Intralesional recordings and epileptogenic zone in focal polymicrogyria

Purpose: Polymicrogyria (PMG) is recognized as an epileptogenic lesion but few data concerning organization of the epileptogenic zone (EZ) are available. Methods: We analyzed the distribution of the EZ according to Stereo-EEG (SEEG) with intralesional recordings in four patients evaluated for intractable partial epilepsy associated with focal unilateral PMG, involving the posterior temporal region in two, the perisylvian area in one and the temporoparietal junction in the other. All had ictal scalp EEG, high-resolution structural and functional MRI, fluorodeoxyglucose positron emission tomography (FDG-PET), and SEEG. For each patient, several depth electrodes were implanted both within the PMG and in extralesional areas. Results: In three patients, the PMG displayed high-frequency spiking activity. However, interictal and ictal recordings demonstrated a large epileptogenic network, which was more widespread than the PMG, including the mesial temporal structures in two. In another patient, interictal spiking and seizure onset site were located within the hippocampus and outside of the PMG, although it was rapidly involved during seizure spread. Overall, EZ was considered to be larger than the PMG in all patients although hypometabolic areas detected by PET were concordant with EZ. Three patients underwent extensive surgery including the PMG and are seizure free with a follow-up > 2 years. Discussion: Although intralesional recordings demonstrated intrinsic epileptogenicity in PMG, our data provide evidence that unilateral focal PMG belongs to a large epileptogenic network extending beyond the MRI lesion. SEEG may be helpful for planning surgery with favorable outcome, providing large resections are feasible, even in apparently focal PMG.