Abstracts

Rationale: The role of the corpus callosum (CC) in interhemispheric seizure propagation is well documented but a noninvasive individual assessment in humans remains unavailable. Here we present a protocol that attempts to characterize the regional changes in the CC associated with the propagation of epileptic activity using MEG-guided tractography. Using this protocol we tested whether interhemispheric generalization is associated to regional changes in the microstructural integrity of the CC.Methods: Clinical and radiologic data from 11 patients (4 patients with only partial seizures, 7 with generalized seizures) was reviewed. All these patients were diagnosed with refractory epilepsy and underwent presurgical evaluations, including prolonged vEEG monitoring, MEG and Diffusion Tensor Imaging (DTI). From the simultaneous EEG-MEG recordings, a total of 83 interictal epileptiform discharges (present on both EEG and MEG tracings) were selected and their propagation patterns were classified as either intrahemispheric or interhemispheric. For each spike, MEG-derived equivalent current dipoles around the time of determined peak were calculated and the derived localization was used as a seed for fiber tracking. Fractional anisotropy (FA) and values of diffusivity (mean, axial and radial) were calculated for the whole CC and for those areas where MEG-guide tracts crossed the CC (mid-sagittal plane). Results: Out of the 83 analyzed interictal discharges, 51 were labeled as intrahemispheric spikes (with intralobar or interlobar propagation) and 32 as interhemispheric. Areas of the CC defined from the fiber tracking of interhemispheric discharges showed reduced relative FA values (region/whole CC) as compared to those with intrahemispheric propagation (p < 0.05).Conclusions: Microstructural white matter alterations occur in the corpus callosum when frequent interhemispheric generalization takes place. Here we present the first non-invasive protocol (MEG-guided tractography, MEG-GT) that allows identifying areas of the CC that are mediating in the generalization of interictal events. The clinical correlation and the value of this protocol in predicting surgical outcome following partial or complete callosotomy will be further investigated.