The Commission of Classification and Terminology of the ILEA (Chair: Dr. Ingrid E. Scheffer) and the Commission on Epidemiology (Chairs: Drs. Ettore Beghi and Dale Hesdorffer) charged a Task Force, chaired by Dr. Daniel H. Lowenstein and Dr. Eugen Trinka, with clinical researchers and epidemiologists, to revise the classification of SE in 2009. Members were Dr. Hannah Cock (UK), Dr. Hesdorffer (USA), Dr. Lowenstein (USA), Dr. Andrea O. Rossetti (Switzerland), Dr. Scheffer (Australia), Dr. Shlomo Shinnar (USA), Dr. Simon Shorvon (UK), and Dr. Trinka (Austria). This group aimed to achieve a unifying definition and classification of SE serving all purposes [8]. Because current knowledge regarding the pathophysiology and underlying neurobiology of SE is far from complete, the Task Force recognized that a proposed definition should include two dimensions: first, a conceptual approach based on the scientific evidence, and second, an operational frame to guide management.

A classification refers to the way in which items are organized and should be based ideally on the underlying neurobiology to form natural classes or entities [9]. Again, knowledge of the different types of SE and its underlying mechanisms are, at best, marginal so any classification would be a compromise between a conceptual, scientific (drawing on what is known) classification and a pragmatic empirical classification [10]. The Task force issued the following definition: “Status epilepticus is a condition resulting either from the failure of the mechanisms responsible for seizure termination or from the initiation of mechanisms, which lead to abnormally prolonged seizures (after time point t1). It is a condition, which can have long–term consequences (after time–point t2), including neuronal death, neuronal injury, and alteration of neuronal networks, depending on the type and duration of seizures” [8].

There are two novel aspects included in this definition: First, conceptually, SE does not represent only a “failure of seizure suppressing mechanism,” as maintained in 2006. Recent advances in the basic understanding of SE have made it clear that there is likely a multitude of simultaneous, parallel processes underlying SE, suggesting that initiation of perpetuating mechanisms seems to be at least as important as failure of suppressing mechanisms. Second, the two time-points, t1 and t2, are highly relevant clinically (Fig. 2.1): time-point t1 indicates when a seizure is abnormally prolonged and unlikely to stop spontaneously in a given time. There is good evidence from clinical research that this is at 5 min for generalized tonic–clonic seizures [11–14]. There is some evidence that t1 is at about 10 min in focal seizures with or without impairment of consciousness [14]. Thus, in general, t1 is the time when treatment should be started. In individual patients there may be some variability based on prior history and comorbidities. Time-point t2 marks the time at which neuronal damage or alteration of neuronal networks may begin, highlighting the need for aggressive treatment to prevent SE from reaching this stage. Again, as knowledge is incomplete, but given the experimental evidence indicating irreversible brain damage after prolonged seizures [15] and the potential threat of brain damage in humans, the Task Force proposed the continued use of 30 min as time-point t2 in convulsive SE. There is only limited (or no) evidence for when neuronal damage takes place in other forms of SE [8, 15]. Imaging studies in humans suggest that cytotoxic edema occurs after prolonged seizure activity, so t2 was set at 60 min for focal status with impairment of consciousness. The timelines were set primarily for operational purposes, and the timing of the onset of brain damage may vary considerably with age, intensity of SE, and other clinical circumstances. It was the intention of the Task Force to choose time t2 to help provide a practical safe guideline for clinical purposes [8].

The framework for the new 2015 classification was built on four axes: (1) semiology, (2) etiology, (3) EEG correlates, and (4) age. The backbone of the classification is the semiology. Here, the different clinically identifiable forms of SE were divided along two taxonomic aspects: motor activity and impairment of consciousness, yielding two major groups: (A) SE with prominent motor symptoms, including all convulsive forms, and (B) SE without prominent motor symptoms representing the nonconvulsive forms of SE (NCSE) (Table 2.3). Each group can be divided again according to the degree of impairment of consciousness, which is highly relevant clinically. NCSE with coma represents a life-threatening condition that requires urgent and intensive treatment, whereas NCSE without coma occurs most often in the form of absence SE or focal SE with impairment of consciousness (previously called “complex partial SE”), which are often far easier to control than the forms of NCSE associated with coma [16].