Abstracts

Spatial Co-localization of Pathological High frequency Oscillations with Seizure Onset Zone in Neocortical Epilepsy

Abstract number : 2.139
Submission category : 3. Clinical Neurophysiology
Year : 2011
Submission ID : 14875
Source : www.aesnet.org
Presentation date : 12/2/2011 12:00:00 AM
Published date : Oct 4, 2011, 07:57 AM

Authors :
S. Wang, J. Bulacio, Z. Wang, J. Gonzalez-Martinez, W. Bingaman, A. Alexopoulos, I. Najm, N. So

Rationale: High frequency oscillation (HFO, 80-500Hz) are found to be tightly related to the seizure onset or epileptogenic zone. Previous studies were mainly performed on patients with mesial temporal epilepsy. The HFO profile in neocortical epilepsy is limited and not well characterized. Our study aims to investigate the spatial distribution and characteristics of interictal HFO in neocortical epilepsy.Methods: The patients studied underwent invasive EEG by stereostatic EEG (SEEG) or subdural grid and depth electrodes. After a first few seizures were recorded, 50-60 electrode contacts were selected and recorded at a sampling rate of 2000Hz for 1-5 days. Only patients whose seizures arose from a defined region in the neocortex were included for study. The EEG onset zone and primary propagation areas were determined by routine visual analysis. Interictal HFOs were analyzed during 10 min EEG in slow wave sleep. HFOs were visually inspected by Jacob s (2010) methods using the Multiview software and further confirmed by Morlet wavelet time frequency analysis. HFOs were divided into ripple (80-250 Hz) and fast ripple (250-500 Hz). HFOs were also differentiated whether occurring with epileptiform discharges (spike, sharp wave or paroxysmal fast activity) or not. We correlated the distribution of HFOs with the corresponding ictal onset zone.Results: 22 patients were studied, including 12 patients (Group A) implanted with subdural grid and depth electrodes and 10 patients (Group B) with SEEG. Ripples were found in various regions in the neocortex. The somatosensory, visual cortex and primary motor cortex often showed spontaneous ripple at a frequency between 80-150 Hz, and are considered to be physiological since they did not change at seizure onset. In addition, they were never associated with epileptiform discharges. Ripples occurring with epileptiform discharges were found in 10 out of 12 patients in group A and 7 of 10 patients in group B. These ripples were considered as pathological since they were located at the seizure onset electrodes or primary propagation areas in a highly specific manner. In contrast, ripples that occurred without epileptiform discharges occurred equally frequently within or outside the seizure onset zone and primary propagation areas. Ripples with epileptiform discharges have a higher frequency, higher amplitude and shorter duration compared to the ripples without epileptiform discharges. Three patients in group A and only one patient in group B showed fast ripple. Fast ripples were found exclusively at seizure onset electrodes.Conclusions: Physiological spontaneous ripples can occur in the neocortex. Pathological ripple occurring with epileptiform discharges reliably localize to the seizure onset zone in neocortical epilepsy. Fast ripples are infrequent in epileptic neocortex, however, it occurred only in the seizure onset zone with high specificity.
Neurophysiology