Simultaneous intracranial and extracranial recording of interictal epileptiform activity in patients with drug resistant partial epilepsy: Patterns of conduction and results from dipole reconstructions

Ten patients with complex partial epileptic seizures undergoing invasive video/EEG-monitoring were recorded with a combination of 10 subdural strip electrode contacts (subtemporal + lateral temporal), and 22 extracranial recording sites. In each patient several spikes with different intracranial distributions were identified, and spikes with similar distributions were averaged together with their extracranial activity. Dipole analysis of the extracranial activity was performed with the BESA(R) program (Scherg, Garching Instrumente, Munchen). In the horizontal plane subtemporal spikes gave oblique posterior, and lateral temporal spikes straight lateral or oblique anterior dipole orientations. In the coronal plane all spikes had an elevated orientation, most pronounced for medial subtemporal spikes. Dipole locations did not separate as well as dipole orientations. In our opinion our results would be difficult to explain without postulating a substantial degree of volume conduction from deep temporal areas to the surface. We conclude that dipole analysis of the interictal epileptiform activity does provide substantial information about which parts of the temporal lobe are involved in the epileptogenic process, making the method a useful tool in the preoperative investigation of patients with drug resistant partial epilepsy.