The most important indication for electroencephalography (EEG) in critically ill patients is to evaluate fluctuating or persistently abnormal mental status (or other focal neurological deficits) that cannot otherwise be explained. Commonly, these symptoms are a manifestation of physiological diffuse cerebral dysfunction (encephalopathy), or they may be due to seizure activity without apparent clinical manifestations. Such “nonconvulsive” seizures (NCS), that may only be detected by EEG, occur in at least 8–10% of critically ill patients. Continuous or frequent NCS is called nonconvulsive status epilepticus (NCSE), and may result in secondary neurological injury, including neuronal death or alteration of neuronal networks. Left untreated, NCSE can become increasingly refractory to treatment. EEGs may be indicated in acute brain injury to detect seizure activity. They are useful in monitoring the depth of anesthesia and in the management of refractory status epilepticus. EEGs may also be used in the intensive care unit to characterize paroxysmal clinical events and in prognostication after cardiac arrest or determining brain death.

SE is defined as 5 minutes or longer of continuous clinical and/or electrographic seizure activity or recurrent seizures without interval recovery; t1 refers to the time point beyond which there is failure of mechanisms responsible for seizure termination or initiation of mechanisms which lead to abnormally prolonged seizures, and t2 refers to the time point beyond which there are long term consequences due to neuronal injury, death, and alteration of neuronal networks. Semiologically, SE can be classified as with or without prominent motor features. Convulsive SE may evolve into NCSE in a significant minority after convulsive activity ceases. NCSE may be diagnosed on EEG in a significant minority of critically ill patients. EPC may not be associated with ictal activity on surface EEG. De novo absence SE may be seen in older individuals in the setting of abrupt benzodiazepine withdrawal. They may have a previous or family history of IGE.