Electroencephalography and evoked potentials in critically ill patients

Electrophysiological testing of the central nervous system (CNS) remains an important diagnostic tool in critically ill patients. Rather than producing static images of the brain and spinal cord, somatosensory evoked potentials (SSEP) and electroencephalography (EEG) measure electrical surrogates of neurological function in real time. An electroencephalogram continuously measures and displays voltage differences between pairs of scalp electrodes, while SSEP measures evoked conduction of an electrical stimulus along the sensory pathway. In critically ill patients with limited neurological exams and inability to safely transport for radiographic studies, SSEP and EEG can provide an important window into brain function. In addition, real-time measurement of changes in brain electrical activity can allow continuous monitoring for response to therapeutic interventions, secondary injury, and state-dependent changes. The primary indications for EEG and SSEP in critically ill patients are: monitoring for seizure activity in high-risk populations, differentiation between metabolic encephalopathy and non-convulsive status epilepticus (NCSE) as a cause of altered mental status, monitoring of anesthetic depth, prognostication after global brain injuries, and determination of brain death. Continuous monitoring of EEG allows rapid, remote detection of clinically silent seizure activity, which has been demonstrated to occur frequently in patients with severe traumatic brain injury, global anoxic injury, and unexplained coma. For therapeutic interventions for intracranial hypertension or refractory status epilepticus, continuous EEG monitoring is also important for defining therapeutic targets such as seizure suppression and burst-suppression. The SSEP has become an important adjunctive prognostic tool, especially in comatose survivors of cardiac arrest. In this population, bilateral absence of cortical potentials has nearly 100% specificity for poor outcomes. The intensive care unit (ICU) provides a number of challenges to acquisition and interpretation of electrophysiological signals. In particular, differentiation between non-convulsive seizures, metabolic encephalopathy patterns, and injury patterns in patients with diffuse brain injuries can be especially difficult to distinguish from one another. The ICU also provides an abundance of sources of electrical interference which can produce artifacts.