In the intensive care unit (ICU) patients are critically ill and require multimodal continuous physiological monitoring. Like telemetry and pulse oximetry for the cardiorespiratory systems, EEG allows for real-time neurophysiological monitoring. EEG measures electrical activity in the brain and can sensitively detect changes in brain functioning and structure (Nuwer 1994). When EEG is set up to run “continuously,” it allows for prolonged simultaneous recording of electrical brain activity and clinical behavior. This is most often used in the ICU setting to identify changes in brain function to correlate with altered mentation or atypical movements, specifically to evaluate for evidence of nonconvulsive seizures (NCS) or nonconvulsive status epilepticus (NCSE). These seizures can be difficult to diagnose as they can be brief and without clinical correlation other than coma or confusion and can only be diagnosed using EEG (32; 85; 30).
The importance of their proper diagnosis is their reported association with significant morbidity and mortality (107; 87) and the fact that delay in the diagnosis and treatment of nonconvulsive seizures and nonconvulsive status epilepticus can lead to worse outcomes (32). Nonconvulsive seizures and nonconvulsive status epilepticus can occur in different settings, including the neurosurgical, neurologic, medical, and surgical intensive care units (66; 70; 50; 51; 21; 36) and in pediatric and adult cases (30; 93; 24). This has led to more widespread use of continuous EEG (cEEG).
Continuous EEG is valuable in detection to facilitate diagnosis and following a response to treatment of seizures. Many have also found utility in using continuous EEG for prognostication and guiding clinical and sedative management in those with traumatic or hypoxic-ischemic brain injuries and in the detection of ischemia warranting intervention in the setting of subarachnoid hemorrhage (32; 87; 31).
This article will review EEG monitoring in the intensive care unit setting, highlight clinical indications and goals of its use, discuss important EEG patterns that may be helpful in clinical diagnosis and prognosis, consider duration of EEG monitoring, and present recent information in this evolving field.
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• Continuous EEG (cEEG) monitoring is defined as EEG monitoring performed for extended periods of time, ranging from hours to days.
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• Continuous EEG is used for a variety of indications, including the detection of nonconvulsive seizure and nonconvulsive status epilepticus, assessing response to therapy in patients who experience status epilepticus, monitoring level of sedation, ischemia detection, and prognostication.
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• Duration of cEEG monitoring differs depending on the clinical situation and EEG patterns present during the early portion of the recording.
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• Continuous EEG is becoming more widespread in its use but there are significant barriers that need to be removed for better access given its utility in clinical management.
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• Quantitative and “rapid” EEG techniques are becoming more widely used and available and may provide more capability to provide timely diagnosis and institution of treatment, regardless of institutional resources, leading to improved outcomes.
Historical note and terminology
Techniques to monitor multiple organ systems for patients in the intensive care unit have been present for some time but neurologic monitoring historically had been done through “neuro-checks.” This alone is well-known to be a suboptimal way to monitor neurologic changes in comatose or sedated patients that would require urgent interventions (40). In the 1980s, many felt that EEG could be used to monitor neurologic patients in the ICU (05; 20) and for prognosis in coma (06; 08). Up until that point, compressing this data would be exceedingly difficult and arduous until digital EEG technology was introduced (39). Over the past 20 years, cEEG use has expanded, and quantitative EEG techniques have become available to further aid in the interpretation of EEG data (86).