The American Clinical Neurophysiology Society (ACNS) has created a uniform EEG terminology for describing these EEG patterns in critically ill patients, aiming to classify these patterns with a universal nomenclature, improve interrater reliability and facilitate research. In the light of widespread use of continuous EEG monitoring for critically ill patients in the last several decades, a spectrum of rhythmic and periodic patterns have been described. Chatrian and colleagues later described periodic lateralized epileptiform discharges (PLEDs) in patients with acute focal brain lesions in 1964. Periodic discharges were initially described by Cobb and Hill in patients with subacute progressive encephalitis in 1950. Here we review the recent advancements in their clinical significance, risk stratification and treatment algorithm. When available and necessary, correlation of IIC with biomarkers of neuronal injury, such as neuronal specific enolase (NSE), neuroimaging, depth electrode recording, cerebral microdialysis and oxygen measurement, can be assessed for the consideration of ASM treatment. In practice, a diagnostic trial with preferably a non-sedative anti-seizure medication (ASM) can be considered with the end point being both clinical and electrographic improvement. Currently there are no evidence-based guidelines on when and if to treat patients with IIC patterns, and particularly how aggressively to treat, presenting a challenging electrophysiological and clinical conundrum. As such, IIC commonly includes the rhythmic and periodic patterns occurring at a rate of 1–2.5 Hz without spatiotemporal evolution and clinical correlates. Generally speaking, rhythmic and periodic patterns are categorized as interictal patterns when occurring at a rate of 2.5 Hz with spatiotemporal evolution. They may be associated with cerebral metabolic crisis and neuronal injury, therefore not clearly interictal either, but lie along an intervening spectrum referred to as ictal-interictal continuum (IIC). These patterns share the rhythmic and sharp appearances of electrographic seizures, but often lack the necessary frequency, spatiotemporal evolution and clinical accompaniments to meet the definitive criteria for ictal patterns. It has also brought about a renaissance in a wide range of rhythmic and periodic patterns with heterogeneous frequency and morphology. Continuous electroencephalogram (cEEG) has become an indispensable technique in the management of critically ill patients for early detection and treatment of non-convulsive seizures (NCS) and non-convulsive status epilepticus (NCSE).
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