized procedures in the use of BCIs will also be needed to find the most suited technique for each individual patient.

Hypnotic modulation of resting state fMRI default mode and extrinsic network connectivity

Resting state fMRI (functional magnetic resonance imaging) acquisitions are characterized by low-frequency spontaneous activity in a default mode network (encompassing medial brain areas and linked to self-related processes) and an anticorrelated "extrinsic" system (encompassing lateral frontoparietal areas and modulated via external sensory stimulation). In order to better determine the functional contribution of these networks to conscious awareness, we here sought to transiently modulate their relationship by means of hypnosis. We used independent component analysis (ICA) on resting state fMRI acquisitions during normal wakefulness, under hypnotic state, and during a control condition of autobiographical mental imagery. As compared to mental imagery, hypnosis-induced modulation of resting state fMRI networks resulted in a reduced "extrinsic" lateral frontoparietal cortical connectivity, possibly reflecting a decreased sensory awareness. The default mode network showed an increased connectivity in bilateral angular and middle frontal gyri, whereas its posterior midline and parahippocampal structures decreased their connectivity during hypnosis, supposedly related to an altered "self" awareness and posthypnotic amnesia. In our view, fMRI resting state studies of physiological (e.g., sleep or hypnosis), pharmacological (e.g., sedation or anesthesia), and pathological modulation (e.g., coma or related states) of "intrinsic" default mode and anticorrelated "extrinsic" sensory networks, and their interaction with other cerebral networks, will further improve our understanding of the neural correlates of subjective awareness.

Predictors of short-term outcome in brain-injured patients with disorders of consciousness

OBJECTIVES

To investigate predictors of recovery from the vegetative state (VS) and minimally conscious state (MCS) after brain injury as measured by the widely used Disability Rating Scale (DRS) and to explore differences in rate of recovery and predictors of recovery during inpatient rehabilitation in patients with non-traumatic (NTBI) and traumatic brain injury (TBI).

DESIGN

Longitudinal observational cohort design and retrospective comparison study, in which an initial DRS score was collected at the time of study enrollment. Weekly DRS scores were recorded until discharge from the rehabilitation center for both NTBI and TBI patients.

SETTING

Seven acute inpatient rehabilitation facilities in the United States and Europe with specialized programs for VS and MCS patients (the Consciousness Consortium).

PARTICIPANTS

One hundred sixty-nine patients with a non-traumatic (N=50) and a traumatic (N=119) brain injury who were in the VS or MCS states.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

DRS score at 13 weeks after injury; change in DRS score over 6 weeks post-admission; and time until commands were first followed (for patients who did not show command-following at or within 2 weeks of admission).

RESULTS

Both time between injury and enrollment and DRS score at enrollment were significant predictors of DRS score at week 13 post-injury but the main effect of etiology only approached significance. Etiology was however a significant predictor of the amount of recovery observed over the 6 weeks following enrollment. Time between injury and enrollment was also a good predictor of this outcome, but not DRS score at enrollment. For the time until commands were first followed, patients with better DRS scores at enrollment, and those with faster early rates of change recovered command following sooner than those with worse DRS scores or slower initial rates of change. The etiology was not a significant predictor for this last outcome. None of these predictive models explained sufficient variance to allow their use in individual clinical decision making.

CONCLUSIONS

Time post-injury and DRS score at enrollment are predictors of early recovery among patients with disorders of consciousness, depending on the outcome measure chosen. Etiology was also a significant predictor in some analyses, with traumatically injured patients recovering more than those with non-traumatic injuries. However, the hypothesized interaction between etiology and time post-injury did not reach significance in any of the analyses suggesting that, within the time frame studied, the decline in prognosis with the passage of time was similar in the two groups.

[Functional neuroimaging (fMRI, PET and MEG): what do we measure?]

Functional cerebral imaging techniques allow the in vivo study of human cognitive and sensorimotor functions in physiological or pathological conditions. In this paper, we review the advantages and limitations of functional magnetic resonance imaging (fMRI), positron emission tomography (PET) and magnetoencephalography (MEG). fMRI and PET measure haemodynamic changes induced by regional changes in neuronal activity. These techniques have a high spatial resolution (a few millimeters), but a poor temporal resolution (a few seconds to several minutes). Electroencephalogram (EEG) and MEG measure the neuronal electrical or magnetic activity with a high temporal resolution (i.e., milliseconds) albeit with a poorer spatial resolution (i.e., a few millimeters to one centimeter). The combination of these different neuroimaging techniques allows studying different components of the brain's activity (e.g., neurovascular coupling, electromagnetic activity) with both a high temporal and spatial resolution.

[Pain assessment in non-communicative patients]

Pain is a subjective experience. Its assessment is based on the subject's direct verbal report. This method of assessment is, however, impossible in patients who cannot communicate their feelings. In this context, indirect measurements such as behavioral observations or physiological measurements are needed. To facilitate the assessment of pain in non-communicative patients, numerous standardized behavioral scales have been developed. The aim of this review is to discuss the main validated pain scales employed in end-stage dementia, newborn and preverbal children, and severely brain damaged patients with a disorder of consciousness such as coma, the vegetative state or the minimally conscious state.

[Behavioural assessment and functional neuro-imaing in vegetative state patients]

Currently, there remains a high rate of misdiagnosis of the vegetative state. This should incite clinicians to use the most sensitive "coma scales" to detect signs of consciousness in these patients. The gold standard remains the Glasgow Coma Scale (GCS, Teasdale and Jennet, 1974), with the Glasgow Liège Scale (GLS, Born, 1988) adding standardized assessment of brainstem reflexes. New sensible behavioral assessment tools for use in the acute neurocritical care setting include the Full Outline of UnResponsiveness (FOUR, Wijdicks et al., 2005). The Coma Recovery Scale-Revised (CRS-R, Giacino and Kalmar, 2004) specifically tests the diagnostic criteria differentiating vegetative from minimally conscious patients. Detecting signs of consciousness also depends on the employed methodology. We showed that for the assesment of the presence of visual pursuit, using a moving mirror is better suited than using a moving object or person. The clinical diagnosis can be confirmed by cerebral positron emission tomography studies objectively quantifying residual metabolic activity in vegetative and minimally conscious patients. Ongoing studies evaluate the prognostic value of functional magnetic resonance imaging studies in these challenging patient populations.