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.

This chapter describes the diagnosis, types, and management of nonconvulsive status epilepticus using a case-based approach. Generalized status epilepticus including nonconvulsive status epilepticus (NCSE) is a medical emergency that is best managed in an intensive care unit with continuous EEG monitoring. Refractory status epilepticus (RSE) is defined as the failure of seizure activity to terminate despite initial benzodiazepines and additional intravenous antiseizure medications. Refractory status epilepticus is an indication for intravenous anesthetics such as midazolam and propofol with the goal of titrating to burst suppression. Highly epileptiform bursts seen are associated with status epilepticus recurrence despite treatment with intravenous anesthetics, a condition known as super-refractory status epilepticus (SRSE). Additionally, this chapter also describes how to recognize status epilepticus cessation and an uncommon form of NCSE called absence status epilepticus.

This chapter focuses on several of the most common actionable EEG abnormalities. This includes defining and describing epileptiform discharges, which are abnormal EEG waves that serve as markers of increased seizure risk. This also includes seizures themselves, and their characteristics and electrographic criteria. Prolonged and repetitive seizures known as status epilepticus are described, including their specific electrographic characteristics and criteria. The treatment resistant form of status epilepticus known as refractory status epilepticus is also described. Finally, the chapter describes a high risk electrographic phenomenon known as brief potentially ictal rhythmic discharges (BIRDs).