Abstracts

RATIONALE: Magnetic Source Imaging (MSI) is utilized for pre-surgical localization of epileptic foci, but it is often difficult to determine apriori when the MSI data will accurately predict the ECoG-determined zone of sezire origin. To better illucidate this relationship we correlated MSI and ECoG results for patients with neocortical epilepsy. Our primary objective was to understand how MSI data can be incorporated into surgical planning for neocortical epilepsy
METHODS: Twenty-three patients with suspected focal epilepsy underwent MEG/MSI studies at our institution and subsequently had invasive intracranial electrode monitoring (ECoG) to localize the zone of seizure origin for surgical resection. MSI results were retrospectively stratified by the number of IIS recorded during a 4 hour recording session into 3 groups: Class I (no spikes), Class II ([lt]5 spikes), and Class III ([gt]6 spikes). Class III was further subdivided based on the clustering density of IIS: Class IIIA= [gt]4mm mean distance between IIS; Class IIIB [lt]4mm mean distance between IIS. We analyzed these groups to determine whenMSI results correlated with the ECoG-determined zone of seizure origin. In addition, we assessed if the MSI provided critical localization data and correlated with surgical outcome.
RESULTS: Twenty-three patients with MSI studies underwent invasive monitoring, including 19 with suspected neocortical epilepsy and 4 with mesial temporal lobe epilepsy. Depth electrodes were utilized in 9 cases, subdural grids in 9 cases, depth electrodes followed by subdural grids and strips in 4 cases, and intraoperative ECoG only in 1 case. ECoG was able to localize the zone of seizure origin in 16/23 (70%) of these cases. In 11 of the 16 (69%) cases in which the ECoG was able to localize the zone of seizure origin, the MSI IISs were classified as Class IIIB (many, diffuse) and regionally correlated to the MSI localization in all cases (i.e. same lobe). In contrast, no Class IIIB cases were identified when ECoG was unable to localize the zone of seizure origin. This difference trended toward but did not achieve statistical significance (p [lt].0.23), presumably due to the relatively small number of cases available for analysis. In 3 cases (all Class IIIB) MSI was used to focus invasive electrodes in locations that would not have otherwise been targeted and provided unique localization data not evident from other imaging modalities that strongly influenced the surgical management of the patient. The classification of MSI findings into subgroups and subsequent statistical analysis generated a model predicting that Class IIIB MSI data are likely to provide reliable information to guide surgical placement of electrodes, but all other groups do not provide reliable enough localization information to guide surgical decision-making.
CONCLUSIONS: When an MSI study revealed six or more IIS densely clustered in a single anatomic location, then the MSI is likely to correspond to the zone of seizure origin identified by ECoG and may be useful to focus the placement of intracranial electrodes. In all other situations MSI does not appear to easily predict the zone of seziure origin, and has minimal influence on surgical planning.
[Supported by: NIH NS20806 from the Epilepsy Branch and NCRR RR13176]