Abstracts

Rationale: There are different neurophysiological markers of the epileptogenic zone in Focal Cortical Dysplasia (FCD): ictal discharges, interictal epileptiform discharges (IEDs) and High Frequency Oscillations (HFOs); but it is not known if one is better [1]. Our questions are: which of these three markers is the most reliable in FCD? Could we predict multifocality based on these parameters?  Methods: We studied 35 consecutive drug-resistant focal epilepsy patients with FCD documented by pathology, who underwent stereo encephalography (SEEG), surgery, and one year of follow-up. We defined two groups depending on surgical outcome: good (ILAE 1-4) and poor (ILAE 5-6) We defined the presence in SEEG channels of: a) IEDs, b) ictal discharges at seizure onset, c) ripples (80-250 Hz) and d) fast ripples (>250 Hz). We defined typical interictal channels as spike or polyspike exceeding 2 Hz or spike or polyspike interrupted by flat periods below 2 Hz. Atypical interictal discharges were defined as frequent IEDs but without fulfilling the typical criteria. We also distinguish asynchronous IEDs in different channels as a marker of multifocality. HFOs were marked using an automatic detector during one hour of slow wave sleep. We classified ripples and fast ripples as occurring with low/high rates for each channel. We considered high rate but not low rate HFOs as marker of epileptogenicity. Based on the superposition of the surgical cavity with the electrodes implanted we considered two groups of channels: those in resected regions and those in non-resected regions. We considered a marker accurate if it was absent from non-resected channels in seizure-free patients (if a marker is present in a resected channel in a seizure-free patient, it could still have been in healthy tissue). Results: We analyzed 21 patients with FCD type II (mean age: 30.6 ± 7.6 years; mean duration of epilepsy: 19.1 ± 12.0 years). A good outcome was seen in 11 patients (52%). For all patients, 171 channels were resected, 165 had ictal discharges, 141 typical interictal discharges, 84 atypical interictal, 68 ripples and 44 fast ripples.  Fast ripples and typical interictal discharges, in second place, were the markers that remained in the smallest percentage of channels in the good outcome group, while the atypical interictal pattern was the marker that remained in the highest percentage of channels (fig.1). Permutation tests found significant associations between outcome and multifocal interictal IEDs (p Conclusions: Fast ripples were the most specific marker of epileptogenic tissue in FCD patients. However, their presence in few channels makes them of limited practical use on their own. The most clinically relevant neurophysiological marker for the delineation of the epileptogenic zone was therefore typical interictal discharges. Multifocality based on IEDs was more frequently associated to a poor outcome.   References: 1. Epilepsia. 2013;54(8):1428-36.  Funding: This work was supported by grant 143208 of the Canadian Institutes of Health Research