There are as many types of status epilepticus as there are types of epileptic seizures. Myoclonic status epilepticus is defined as a condition in which generalized myoclonic jerks are repeated continuously or occur in clusters lasting for a sufficiently long period, usually >30 minutes.

Myoclonic status is divided into two types—pure and symptomatic.^12,31 The former is observed in epileptic patients. The latter is associated with degenerative encephalopathies, that is, the dyssynergia cerebellaris myoclonica of Ramsay-Hunt, progressive myoclonus epilepsy, and, especially, Lafora disease; with various lipidoses or metabolic (hepatic and renal) encephalopathies; with toxic encephalopathies; and with severe anoxic encephalopathies. However, Treiman⁴² argued that symptomatic myoclonic status epilepticus should be considered a subtle presentation of generalized convulsive status and that the term “myoclonic status epilepticus” should be used only for pure myoclonic status. Therefore, this chapter discusses only pure myoclonic status epilepticus.

Pure myoclonic status epilepticus is seen in patients with generalized epilepsy, especially in types that exhibit myoclonic seizures. Pure myoclonic status is further subclassified into primary and secondary forms.¹²

Primary myoclonic status epilepticus is regarded as the prototype of myoclonic status epilepticus and is observed in patients with one of the primary (idiopathic) generalized epilepsies, such as juvenile myoclonic epilepsy.³

Secondary myoclonic status epilepticus occurs in children with one of the secondary (symptomatic) generalized epilepsies,¹² such as myoclonic-astatic epilepsy, Lennox-Gastaut syndrome, and epilepsy with myoclonic absences. This secondary form has also been reported as minor epileptic status^5,6,36 and has been observed in myoclonic epilepsy (“myoclonic status”) in nonprogressive encephalopathies,⁸ and in association with obtundation status in severe myoclonic epilepsy in infancy.¹⁰ Status epilepticus associated with negative myoclonus is also classified as a type of secondary myoclonic status epilepticus and is seen in epilepsy with continuous spike-waves during slow-wave sleep (CSWS)⁴⁰ and its related epilepsies.^22,26,29 The Commission on Classification and Terminology of the International League Against Epilepsy classifies severe myoclonic epilepsy in infancy and epilepsy with CSWS into “epilepsies and syndromes undetermined whether focal or generalized.”⁷

Although the incidence and prevalence of the primary and the secondary types of myoclonic status epilepticus have not been precisely delineated, the primary type is regarded as relatively rare.^11,12 In contrast, the secondary type of myoclonic status is far more frequent.¹²

There are various etiologies of pure myoclonic status epilepticus, and the underlying pathophysiology of myoclonic status epilepticus is unclear.

Recently, seizure aggravation by antiepileptic drugs (AEDs), especially a worsening of seizures resulting from an inappropriate choice of an AED, has been attracting much attention.^13,30 Although in this case AEDs may provoke aggravation by a purely pharmacodynamic mechanism, the precise biological mechanisms involved are unclear. Aggravation by AEDs is probably more common in children than in adults.^13,30 Potential AEDs include carbamazepine, phenytoin, phenobarbital, vigabatrin, lamotrigine, gabapentin, and tiagabine.^13,30 Generalized seizures such as myoclonic seizures and absences are aggravated,^30,35,37 sometimes evolving into nonconvulsive status epilepticus such as myoclonic status.¹³ Although such aggravation usually occurs in generalized epilepsies, it is also observed in severe myoclonic epilepsy in infancy,¹⁰ in epilepsy with CSWS and its related epilepsies,^22,26,29 and even in localization-related epilepsies.^28,35,37

Clinical and electroencephalogram (EEG) findings suggest that perhaps the secondary type of myoclonic status epilepticus should be considered a modified form of atypical absence status.¹² In some cases diagnosed as absence status, however, myoclonus constitutes the main symptom rather than a consciousness disturbance.^41,43 Differentiation between myoclonic status epilepticus with absence features and absence status epilepticus with myoclonic features remains a problem in the classification of status epilepticus.

Several investigators have attempted to classify myoclonus based on its pathophysiology.^17,18,23 Electrophysiologic examinations of myoclonus, such as jerk-locked averaging, somatosensory-evoked potentials (SEPs), enhanced long-latency reflexes (C reflexes), magnetoencephalography, and transcranial magnetic stimulation, have been performed to investigate the pathophysiology of myoclonus.^33,34

Shibasaki et al.^33,34 classified myoclonus into three types (cortical, subcortical, and spinal myoclonus) based on the presumed physiologic mechanism underlying its generation. They emphasized the existence of cortical reflex myoclonus in which
a myoclonus-related cortical spike was detected by jerk-locked averaging, and a giant SEP accompanied by an enhanced C reflex was observed. They speculated that the sensory and motor cortices mediated the occurrence of cortical reflex myoclonus. However, the pathophysiology of myoclonic seizures in epilepsy has not been fully elucidated.

In cortical myoclonus, the EEG usually shows multifocal or generalized spike-wave or multiple spike-wave discharges with or without associated myoclonus. On the conventional polygraph, however, the temporal and spatial relationships between myoclonus and its EEG correlate are often difficult to determine quantitatively.

Simultaneous EEG and electromyogram (EMG) polygraphic recording demonstrates that muscular inhibition, namely, negative myoclonus, can arise from the central nervous system in addition to positive myoclonus, which refers to sudden, brief, shock-like involuntary movements caused by muscular contractions. Negative myoclonus of cortical origin also can be associated with an EEG spike or spike-wave complex. Again, however, it is difficult to determine precisely the temporal and spatial relationships between the EMG silent period and the associated EEG spike on the conventional polygraph. Furthermore, because the silent period tends to be preceded or followed by an abrupt EMG discharge (positive myoclonus),⁴⁶ it is often difficult to judge whether the detected EEG spike is directly related to the positive component or the negative component of the EMG discharge. Myoclonic status epilepticus characterized by negative myoclonus has also been described.^22,26,28,29

In the primary type of myoclonic status epilepticus, consciousness is basically preserved in spite of long-lasting seizures. A polygraphic recording of the myoclonic jerks³¹ shows brief muscular contractions of 100- to 200-msec duration repeated at a rate of 3 to 6 per second. These contractions are diffuse, predominating over the proximal muscles of the extremities, and they are bilaterally synchronous in homologous regions.