Abstracts

Rationale: Cortical dysplasia is a major cause of pharmacoresistant epilepsy. However, small focal cortical dysplasia (FCD) is known to be often overlooked by MRI. In this study, we reviewed patients with small FCD to test whether EEG or MEG spike source localization help to assure the presence of the lesion. Methods: The study included 5 patients with leg sensori-motor seizures. In all patients, MRIs had been reported as normal before referral, while high-field MRI in our hospital showed subtle abnormalities suggesting small FCD. EEG was measured by 42-channel electrodes with approximately double density of standard 10-20 system. MEG was simultaneously recorded using a 160- or 200- channel axial gradiometer system (MEG vision PQ1160C, Yokogawa Electric, Tokyo, Japan). Typical interictal EEG and MEG spikes were identified visually and used as templates to search for similar spatiotemporal spike patterns. Using a spike detection software (BESA Research 5.3, BESA GmbH, Germany), similar spikes were detected, averaged, and high-pass filtered (1.6 Hz) to enhance the spike onset. Single equivalent current dipole model was used to localize the onset-related source. We measured the distance between the center of MRI lesion and the source location of EEG and MEG spikes, respectively. Results: In one patient, the EEG dipole was close to the lesion, while the MEG far from it but correlated with seizure semiology. In another patient, the MEG was close to the lesion as well as correlated with semiology, but the EEG far from the lesion. In the rest 3 patients, both EEG and MEG were close to the lesion and correlated with semiology. Conclusions: The source localization of MEG and EEG spikes was helpful in the detection of small FCD. Either EEG or MEG localized small FCD in all cases, suggesting combination efficacy of EEG and MEG. Mis-localization observed in two cases implies several cautions: 1) EEG and MEG could only detect propagated spike activities; 2) MEG detects only tangential current to the scalp; and 3) EEG predominantly detects radial current.