Clinical neurophysiology in acute coma and disorders of consciousness

Over the last decade, significant improvements have been made in understanding and categorizing coma and disorders of consciousness. Although imaging techniques have been paramount in exploring disorders of consciousness, electrophysiologic techniques continue to be important for studying brain function in behaviorally unresponsive patients. In acute coma, electroencephalogram and evoked potentials have important roles in excluding nonconvulsive seizures, determining prognosis, monitoring for signs of improvement or worsening, and examining for markers of conscious response to external stimuli. Absence of cortical SSEPs is the most specific marker of poor prognosis after cardiac arrest. Recognition of stimulus-induced epileptiform discharges and clinical seizures has further blurred the lines along the ictal-interictal spectrum in coma. For chronic disorders of consciousness, more experimental techniques, such as cognitive event-related potentials and long-latency evoked potentials, have demonstrated an expanded role in determining prognosis and examining for indicators of consciousness. Like functional magnetic resonance imaging, these specialized techniques have demonstrated signs of preserved cognition in patients who otherwise appear unconscious. Future directions for clinical electrophysiologic testing in disorders of consciousness are likely to include automated and quantitative signal processing techniques and better standardization of cognitive event-related potentials.