The most common reason for initiating a cEEG study in the intensive care unit (ICU) is for the detection of subclinical or NCS. CEEG is the only type of monitor capable of detecting these types of seizures and, therefore, uniquely suited to this job. The most important population to monitor are those patients who are encephalopathic and were known or strongly suspected to have experienced generalized convulsive status epilepticus (GCSE). The time to recovery of a normal level of consciousness varies greatly, but if the patient does not appear to show improvement within 30 min, cEEG monitoring will almost certainly be needed. Multiple studies in adults and children have shown high rates of NCS (43–57 %) and NCSE (13 %) after clinical seizure activity has stopped with or without the use of abortive medications [1, 2]. In clinical practice, these patients should take precedence over others that will require cEEG monitoring. If cEEG resources are limited and monitoring cannot be initiated promptly, it may be necessary to transfer the patient to an institution with these capabilities. If cEEG is not available, an emergency 30 min EEG may be helpful, but it is likely to not meet the needs of the patient. The American Clinical Neurophysiology Society (ACNS) and Neurocritical Care Society recommend initiating the study as soon as possible and within 60 min, if possible [1, 3]. CEEG monitoring will be required for multiple days or longer if NCS or NCSE is detected after GCSE. Though the technique for monitoring and review does not change, the purpose of cEEG is now directed at terminating seizure activity and ensuring that it does not recur.

Refractory (RSE) and super refractory status epilepticus (SRSE) require IV anesthetic agents such as midazolam, propofol, and pentobarbital. Once RSE or SRSE has been diagnosed and IV anesthetics started, cEEG monitoring is required not only to monitor for the termination of seizure activity but also to titrate to the desired depth of anesthesia whether it is seizure, burst, or total EEG suppression. During the withdrawal of IV anesthetics, cEEG monitoring is needed to ensure that NCSs do not reemerge. In SRSE, the rate of seizure reoccurrence is unfortunately high (greater than 50 %), and close monitoring is necessary to confirm that treatment has been effective [4]. If monitoring is anticipated to last for many days, different tools could be used if they are available. Computed tomography (CT) and magnetic resonance imaging (MRI) compatible electrodes, like disposable plastic, subdermal needle, and wire electrodes, may be used in place of non-disposable gold-plated electrodes if neuroimaging is needed. Quantitative EEG (qEEG) software can be used to facilitate review of long periods of data particularly if a reproducible seizure pattern is found.

CEEG for the detection of seizure activity should not be limited to those with GCSE. In the setting of supratentorial brain injury, many encephalopathic patients are at risk for NCSs. Though clinical seizure activity noted prior to the onset of encephalopathy increases the risk, the rates of detecting subclinical seizure activity in this population remain relatively high. The patients most likely to experience NCS include those with prior epilepsy, intracranial hemorrhage (ICH), moderate-severe traumatic brain injury (TBI), central nervous system (CNS) infections, hypoxic-ischemic-related injury, and brain tumors and those who have undergone a recent neurosurgical procedure [1]. If there is suspicion for NCS, early application of cEEG is critical to identify and treat seizures as they become refractory to abortive agents without prompt recognition and treatment. Though little outcome data is available, it is likely that detection and treatment of seizures may reduce any secondary brain injury that may occur as a result of the NCS [5]. As with patients after GCSE, routine EEG will be inadequate. Therefore, patients with known brain injury and an unexplained encephalopathy should be considered “high risk” and undergo cEEG monitoring as soon as possible.

Though high rates of NCS and NCSE are well recognized in those with brain injury and encephalopathy, acutely ill medical and surgical patients with altered mental status may also be at high risk for seizures. Many acute illnesses, especially sepsis, with single or multi-organ failure and encephalopathy are associated with NCS and NCSE. Similarly, patients who are found to have epileptiform patterns, such as lateralized or generalized periodic discharges, on routine EEG are also at high risk for seizures. Therefore, in these cases cEEG should be strongly considered if resources are available [1, 6].

CEEG has proven critical in the ICU for spell characterization. Similar to studies performed outside the ICU, characterizing paroxysmal events is a common use for cEEG. Stereotyped motor movements presumed to represent seizure are a frequent request for a routine EEG. However, cEEG with audio and video has the advantage of detecting multiple events over several hours and is crucial for determining the etiology of these events. Many such movements are seen that resemble seizure activity but are not epileptic in nature, including clonus, tremors, and intermittent posturing from herniation. This is a common and expected use for cEEG monitoring. When monitoring for spell characterization, capturing several events is encouraged to properly define their etiology and determine their clinical significance. However, once the desired events have been captured, monitoring may no longer be needed.

Routine EEG has been used for several decades as a prognostication tool, particularly after cardiac arrest, but cEEG is becoming useful for this purpose as well. Though there is no evidence as of yet to suggest that cEEG would necessarily be more helpful than a routine study, compelling information has been gained from experience with cEEG monitoring. In most patient populations studied, a lack of EEG reactivity in the absence of significant sedation is consistently associated with a poor prognosis [1, 6]. EEG reactivity has been defined as a change in background frequency and/or amplitude when an external stimulus is applied [6, 7]. EEG reactivity is best determined using a standardized stimulation protocol, but a combination of auditory and tactile stimulation is probably all that is needed in most circumstances. The association between poor prognosis and a lack of reactivity is best documented in comatose post-cardiac arrest patients, but it is present in those with TBI, SAH, and sepsis as well. Though less well studied, a wide range of other prognostic findings can be found during cEEG monitoring. In cardiac arrest, burst suppression patterns are associated with a poor outcome, whereas a continuous reactive record is associated with a good outcome. In sepsis, the appearance of lateralized periodic discharges (LPD) and seizures may be associated with a poor outcome, but this association is less robust [1, 6, 8].