Abstracts

Rationale: Atonic seizures are common in some epileptic syndromes beginning in infancy or early childhood but rarely described in structural epilepsy. We aimed to systematize the electro-clinical features of atonic seizures in surgically remediable pediatric patients and addressed the question if their occurrence may reflect particular organization of the epileptogenic zone (EZ).Methods: From a total of 186 surgically treated pediatric epilepsy patients explored at the Strasbourg University and at the Kork epilepsy centers over a period between January 1998 and December 2010 we identified 8 with a history of atonic seizures. 4 of them were included in this study according to the following criteria: structural epilepsy started under the age of 16 years, atonic seizures documented by scalp video-EEG, resective epilepsy surgery with a minimum follow-up of 12 months. We analyzed clinical history, video-EEG and neuroimaging and correlated the findings with the EZ defined by scalp or invasive EEG and outcome after surgery.Results: All patients were male with mean age of 15 months at epilepsy onset; atonic seizures were first documented at mean age of 19 months. A total of 20 atonic seizures were analyzed. The negative motor semiology comprised an axial atonia with head and arms dropping and loss of postural tone leading to a fall. This core ictal phenomenon was preceded by few bilateral myoclonic jerks in one and by a brief behavioral arrest in three other cases. The mean latency before the appearance of atonic phenomenon was 6 sec. The fall duration did not exceed 2 sec in all but one patient. The atonic phase was followed by contralateral or bilateral dystonic arm posturing in two cases. Ictal EEG activity was either immediately generalized or briefly localized over a pre-central region with fast bilateral spread. Neuroimaging revealed extended lesions situated in the dorsolateral premotor region in front of or adjacent to the primary motor cortex. The lesional cortex included the primary negative motor area (NMA) in one case. This patient showed particularly short latencie before the appearance of atonic phenomenon and brief seizure duration. Two children were reevaluated at the age of 7 and 16 years respectively. The ictal semiology comprised asymmetric tonic posturing and head deviation but no more atonic seizures while ictal EEG pattern did not change. The SEEG-recordings confirmed the EZ localized within the lesional and perilesional dorsolateral premotor cortex. All patients became completely seizure-free after surgery. Histopathology showed focal cortical dysplasia in all cases.Conclusions: The atonic seizures in our series were rare, limited to the first years of life and associated with an EZ localized within the dorsolateral premotor region not necessarily including the NMA. These features may reflect interdependencies between the epileptogenic structures and the adjacent functional areas within the motor network characterized by enhanced and wide spread interconnectivity due to the maturation process.