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  • Updated 09.20.2020
  • Released 10.29.1999
  • Expires For CME 09.20.2023

Myoclonic status epilepticus



The term “myoclonic status epilepticus” and its variations, including “prolonged myoclonus,” “status myoclonus,” “status myoclonicus,” and “myoclonic status,” have been used to describe a variety of clinical states that have in common a prolonged period of frequent spontaneous myoclonic jerks. Myoclonic status may be associated with a wide range of etiologies, including anoxic brain injury, toxic-metabolic encephalopathies, and exacerbations of certain epilepsy syndromes. The clinical presentation and significance of frequent myoclonic jerks differs greatly by etiology. In this article, the authors discuss the classification, clinical presentations, etiologies, and management of conditions typically categorized as subtypes of myoclonic status epilepticus. They also discuss controversies of prognostic significance and management of myoclonic status epilepticus following anoxic brain injury, including data to suggest possible clinical and EEG predictors for neurologic outcomes.

Key points

• The term “myoclonic status epilepticus” has been used to describe a wide array of clinicoelectrographic presentations, with varied prognostic and treatment implications.

• Myoclonic status epilepticus has been described in generalized epilepsy syndromes, neurodegenerative disease, infectious or inflammatory neurologic disease, toxic-metabolic states, and following anoxic brain injury.

• The presentation of and approach to myoclonic status epilepticus depends largely on the underlying etiology.

• Although traditionally considered a poor prognosticator following cardiac arrest, the presence of myoclonic status epilepticus should be evaluated in concert with other clinical and neurophysiologic variables when making a prediction of outcome, as a significant minority of these patients will have favorable outcome.

Historical note and terminology

The term myoclonic status epilepticus and its variations, including prolonged myoclonus, status myoclonus, status myoclonicus, generalized myoclonic status epilepticus, and myoclonic status have been used to describe a variety of clinical states that have in common a prolonged period of frequent spontaneous myoclonic jerks but otherwise differ substantially. There is no consensus on the correct application of these terms though the prognostic and treatment implications of frequent myoclonic jerks varies greatly by etiology and electroclinical features. For example, “myoclonic status” and “myoclonic status epilepticus” are often used to describe a clinical presentation initially described by Brett as “minor epileptic status” (13; 70). He described a fluctuating state of frequent myoclonic or atonic seizures sufficient to mimic ataxia and cause severe functional impairment. This occurs predominantly in children with mental retardation and epilepsy and was associated with variable degrees of impaired consciousness. Celesia and colleagues used the term “generalized status myoclonicus” to describe 19 critically ill adults with a very different presentation involving frequent myoclonic jerks (19). They defined “generalized status myoclonicus” as a “fixed and enduring state lasting at least 30 minutes and characterized by continuous generalized, and at times asynchronous, rhythmic myoclonic jerks incessantly repeated at a frequency of 1 to 5 seconds.” Eighteen of these patients were comatose during the frequent myoclonic activity. The majority of patients (67%) had suffered anoxic brain injury, and the others had significant toxic-metabolic disturbances such as renal or liver failure. Celesia and colleagues emphasized that generalized status myoclonicus was a clinical, not electrographic, definition and found that scalp EEGs in these patients varied (although most had a correlate for myoclonic jerks on scalp EEG, 2 did not). In contrast, in a retrospective review of hospitalized patients, Jumao-as and Brenner used a strict electroclinical definition to identify 23 patients with a very similar presentation, which they called “myoclonic status epilepticus” (53). They defined myoclonic status epilepticus as a prolonged (more than 30-minute) period of myoclonic jerks that were correlated with epileptiform discharges on EEG. Despite the different inclusion criteria, the affected patients in this study were very similar to those with “generalized status myoclonicus”: the majority (65%) had encephalopathy from anoxic brain injury. Other etiologies included metabolic encephalopathy (17%), epilepsy (9%), and dementia (8%).

The most commonly cited classification of myoclonic status epilepticus was proposed by Gastaut (39). Gastaut suggested that myoclonic status epilepticus should be divided into “true” and “symptomatic” forms. In this classification, true myoclonic status epilepticus refers to myoclonic status epilepticus in patients with epilepsy and can be further characterized as primary myoclonic status epilepticus or secondary myoclonic status epilepticus, a distinction relating to the original concept of primary versus secondary generalized epilepsy (61). Thus, under the prior International League Against Epilepsy’s classification of epilepsy syndromes, primary myoclonic status epilepticus refers to myoclonic status epilepticus in patients with idiopathic generalized epilepsy, and secondary myoclonic status epilepticus refers to myoclonic status epilepticus in patients with symptomatic or cryptogenic generalized epilepsy (22). Under the newly-adopted classification system, primary myoclonic status epilepticus would be seen in patients with idiopathic or genetic generalized epilepsy, and because the concepts of “symptomatic” and “cryptogenic” have been removed, secondary myoclonic status epilepticus would best correspond to myoclonic status epilepticus in patients with developmental and epileptic encephalopathies (of genetic or unknown cause) (06; 84). The 1989 classification system is used below, as it more clearly corresponds to Gastaut’s organization.

In Gastaut’s classification system, symptomatic myoclonic status epilepticus was reserved for myoclonic status epilepticus that occurs as a result of infectious, inflammatory, neurodegenerative, toxic-metabolic, or anoxic brain disease. Because it occurs in patients without a prior diagnosis of epilepsy, it is the symptomatic form of myoclonic status epilepticus that several authors (including Gastaut) have proposed should be renamed “status myoclonus” or “status myoclonicus” (39; 19; 100). However, as myoclonic activity involving the cortex can be considered an epileptic seizure, we think it is reasonable to use “myoclonic status epilepticus” as an umbrella term to describe frequent myoclonus of presumed or certain cortical origin regardless of etiology. This applies to all conditions under Gastaut’s original classification of myoclonic status epilepticus, although many subtypes involve myoclonus of cortical or subcortical origin. We exclude from this classification conditions in which myoclonus is never primarily cortical in origin, such as propriospinal myoclonic status and opsoclonus-myoclonus.

Myoclonic status epilepticus classification
The term myoclonic status epilepticus has been applied to many disparate clinical presentations with a wide range etiologies. Gastaut’s proposed classification system divides myoclonic status epilepticus into “true myoclonic statu...

There is no definition of the duration or frequency of myoclonic jerks required to qualify as myoclonic status, but they should occur frequently and long enough to significantly impair functioning. A reasonable general definition might be that myoclonus must occur either (1) at least once every 10 seconds for longer than 10 minutes or (2) at least once a minute for longer than 30 minutes.

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