Electroencephalography (EEG) is very important, noninvasive, bedside diagnostic procedure, for relatively quick differentiating the etiology and therapeutically efficacy in critically ill patients, with variety of cerebral injures and altered states of consciousness.

It is essential to differentiate artifact from pathophysiologic EEG changes that would suggest encephalopathy, epileptiform activity or seizures. There are particular patterns typical of deepening encephalopathy, as well as, coma patterns, that have diagnostic and prognostic significance. Epileptiform patterns, including periodic lateralized epileptiform discharges, bilateral independent periodic lateralized epileptiform discharges and generalized periodic epileptiform discharges, present particular challenges, as there is a gray-zone between interictal patterns and the evolving patterns of nonconvulsive seizures.

EEG is the only procedure for diagnosing nonconvulsive status epilepticus, showing generalized slow waves for idiopathic generalized epilepsy or regional spikes or sharp waves in focal epilepsy. Furthermore, it is the most frequently used test worldwide for confirmation of brain death. Together with polysomnography, EEG is used for showing cyclic variations and pathological changes during sleep and their relation to eye movements, body motility and dreaming.

Accurate use of EEG in the intensive care unit requires optimal EEG technical expertise in performing the study, as well as, appropriate interpretation by a trained electrophysiologist.