Abstracts

Rationale: Periventricular nodular heterotopia (PVNH) is a malformation of cortical development associated with drug-resistant epilepsy with frequent poor surgical outcome due to the complex epileptogenic cortical network which can involve the nodule, the overlying cortex, or both. Single-pulse direct electrical stimulations (SPES) during intracerebral EEG monitoring allow the investigation of cortical effective functional connectivity between stimulated and responsive cortices by eliciting cortico-cortical evoked potentials (CCEPs). We used SPES in order to determine PVNH connectome and to assess its relation with the epileptogenic network.  Methods: We retrospectively studied patients with PVNH explored with stereo-electroencephalography (SEEG) in Lyon and Marseille epilepsy centers for whom 1Hz or 0,2Hz SPES had been performed, at least within the nodule.Outbound connectivity (regions where CCEPs were elicited by the nodule stimulation) and inbound connectivity (regions which stimulation elicited CCEPs in the nodule) 3D maps were created.SEEG electrode contacts were then classified into three groups: heterotopic (located within or near the visible borders of the nodule on MRI), connected (located in normotopic cortex with functional connections with the nodule revealed by SPES) and not connected. Seizures were collected, visually analyzed and computed to calculate the epileptogenicity index (EI) as described in Bartolomei al. Brain 2008;131:1818–1830 in order to quantify contacts implication in the seizure-onset zone. Based on EI values, seizures were classified as heterotopic, normotopic and normo-heterotopic. Results: 11 patients (31 yo +/-11) were analyzed: 7 with bilateral PVNH, 3 with unifocal PVNH and 1 with unilateral bifocal PVNH. All were analyzed for outbound connectivity. SPES were also performed in non-heterotopic cortical regions in 5 patients, allowing to evaluate inbound connectivity.90 outbound and 32 inbound connections were found. 2 nodules stimulation showed no significant connections while most nodules had numerous and widespread connections within the hemisphere, even in lobes that do not contain the PVNH. Three patients showed connections between posterior PVNH and the homolateral hippocampus and two patients showed connections between posterior PVNH and the contralateral hemisphere. Interestingly, those distant-connected regions showed high EI values.A total of 43 seizures were analyzed: 19 were normo-heterotopic and 22 were purely normotopic and only 2 were purely heterotopic. Conclusions: Our study revealed that PVNH had widespread inbound and outbound connections with the overlying cortex, within the same lobe but also with distant regions, including contralateral hemisphere and the hippocampus. Those connected regions usually showed high EI values supporting the hypothesis of a diffuse epileptogenic network in patient with PNVH. Funding: No funding