Abstracts

Rationale: Magnetic source imaging (MSI) is a non-invasive test based on magnetoencephalography spike localization that can supplement or potentially replace the use of intracranial EEG (ICEEG) for seizure localization in some epilepsy surgery patients. This study evaluated whether MSI can supplement ICEEG by effecting electrode placement to improve sampling of the seizure onset zone(s). Methods: Of 264 consecutive epilepsy surgery candidates (between 2001-2006) 160 patients likely to require ICEEG were prospectively enrolled on the basis of insufficient localization from video and ictal scalp EEG (VEEG), and MRI results. Prior to presenting MSI results, decisions were made as to whether to proceed with ICEEG, and if so, where to place electrodes such that the hypothetical seizure onset zone would be sampled. Next MSI results were provided. The only changes to the original plan that could be made were whether electrode coverage should be added to cover a region indicated by MSI that was not included in the original plan. Results: Seventy-seven of the 160 patients had ICEEG recordings to localize seizures. MSI indicated additional electrode coverage in 18 of 77 (23%) cases. Sixty-two patients underwent surgical resection based on ICEEG recording of seizures. The remaining cases were either not satisfactorily localized, seizure onset zone overlapped with functional brain tissue that if removed would cause an unacceptable neurological deficit, or no seizures were recorded during at least 7 days of monitoring. Two of the 18 patients of interest did not have surgery: one with orbital frontal seizures appearing to arise from MSI indicated additional electrode coverage (still insufficiently sampled for satisfactory surgical planning), and the second had seizure localization overlapping with posterior lateral temporal language cortex, also in a region of additional coverage indicated by MSI. By Wilcoxon Signed-Ranks Test based on Engel I-IV classification, post surgical outcome (mean=3.4 years, minimum > 1 year) was not significantly different between those patients who had additional ICEEG electrode coverage (n=16) and those with no added electrodes (n=46). For the entire group (n=62) tightly localized MSI was significantly associated with seizure-free outcome (P < 0.05). Conclusions: MSI spike localization increases the chance that the seizure onset zone is sampled when patients undergo ICEEG for presurgical epilepsy evaluations. The clinical impact of this effect--improving yield of ICEEG in regard to proportion of diagnostically conclusive evaluations--should be considered in clinically defined surgery candidates that do not have satisfactory indication of epilepsy localization from other non-invasive tests.