EEG Assessment of Consciousness Rebooting from Coma

The bi-dimensional arousal-awareness operational framework of defining consciousness assumes a graded level of both arousal and awareness. At the lower end of both the arousal and awareness scales is coma, defined as a state of unresponsiveness, in which a patient cannot be awaken. Recovery from coma following brain injury occurs through a sequence of disorders of consciousness (DOC) ranging from unresponsive wakefulness syndrome (UWS)/vegetative state (VS) which shows no signs of awareness to minimally conscious states (MCS) from which consciousness can emerge. The graded consciousness recovery parallels scales of brain injury severity suggesting that consciousness recovers by turning on hierarchical services. This sequence resembles a computer booting process, which may hang at a specific intermediate levels of consciousness such as UWS/VS or MCS. The recovery sequence can be assessed by electroencephalography (EEG), a large-scale measure of fluctuations in electrical activity of the brain. Resting state EEG assesses the graded recovery of brain oscillations from isoelectric line to burst-suppression (BS) to EEG slowing to wakefulness alpha activity and faster rhythms. Given that some degree of connectivity is required for consciousness, the assessment of DOC can be improved by testing the EEG in controlled stimulation paradigms addressing either the event related potential (ERP) or the event related changes in background EEG rhythms referred to as reactivity, which can be evoked even in BS. This grading of EEG changes provides the rationale for developing EEG indexes for monitoring the booting process during coma recovery to improve the diagnosis and prognostication of DOC.