Across the consciousness continuum-from unresponsive wakefulness to sleep

Abstract

Advances in the development of new paradigms as well as in neuroimaging techniques nowadays enable us to make inferences about the level of consciousness patients with disorders of consciousness (DOC) retain. They, moreover, allow to predict their probable development. Today, we know that certain brain responses (e.g., event-related potentials or oscillatory changes) to stimulation, circadian rhythmicity, the presence or absence of sleep patterns as well as measures of resting state brain activity can serve the diagnostic and prognostic evaluation process. Still, the paradigms we are using nowadays do not allow to disentangle VS/UWS and minimally conscious state (MCS) patients with the desired reliability and validity. Furthermore, even rather well-established methods have, unfortunately, not found their way into clinical routine yet. We here review current literature as well as recent findings from our group and discuss how neuroimaging methods (fMRI, PET) and particularly electroencephalography (EEG) can be used to investigate cognition in DOC or even to assess the degree of residual awareness. We, moreover, propose that circadian rhythmicity and sleep in brain-injured patients are promising fields of research in this context.

Keywords: circadian rhythms; disorders of consciousness (DOC); electroencephalography (EEG); resting state; sleep.

Figure 1

Two factors contributing to consciousness: arousal and awareness. In healthy sleep, arousal and awareness covary with the exception of rapid-eye movement (REM) sleep while in brain death and coma, arousal and awareness are not detectable. In the vegetative state (VS) arousal is preserved in the absence of evidence for awareness. In the minimally conscious state (MCS), both dimensions are present and behavioral evidence of awareness is reproducible albeit inconsistent. In the locked-in syndrome (LIS), both dimensions are more or less fully preserved despite complete loss motor functions (adapted with permission from Giacino et al. (2009)).

Figure 2

Frontal theta event-related (de-)synchronization (ERD/ERS) to counted own names. Time–frequency difference plots [targets (SON)-passively listened other names] for the “count own name”-condition. The dashed lines mark the presentation of the stimuli (names) and the rectangles the area with the highest difference in the theta-range. Note the increasing processing delay in theta power over groups as well as the alpha ERD in controls only (adapted with permission from Fellinger et al., 2011).

Figure 3

Sleep pattern from a healthy human. An exemplary hypnogram depicting the different sleep stages over 8 h of nocturnal sleep. On the right side of the figure, typical EEG graphoelements of every sleep stage have been listed. In addition to certain EEG patterns, some of the sleep stages also have characteristic EMG and EOG activity patterns. Early sleep is predominated by N3, whereas later sleep is characterized by a relatively high amount of R. One observes during wake—high muscle tone, high frequency EEG (alpha-beta) and blinks; during light N1—eye rolling and vertex sharp waves; during light N2—sleep spindles and k-complexes; during deep N3—slow oscillations; and during R—rapid eye movements, PGO waves, saw-tooth waves and muscle atonia with concurring rare muscle twitches. Abbreviations: EEG, Electroencephalography; EMG, Electromyography; EOG, Electrooculography; R, Rapid Eye Movement Sleep; REMs—Rapid Eye Movements; PGO, Ponto-Geniculo-Occipital Waves.