Influence of magnetic source imaging for planning intracranial EEG in epilepsy

Abstract

Background: Magnetic source imaging (MSI) is used routinely in epilepsy presurgical evaluation and in mapping eloquent cortex for surgery. Despite increasing use, the diagnostic yield of MSI is uncertain, with reports varying from 5% to 35%. To add benefit, a diagnostic technique should influence decisions made from other tests, and that influence should yield better outcomes. We report preliminary results of an ongoing, long-term clinical study in epilepsy, where MSI changed surgical decisions.

Methods: We determined whether MSI changed the surgical decision in a prospective, blinded, crossover-controlled, single-treatment, observational case series. Sixty-nine sequential patients diagnosed with partial epilepsy of suspected neocortical origin had video-EEG and imaging. All met criteria for intracranial EEG (ICEEG). At a surgical conference, a decision was made before and after presentation of MSI. Cases where MSI altered the decision were noted.

Results: MSI gave nonredundant information in 23 patients (33%). MSI added ICEEG electrodes in 9 (13%) and changed the surgical decision in another 14 (20%). Based on MSI, 16 patients (23%) were scheduled for different ICEEG coverage. Twenty-eight have gone to ICEEG, 29 to resection, and 14 to vagal nerve stimulation, including 17 where MSI changed the decision. Additional electrodes in 4 patients covered the correct: hemisphere in 3, lobe in 3, and sublobar ictal onset zone in 1. MSI avoided contralateral electrodes in 2, who both localized on ICEEG. MSI added information to ICEEG in 1.

Conclusion: Magnetic source imaging (MSI) provided nonredundant information in 33% of patients. In those who have undergone surgery to date, MSI added useful information that changed treatment in 6 (9%), without increasing complications. MSI has benefited 21% who have gone to surgery.

Figure 1 Flowchart of patient decision algorithm The pre–magnetic source imaging (MSI) decision was blinded to MSI. The patients were assigned to one of three treatments. The blind was then broken, and the MSI information was presented. Then a post-MSI decision was made, and the patients were reassigned to one of three treatments. The post-MSI treatments are primed. One-third of the patients were reassigned to a different treatment by the post-MSI decision. VEEG = video-EEG; ICEEG = intracranial EEG; VNS = vagal nerve stimulation.

Figure 2 Patient 33: MSI added information to ICEEG These figures show the actual magnetic source imaging (MSI) data presented at the conferences. (A) First MSI, before intracranial EEG (ICEEG) and before first surgery. The MSI shows a cluster of spike dipoles in the right prefrontal region. MRI in top row: Left to right: Axial, coronal, and sagittal images. Bottom left: Typical spike in MEG. Calibrations: 10 pT, 270 msec. Bottom right: Isocontour maps and butterfly plot of tracings: Blue magnetic flux exiting head, red magnetic flux entering head. MRIs are radiology convention (with left at right of image); spike tracings and MEG map are electrophysiology convention (left at left). (B) Second MSI 18 months later, after first ICEEG and first surgery of right anterior temporal lobectomy (visible on sagittal section), but before second ICEEG and second surgery of right frontal resection. MSI shows similar cluster of spike dipoles in right prefrontal region. Figure setup same as in A. Calibrations: 12 pT, 404 msec.