Abstracts

Rationale: Infrequently, in the evaluation of candidacy for temporal lobe epilepsy surgery, bilateral independent ictal onsets are confirmed on scalp EEG. Mechanisms of independent bilateral expression are poorly understood. Although mesial sclerosis is the hallmark lesion, cryptogenic cases are represented in abundance. Multiple video EEG recording sessions are often required for confirmation (Koukou et al 2007). As many as 25% of surgical failures have been attributed to the elaboration of contralateral 'discordant ' seizures in post-operative assessment (Hennessy et al 2000). We reviewed our experience to compare with other centers' series.Methods: We queried our epilepsy monitoring database between 8/03 and 12/06 to identify cases of confirmed independent temporal lobe ictal onsets based on scalp EEG criteria . Retrospective chart review of verified cases was undertaken to identify risk factors, lesional basis (if any), therapeutic history and circumstance of detection.Results: 8 cases of bilateral independent temporal lobe onsets were found. 6/8 were cryptogenic/ non-lesional. 1/8 was associated with severe antecedent head trauma and massive bitemporal encephalomalacia on MRI. 1/8 was a patient with presumed genetically based partial seizures and mental retardation that is shared with an identical twin and older brother. 3/8 required a second monitoring experience to establish bilateral ictal behavior, including one case revealed after surgical failure for presumed unitemporal epilepsy. In one unique case, 7 consecutive seizures clustered left temporal, followed by exactly alternating right and left seizures for 14 more ictal events. All patients were refractory to multiple anticonvulsant regimens as well as vagal nerve stimulation. 2/8 are undergoing presurgical assessment for potential resection of the site of most frequent onset. Conclusions: Multiple risk factors and pathologic substrates of bilateral independent foci were identified . All cases share a history of resistance to non-resective strategies. Sequential emergence in several cases implies unique facilitory and inhibitory cyclic behaviors . Functional relationships have been documented between symmetric temporal lobe ictal zones, most commonly mutually suppressive (Chkhenkeli et al 2007). Suppressive reciprocal relationships may account for sampling error as well as the 'cluster effect' (Haut et al 1997)), necessitating the recording of a large number of seizures to confirm bilaterality. Not assuming simultaneity in genesis, the mechanisms and chronology of unitemporal to bitemporal expression is not understood. The surgical strategy of unitemporal resection has a variable outcome, including fatigue of the discordant side as well as reports of increased autonomy and refractoriness of the remaining contralateral ictal zone. Due to elusive diagnosis coupled with therapeutic resistance, bilateral temporal lobe epilepsy may be considered the maximal negative expression of temporal-based epilepsies and is likely more common than currently appreciated.