Comparison of interictal EEG and MEG II: Loss of deep cortical fissures in polymicrogyria requires simultaneous EEG with MEG source analysis

Purpose: Multiple source analysis of interictal EEG and MEG spikes was applied to the determination of irritative zones in polymicrogyria (PMG). PMG is characterized by a marked loss of deep cortical fissures. Since MEG signals are predominantly generated from tangentially orientated neurons in fissures, an MEG/EEG discrepancy was assumed.

Patients: We studied 7 patients (age 7.5 to 19 years) with localization-related epilepsy and unilateral polymicrogyria (PMG) as defined by anatomical MRI.

Methods: 122-channel whole-head MEG and 32-channel EEG were recorded simultaneously for 25 to 40 minutes. Using the BESA program, interictal spikes were identified visually and used as templates to search for similar spatio-temporal spike patterns throughout the recording. Detected similar spikes (r>0.85) were averaged, high-pass filtered (5Hz) to enhance spike onset, and subjected to multiple spatio-temporal source analysis. Source localization was visualized by superposition on T1-weighted MRI and compared to the lesion.

Results: 9 spike types were identified in 7 patients (2 types in 2 patients). Sources modeling the spike onset activity were localized within the visible lesion in 8/9 EEG and 7/9 MEG spike types. MEG spike onset was significantly preceded by EEG in 2 spike types (19 and 25 ms). The radially oriented onset activity was shown by EEG, while MEG localized already propagated activity. This lead to a mislocalization of the earliest MEG spike activity to the normal hemisphere in one case, while the preceding radial EEG onset activity was localized within the lesion. Distances between EEG and MEG onset sources varied between 9 to 47mm in the 8 spike types with concordant lateralization.

Conclusion: Discrepancies in MEG and EEG were related to the lack of deep fissures in PMG. In two cases, MEG was blind to the onset of radial interictal spike activity and localized propagated spike activity. In two other cases, MEG localized to the more peripheral parts of the irritative zone. Simultaneous EEG recordings with MEG and multiple source analysis are required to avoid problems of MEG interpretation.