Myoclonic-astatic epilepsy of early childhood--clinical and EEG analysis of myoclonic-astatic seizures, and discussions on the nosology of the syndrome.

PURPOSE: The aim of this study is to elucidate the clinical and neurophysiological characteristics of the myoclonic, myoclonic-astatic, or astatic seizures in patients with myoclonic-astatic epilepsy (MAE) of early childhood, and to discuss on the nosology of this unique epileptic syndrome.
SUBJECTS: The subjects included 30 patients, who fulfilled the following modified International League Against Epilepsy (ILAE) criteria for MAE, and whose main seizures were captured by video-electroencephalographs (EEG) or polygraphs. The modified ILAE criteria includes: (1) normal development before onset of epilepsy and absence of organic cerebral abnormalities; (2) onset of myoclonic, myoclonic-astatic or astatic seizures between 7 months and 6 years of age; (3) presence of generalized spike- or polyspike-wave EEG discharges at 2-3 Hz, without focal spike discharges; and (4) exclusion of severe and benign myoclonic epilepsy (SME, BME) in infants and cryptogenic Lennox-Gastaut syndrome based on the ILAE definitions.
RESULTS: The seizures were investigated precisely by video-EEG (n=5), polygraph (n=2), and video-polygraph (n=23), which identified myoclonic seizures in 16 cases (myoclonic group), atonic seizures, with or without preceding minor myoclonus, in 11 cases (atonic group), and myoclonic-atonic seizures in three cases. All patients had a history of drop attacks, apart from ten patients with myoclonic seizures. Myoclonic seizures, involving mainly the axial muscles were classified into those with mild intensity not sufficient to cause the patients to fall (n=10) and those that are stronger and sufficient to cause astatic falling due to flexion of the waist or extension of the trunk (n=6). Patients in the atonic group fell straight downward, landed on their buttocks, and recovered immediately. Analysis of the ictal EEGs showed that all attacks corresponded to the generalized spike or polyspikes-and-wave complexes. In the atonic form, the spike-and-wave morphology was characterized by a positive-negative-deep-positive wave followed by a large negative slow wave. In two patients, the intensity of the atonia appeared to correspond to the depth of the positive component of the spike-and-wave complexes. We did not detect any significant differences in the clinical and EEG features and prognosis, between the atonic and myoclonic groups.
CONCLUSIONS: Although the determination of exact seizure type is a prerequisite for diagnosing an epileptic syndrome, the strict differentiation of seizure type into either a myoclonic or atonic form, does not appear to have a significant impact on the outcome or in delineating this unique epileptic syndrome. At present, we consider it better to follow the current International Classification of Epileptic Syndromes and Epilepsies until a more appropriate system than the clinico-electrical approach for classifying patients with MAE is available.