gms | German Medical Science

Objective: In order to determine the resection extent in epilepsy surgery it is necessary to estimate the epileptogenic zone (EZ). Focal cortical dysplasias (FCD) are subtle epileptogenic lesions increasingly distinguishable in MRI by malformation of the border between the cortex and white matter (Blümcke et al., 2011). Yet, morphological definition of the EZ alone is controversial. One possible electrophysiologic correlate for the EZ are high frequency oscillations (HFO), which can be recorded in intracranial EEG (Zijlmans et al. 2012). The purpose of this study is the detection of HFO with the use of stereotactic guided micro-electrode recordings (MER) to improve accuracy of EZ definition.

Methods: In 5 patients with refractory focal epilepsy, pre-surgical workup included MRI-guided implantation of stereotactic-EEG electrodes (SEEG) into FCD. Stereotactic MER was done in general anesthesia with single channel sharp tungsten electrode, driven forward in 1mm steps along trajectory for implantation of SEEG electrode, a usual technique of basal ganglia mapping during deep brain stimulation surgery. Stereotactic planning and manual segmentation of the lesion was performed with IPlan Stereotaxy Software (Brainlab, Germany). Signal recording was done on MER System (Inomed, Germany). The MER data were analyzed in MATLAB software (Mathworks) with Fieldtrip-Toolbox. Through wavelet-based time-frequency analysis the HFO were detected and the location was correlated to outlines of MRI finding.

Results: We detected high-amplitude spike bursts with oscillation frequencies from 250 to 500 Hz in 4/5 patients with FCD IIb. The characteristics of the HFO reassembled previously published HFO activity in intracranial EEG recordings (Brazdil et al, 2010). Location of MER positive HFO on MRI shows good overlap with MRI-suspected FCD region.

Conclusion: This study indicates the feasibility of intraoperative HFO detection in FCD with MER mapping technique. This potentially presents a technique of intraoperative electrophysiological mapping of cortical epileptogenic zones both in SEEG implantations and in resective surgery.