Electromagnetic source imaging in presurgical workup of patients with epilepsy: A prospective study

Abstract

Objective: To determine the diagnostic accuracy and clinical utility of electromagnetic source imaging (EMSI) in presurgical evaluation of patients with epilepsy.

Methods: We prospectively recorded magnetoencephalography (MEG) simultaneously with EEG and performed EMSI, comprising electric source imaging, magnetic source imaging, and analysis of combined MEG-EEG datasets, using 2 different software packages. As reference standard for irritative zone (IZ) and seizure onset zone (SOZ), we used intracranial recordings and for localization accuracy, outcome 1 year after operation.

Results: We included 141 consecutive patients. EMSI showed localized epileptiform discharges in 94 patients (67%). Most of the epileptiform discharge clusters (72%) were identified by both modalities, 15% only by EEG, and 14% only by MEG. Agreement was substantial between inverse solutions and moderate between software packages. EMSI provided new information that changed the management plan in 34% of the patients, and these changes were useful in 80%. Depending on the method, EMSI had a concordance of 53% to 89% with IZ and 35% to 73% with SOZ. Localization accuracy of EMSI was between 44% and 57%, which was not significantly different from MRI (49%-76%) and PET (54%-85%). Combined EMSI achieved significantly higher odds ratio compared to electric source imaging and magnetic source imaging.

Conclusion: EMSI has accuracy similar to established imaging methods and provides clinically useful, new information in 34% of the patients.

Classification of evidence: This study provides Class IV evidence that EMSI had a concordance of 53%-89% and 35%-73% (depending on analysis) for the localization of epileptic focus as compared with intracranial recordings-IZ and SOZ, respectively.

Figure 1. Methodologic flowchart of EMSI

EMSI was performed using the following 4 steps: (1) review of the magnetoencephalography and EEG recordings and visual identification and marking of EDs belonging to the same cluster. These EDs were used for template-matching. Each detected spike was visually checked and artifacts were discarded. (2) EDs within each cluster were averaged to improve signal-to-noise ratio (critical for spike onset activity relative to the background activity). Sequential topographic plots of the ascending phase and principal components analysis was used to identify propagation. (3) Individual head model was created for each patient, and the EEG electrodes were aligned to the scalp. (4) Source modeling was performed using 2 different inverse-solution strategies: equivalent current dipole and distributed source models where yellow indicates maximum intensity. ED = epileptiform discharge; EMSI = electromagnetic source imaging; IED = interictal epileptiform discharge.

Figure 2. Electromagnetic source imaging

Electromagnetic source imaging (equivalent current dipole and distributed source model) for a patient with frontal (A and B) and temporal (C and D) focus. Analysis was done using CURRY (A and C) and BESA (B and D) software. Figures in appendix 2 (data available from Dryad, doi.org/10.5061/dryad.p4r01pq) show preoperative sources coregistered with postoperative MRI for these patients.

Figure 3. Flowchart of the presurgical evaluation for the 141 recruited patients

Red arrows and boxes indicate that operation was not offered and green arrows and boxes indicate that operation was indicated, by the MDT. *At this stage in the flowchart, the MDT made 2-step decisions: first blinded to EMSI, then including EMSI results. **One patient died of acute myocardial infarction and one patient died of sudden unexpected death in epilepsy. EMSI = electromagnetic source imaging; ICR = intracranial recording; MDT = multidisciplinary team; OP = operation.

Figure 4. Clinical utility of EMSI

In 34% of patients (29/85), EMSI changed the management plan. The changes were distributed as follows: stop → implantation of intracranial electrodes, 6/85 (7%); implantation → stop 1/85 (1%), change in the location of implanted electrodes, 14/85 (16.5%); skipping implantation and going directly to operation, 8/85 (9.4%). EMSI = electromagnetic source imaging; IC = intracranial electrodes.