Abstract
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). It has also brought about a renaissance in a wide range of rhythmic and periodic patterns with heterogeneous frequency and morphology. 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. 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). Generally speaking, rhythmic and periodic patterns are categorized as interictal patterns when occurring at a rate of <1Hz, and are categorized as NCS and NCSE when occurring at a rate of >2.5 Hz with spatiotemporal evolution. 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. 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. 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. 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. Here we review the recent advancements in their clinical significance, risk stratification and treatment algorithm.
Highlights
Periodic discharges were initially described by Cobb and Hill in patients with subacute progressive encephalitis in 1950 [1]
The concept of IIC was first coined by PohlmannEden et al in 1996, who described periodic lateralized epileptiform discharges (PLEDs) as “an electrographic signature of a dynamic pathophysiological state in which unstable neurobiological processes create an ictal interictal continuum, with the nature of the underlying neuronal injury, the patient’s preexisting propensity to have seizures, and the coexistence of any acute metabolic derangements all contributing to whether seizures occur or not” [6]
Mitigate the seizure risk associated with IIC patterns Most of IIC patterns are highly associated with increased risk of seizures, when they are > 2.0 Hz and associated with “plus” features
Summary
Periodic discharges were initially described by Cobb and Hill in patients with subacute progressive encephalitis in 1950 [1]. 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. The current use of ictal-interictal continuum has been expanded to include other rhythmic and periodic patterns (i.e. LPDs, GPDs, BIPDs, LRDA and GRDA) [7, 8].
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