Unknown; 0.5% to 1% of selected population of children with severe forms of epilepsy.

Age at onset
1 day to 5 years; peak at 12 months.

2-fold female preponderance.

Neurological and mental state
Abnormal (76%).

(1) chromosomal abnormalities and mainly Angelman syndrome (49%); (2) fetal/neonatal brain hypoxia (20%); and (3) malformations of cortical development and other lesions (31%).

Clinical manifestations
Atypical status epilepticus of myoclonic jerks and subcontinuous absences that are repetitive and long (sometimes for days). The myoclonic jerks involving eyelids, face, and limbs are mostly erratic and asynchronous, becoming more rhythmic and synchronous during the absences. The myoclonic jerks are often inconspicuous and the infants may appear just apathetic and ataxic.

Myoclonic status may be the first seizure manifestation but in others the initial seizures are mostly focal motor seizures, myoclonic absences, massive myoclonias, or, more rarely, generalized or unilateral clonic seizures recurring only during febrile illness in some cases. Tonic seizures are never observed.

Many patients also have frequent and sudden spontaneous massive startle attacks of brief and abrupt loss of postural tone as well as long-lasting bursts of intentional myoclonus or tremor.

Diagnostic procedures
Chromosomal analysis, brain imaging, and EEG.

Inter-ictal EEG
Abnormal background with diffuse or focal slow activity and asymmetrical, mainly frontocentral or posterior, paroxysms of 3 to 6 Hz waves, occasionally with spikes.

Ictal EEG
Variable, with mainly paroxysms of 3 to 6 Hz slow waves with superimposed spikes or paroxysms of <2 Hz spike-slow waves.

Poor, with progressive deterioration to severe neurological and mental deficits. The myoclonic status improves with age, but the patients rarely achieve a relatively normal state.

Differential diagnosis
Progressive encephalopathies such as late infantile form of ceroid-lipofuscinosis, migrating focal seizures in infancy, and Dravet syndrome.

Management options
There is no effective treatment other than benzodiazepines, which transiently interrupt the myoclonic status epilepticus.

This section was adapted from:

The educational kit on epilepsies: The epileptic syndromes By C. P. Panayiotopoulos Originally published by MEDICINAE, 21 Cave Street, Oxford OX4 1BA
First published 2006 and reprinted in 2007. The Educational Kit on Epilepsies was produced through an unrestricted educational grant from UCB Pharma SA.
UCB Pharma SA assumes no responsibility of the views expressed and recommended treatments in these volumes.

Authored By: 
C. P. Panayiotopoulos MD, PhD, FRCP
Authored Date: 
Reviewed By: 
Steven C. Schachter MD
Sunday, June 1, 2008