Abstracts

Rationale: Bilateral interictal epileptiform discharges, reported in 8-70% of patients with unilateral temporal lobe epilepsy (TLE), have low predictive value for the diagnosis of bilateral independent TLE (BiTLE). However, the predictive value of the ictal EEG is not well known. We explored the likelihood of true BiTLE in patients whose ictal scalp EEG suggested BiTLE. Methods: We reviewed our epilepsy monitoring unit database from July 2003 to Dec 2010 to identify patients with an initial diagnosis of presumed BiTLE based on ictal scalp EEG, who subsequently underwent intracranial EEG (IEEG) monitoring. Variables of interest included demographic variables, clinical variables (e.g. epilepsy duration, seizure types, etc), MRI variables, scalp EEG variables (percent lateralization, etc), intracranial EEG variables (ictal onset, etc) and surgical outcomes (Engel classification for seizures, etc). Descriptive statistics were calculated. Results: Of 917 patients admitted to the epilepsy monitoring unit, 35 patients had an initial diagnosis of presumed BiTLE based on ictal scalp EEG. Eighteen of them (50%) (11 males) underwent IEEG with bilateral temporal and some extra-temporal coverage (our study population). Average age and duration of epilepsy were 28.7 and 14.2 years, respectively. Ten patients (56%) had true BiTLE on IEEG, of which three underwent a temporal resection. One of these patients had a tumour (Engel Class I) and the other two had no MRI lesions (1 Engel Class I, 1 Engel Class IV) at >3 year follow up. Eight patients did not have BiTLE on IEEG. Seven of them had unilateral TLE while one had frontal epilepsy. Of the seven with unilateral TLE, five had a temporal resection. Two of these patients had an MRI lesion (Engel class I) and the remaining three had no MRI lesion (Engel Class I in one, Engel Class IV in two). Conclusions: About 40% of patients with ictal scalp EEG patterns simulating BiTLE turn out to have unilateral TLE. IEEG is crucial to determine if patients have BiTLE or even extratemporal epilepsy. However, in those with BiTLE and no lesion on MRI, the role of IEEG to determine surgical candidacy (and laterality) requires further exploration.