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EEG in Status Epilepticus

Overview

Status epilepticus (SE) is a life-threatening, neurologic emergency that the International League Against Epilepsy (ILAE) defines as “seizure that persists for a sufficient length of time or is repeated frequently enough that recovery between attacks does not occur.”
Due to early research that showed irreversible neuronal damage after about 30 minutes of continuous epileptic activity,
the conventional definition of status epilepticus as continuous clinical seizure activity lasting greater than 30 minutes or 2 or more repetitive seizures without recovery of the baseline level of consciousness between attacks was adopted. Although this definition initially referred to clinically obvious or generalized convulsive status epilepticus (GCSE), the advent of continuous electroencephalographic (EEG) monitoring has facilitated the recognition of subtle convulsive and nonconvulsive (NCSE) forms of status epilepticus as well.

However, studies have suggested that this definition might need revision. Work by Jenssen et al
and Shinnar et al
demonstrated that seizures that do not spontaneously resolve within 5-10 minutes are unlikely to terminate without intervention. Additionally, Treiman et al showed that the duration of generalized convulsive status epilepticus before treatment was an important determinant of treatment success, whereas later studies have shown that delayed treatment in nonconvulsive status epilepticus is associated with increased mortality and refractoriness to treatment.
Based on these findings, many authors have suggested a more aggressive definition of status epilepticus that is based on a duration of 5 rather than 30 minutes.

In Europe, the annual incidence of generalized convulsive status epilepticus is estimated to range from 3.6 to 6.6 per 100,000, whereas nonconvulsive status epilepticus is estimated from 2.6 to 7.8 per 100,000.
In the United States, a prospective study that included all forms of status epilepticus (GCSE, subtle convulsive status epilepticus, and NCSE) cited a markedly higher incidence rate of 41 per 100,000.
Mortality rates for status epilepticus range from 3% to 33%, and such variability is heavily dependent on etiology, age, and clinical seizure form.

Any seizure type can develop into status epilepticus, and hence, there are as many types of status epilepticus as there are types of seizures.
Because the type of status epilepticus can often inform on the etiology, prognosis, and treatment of the seizures, distinguishing between these various forms of status epilepticus is crucial. Such classification is based on both the clinical semiology and the findings on the EEG. With the increased recognition of subtle convulsive and nonconvulsive status epilepticus over the past 2 decades,
as well as the controversies regarding periodic patterns that lie on the ictal-interictal continuum, EEG in particular is indispensable in identifying and characterizing the various forms of status epilepticus.

This review presents a concise categorization of the various types of status epilepticus with attention to their particular hallmarks on EEG. It also discusses the more ambiguous periodic patterns (generalized periodic discharges [GPDs]; periodic lateralized epileptiform discharges [PLEDs]; PLEDs with transitional rhythmic discharges [PLEDs Plus]; bilateral, independent PLEDs [BIPLEDs]; and stimulus-induced rhythmic, periodic, or ictal discharges [SIRPIDs]), which have been the focus of debate regarding their potential ictal nature and clinical relevance.
Consideration of these more controversial patterns serves to underscore some of the limitations and uncertainties involved with EEG interpretation in status epilepticus.

For more information, see Status Epilepticus and Pediatric Status Epilepticus.

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