Abstracts

Rationale: Massive, asymmetric brain abnormalities are not uncommon in the intractable epilepsy population referred to tertiary centers, especially in the pediatric age group. In those being considered for epilepsy surgery, precise localization of epileptogenic foci is required, but EEG localization is almost always very difficult. Because it is insensitive to tissue inhomogeneities and has a higher spatial resolution, MEG source localization can be useful in these cases. Methods: We included 4 consecutive medically refractory epilepsy cases whose imaging studies showed massive asymmetric brain abnormalities: Case 1 (21 year old, male): large open lip schizencephaly in the right hemisphere, Case 2 (4, boy): massive dilatation of right lateral ventricles in the setting of sequalae of prematurity, Case 3 (4, boy): after left anatomic hemispherectomy for hemimegalencephaly, and Case 4 (11, boy): after right functional hemispherectomy for hypoxic-ischemic insult and malformation of cortical development. All of these patients underwent video-EEG monitoring (VEEG) and MRI. MEG was recorded by using a 204 planar gradiometer MEG system (Neuromag, Helsinki, Finland) with simultaneous EEG. Source localization of interictal epileptiform discharges was obtained using standard equivalent current dipole (ECD) methods. Results: Case 1: VEEG showed multiregional sharp waves in the left and right hemispheres. Right hemispheric sharp waves were very low voltage on EEG. It also showed one non-localizable EEG seizure. One cluster of spike ECDs was estimated in the right temporal region by MEG. Case 2: VEEG showed generalized slow spike and wave complexes with maximum amplitude in the right frontal region and multiregional sharp waves in the right frontal, right centro-parietal, and left frontal regions. No seizures were captured. Single tight cluster of ECDs was estimated in the right inferior parietal lobe by MEG. Case 3: VEEG showed multiregional sharp waves in the right posterior quadrant. It also showed EEG seizures arising from the right hemisphere. Single tight cluster of ECDs was estimated around the right parieto-occipital sulcus by MEG. Case 4: VEEG showed generalized interictal and ictal epileptiform discharges with maximum amplitude in the left hemisphere. MEG showed a few ECDs estimated in the right medial occipital region as well as many ECDs in the left inferior parietal, parieto-occipital, and temporal regions. Conclusions: The MEG results differed dramatically from the EEG. MEG can detect epileptiform discharges in the atrophic hemisphere, which are decreased or hidden on EEG (Cases 1, 4). MEG can also reveal a single focal irritative zone, even though EEG shows multiregional or generalized epileptiform discharges (Cases 2, 3). MEG source localization is useful for determining precise localization of interictal discharges in patients with massive asymmetric brain abnormalities, a situation where EEG localization is usually unhelpful or misleading.