MEG Node Degree for Focus Localization: Comparison with Invasive EEG

Abstract

Epilepsy surgery is a viable therapy option for patients with pharmacoresistant focal epilepsies. A prerequisite for postoperative seizure freedom is the localization of the epileptogenic zone, e.g., using electro- and magnetoencephalography (EEG/MEG). Evidence shows that resting state MEG contains subtle alterations, which may add information to the workup of epilepsy surgery. Here, we investigate node degree (ND), a graph-theoretical parameter of functional connectivity, in relation to the seizure onset zone (SOZ) determined by invasive EEG (iEEG) in a consecutive series of 50 adult patients. Resting state data were subjected to whole brain, all-to-all connectivity analysis using the imaginary part of coherence. Graphs were described using parcellated ND. SOZ localization was investigated on a lobar and sublobar level. On a lobar level, all frequency bands except alpha showed significantly higher maximal ND (mND) values inside the SOZ compared to outside (ratios 1.11-1.20, alpha 1.02). Area-under-the-curve (AUC) was 0.67-0.78 for all expected alpha (0.44, ns). On a sublobar level, mND inside the SOZ was higher for all frequency bands (1.13-1.38, AUC 0.58-0.78) except gamma (1.02). MEG ND is significantly related to SOZ in delta, theta and beta bands. ND may provide new localization tools for presurgical evaluation of epilepsy surgery.

Keywords: MEG; connectivity; epilepsy; epilepsy surgery; focus localization; graph theory.

Figure 1

Example case of a patient with right fronto-polar focal cortical dysplasia type IIb. The first column shows the iEEG findings; fronto-polar SOZ electrodes are circled. Lower inset shows depth electrodes inserted into the lesion. The circle marker shows the localization from routine MEG analysis based on IEDs. Second column shows MRI postprocessing (voxel-based morphometry, MAP19, combined z-score, thresholded at z > 3) superimposed on the patient’s T1. Third column shows delta band node degree (thresholded at 95th percentile to illustrate maximum, see Supplementary Figure S1 for full distributions). Right-most column shows sagittal slices of node degree in the other frequency bands (thresholded). The patient underwent epilepsy surgery and was seizure-free thereafter for several years; however, the patient then developed recurrent seizures with a reduced frequency.

Figure 2

Overview of localization performance of node degree in different frequency bands for the seizure onset in invasive EEG on a lobar and sublobar (AAL atlas region) level.