Abstracts

Rationale: The question has arisen whether interictal magnetic source imaging (MSI), a non-invasive test, may in the future replace ictal EEG recordings from implanted intracerebral electrodes in the presurgical evaluation of patients with medically intractable epilepsy, hypothesized to be focal. (Knowlton et al 2006). To gather more information to answer this question, we examined the experience at our medical center with MSI in the presurgical evaluation of patients who underwent intracerebral depth electrode placement for chronic invasive EEG recording. The purpose of this study was to compare the MSI results with the intracranial EEG findings, with respect to the localized epileptogenic zone. Methods: Using the UCLA Seizure Disorders Center surgical database, the diagnostic study results of 40 consecutive patients with medically intractable epilepsy who underwent implantation of intracerebral depth electrodes at UCLA Medical Center by one surgeon (I.F.) between May 2000 and May 2005 were retrospectively reviewed. Of those 40 implanted patients, 25 were identified as having undergone pre-implant interictal magnetoencephalography. Results: For these 25 implanted patients, comparing the ictal onset zone(s) identified by their ictal EEG recordings with the irritative zone(s) identified by interictal MSI yielded the following: 9/25 had interictal depth EEG and interictal MSI results which correlated precisely; 7/25 had EEG and MSI findings which identified the same lobe (temporal) as the epileptogenic zone but differed in the region of the temporal lobe identified; and 9/25 did not correlate. Of the 9/25 cases which did not correlate, there were no MEG spikes in three cases, the localized anatomic regions were different in three cases, there were bilateral localizations via MSI yet unilateral ictal onset zone in two cases, and no localized ictal onset zone via invasively recorded ictal EEG in one case. Conclusions: These data suggest that interictal MSI predicts the invasive electrode defined ictal onset(s) in some patients, while in other patients confounding localizations may occur. Identifying which patients would benefit from MSI and for which patients MSI cannot obviate the need for ictal EEG recorded via invasive electrodes will require investigation of a larger number of patients, as well as analysis of post-surgical clinical outcomes confirmatory of the invasive electrode localizations.