EEG assessment of disordered consciousness: a framework and a case study

Abstract

Assessing cognitive abilities in disorders of consciousness (DOC) relies on assessments of overt behaviour, such as the ability to follow commands. Neuroimaging has shown that absence of overt behavior does not necessarily indicate absence of covert cognition, raising questions about behaviour-only assessment. Several electroencephalographic (EEG) markers of higher cognitive functions (event-related potentials; ERPs) have shown the potential to differentiate between DOC states, as well as predict awakening and condition upon emergence. However, no one ERP has emerged with sensitivity and specificity high enough to be widely accepted, showing that further investigation is needed. More recently, evidence has emerged for fluctuations of ERP detectability in DOC over the course of several hours, and for prognostic power of changes in ERP presentation between testing sessions. This investigation builds on such findings towards improving evaluation of cognition in DOC. A testing battery combining several well-known auditory ERPs was administered to a comatose patient over a 24-hour period during two recording sessions one week apart, as well as to a sample of healthy young adults. The patient scored 3 and 6 on the Glasgow Coma Scale (GCS) during the first and second session, respectively. The results show that changes in GCS score were accompanied by changes in ERP detectability. The results also suggest detectability fluctuations over the course of 24 hours, which in turn suggests that repeated testing is necessary for complete evaluation. Future work should validate these findings with a larger sample; additionally, establishing population norms for single-subject prevalence, latency, and amplitude of ERPs would improve confidence in interpreting patient results. With the current understanding of both healthy and DOC ERPs, detecting ERP presence may contribute to a positive DOC prognosis with a degree of confidence, but caution must be exercised in making negative prognoses or high-stakes care decisions based on ERP absence.