Status epilepticus is divided into 2 main subgroups: convulsive and nonconvulsive status epilepticus. Nonconvulsive status epilepticus is used to describe the group of seizures that are characterized by electrographic seizure activity on EEG associated with minimal or no motor movements, typically with associated alteration of awareness (from wandering to comatose). In this article, the authors describe the current thinking and controversies surrounding the definition and classification and the difficulties in diagnosis of nonconvulsive status epilepticus. The authors also provide a treatment framework based on the current literature.
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• Nonconvulsive status epilepticus is a state of ongoing seizure or intermittent seizure activity with minimal or no motor movements and alteration of consciousness.
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• Nonconvulsive status epilepticus occurs both in ambulatory patients with epilepsy and in critically ill patients, especially those with acute brain injury.
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• Ambulatory nonconvulsive status epilepticus (eg, absence nonconvulsive status epilepticus, focal status epilepticus with or without impaired awareness) carries a better outcome than nonconvulsive status epilepticus evolving from convulsive subtle status epilepticus and nonconvulsive status epilepticus in critically ill patients.
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• There is no consensus on the best treatment strategy for nonconvulsive status epilepticus, and most of the recommendations are extrapolated from studies of convulsive status epilepticus.
Historical note and terminology
In perhaps the earliest description of epilepsy with persistent altered consciousness, Trousseau in 1868 noted that repeated petit mal seizures might occur so frequently that one seizure would become confused with the next, simulating a continuous seizure that might persist for days. The term “status epilepticus” (“etat de mal”) was coined in 1824 (45). The idea that seizures could take on a prolonged, nonconvulsive form was highlighted by Bourneville who, in 1878, described 2 forms of status epilepticus: convulsive and meningitic. Meningitic status epilepticus is characterized by subtle or absent convulsions, coma, and autonomic abnormalities. These descriptions might well have constituted nonconvulsive status epilepticus, but the first electroclinical diagnosis occurred in 1945 when Lennox diagnosed absence status epilepticus with EEG correlate in his cousin. Nine years later, Penfield and Jasper described focal status epilepticus with intact awareness, aura continua, in a patient who suffered continual tingling of the left foot, and in 1958 Gastaut published the first case of focal status epilepticus with impaired awareness (18).
Nonconvulsive status epilepticus was initially described in ambulatory patients with chronic epilepsy, but with the evolution of EEG technology it has become evident that nonconvulsive status epilepticus is common among hospitalized patients, especially critically ill patients with unexplained altered mental status. The idea that electrographic seizures can frequently manifest solely as coma in hospitalized patients first became apparent in those presenting with convulsive status epilepticus. In 1984, Treiman and colleagues at UCLA coined the term ”subtle status epilepticus” to describe the late “burned-out” stage of generalized convulsive status epilepticus as consisting of “nystagmus, eye blinking, and/or mild motor movements…with marked impairment of consciousness and continuous or nearly continuous, usually bilateral, ictal patterns on the EEG” (61). They also noted that some patients with this form of nonconvulsive status epilepticus may have had only an isolated convulsion (rather than convulsive status epilepticus) or may have never had a recognized convulsion (60). Subsequent authors have referred to this phenomenon as status epilepticus terminans (22) to distinguish it from other forms of nonconvulsive status epilepticus with little or no anteceding clinical manifestations. The widespread use of digital EEG and prolonged EEG monitoring has made it evident that nonconvulsive status epilepticus without preceding convulsive activity is the more common form of nonconvulsive status epilepticus in hospitalized patients, particularly in children and adults with acute brain injury and patients with severe systemic illness, such as sepsis (14; Drislane 2000; Abend et al 2013; 01).
Initially, nonconvulsive status epilepticus was defined as an ongoing state of nonconvulsive seizure or multiple nonconvulsive seizures without return to baseline for at least 30 minutes (59). A consensus working group refined the definition to account for the heterogeneous etiology, phenomenology, and prognosis of nonconvulsive status epilepticus (Trinka et al 2015). This definition incorporates elements of time (termed T1 and T2) and 4 additional axes (semiology, etiology, EEG correlate, and age). T1 indicates when seizures need to be acutely treated (eg, 10 minutes for focal status epilepticus with impaired awareness and 10 to 15 minutes for absence status epilepticus), and T2 indicates the time at which the likelihood of permanent neuronal injury increases and more aggressive therapy may be justified (more than 60 minutes for focal status epilepticus with impaired awareness and unknown for absence status epilepticus). However, this proposal does not account for age-related differences in the occurrence of T2, where experimental evidence and data from the FEBSTAT trial suggest that the immature brain is less likely to develop permanent injury than the adult brain (38; 43).