Dramatic effect of ethosuximide on epileptic negative myoclonus: implications for the neurophysiological mechanism.

Epileptic negative myoclonus (ENM) is a recently defined epileptic seizure type seen in various epileptic syndromes. Although the long-term prognosis appears to be favorable, the treatment of localization-related epilepsy (LRE) with ENM in childhood is sometimes difficult due to the apparently pharmaco-resistant nature of ENM. We evaluated the effects of antiepileptic drugs (AEDs) in 10 patients with ENM. Carbamazepine was administered to eight patients, none of whom improved. Responses to clonazepam and valproic acid were unpredictable, whereas ethosuximide (ESM) achieved complete control of ENM in all six cases treated with this drug as adjunctive therapy. The pharmacological responses of ENM to CBZ and ESM were quite similar to those of absence seizures. According to the SPECT and ictal EEG findings in addition to the pharmacological responses from this study, we favor to postulate that ENM is produced by a direct inhibitory action on the motor cortex resulting in the interruption of voluntary muscle contraction as generated by sharp-slow wave complexes, compatible with the mechanism considered to underlie absence seizures. ENM are refractory to treatment and persisting if the wrong AEDs, such as PHT or CBZ, are selected at the diagnosis of LRE. We recommended a trial of ESM when ENM develops during the clinical course of LRE regardless of etiology.