Abstracts

Rationale: High-frequency oscillations (HFOs) known as ripples (80-250 Hz) and fast ripples (250-500 Hz) are most likely linked to epileptogenesis and have been found in the seizure onset zone (SOZ) of human ictal and interictal invasive recordings. High-frequency neuromagnetic signals up to 1,500 Hz have also been identified in the somatosensory system in the normal brain using magnetoencephalography (MEG). Therefore, it would be of utmost interest to systematically investigate the differences of the high-frequency neuromagnetic signals between the normal and the epileptic brain. Methods: Forty eight clinical patients with epilepsy (26 female and 22 male, aged 6-18 years, mean age of 10 years) and 60 healthy children (30 female and 30 male, aged 6-18 years, mean age of 11 years) were studied. A CTF OMEGA 275 channel system (VSM MedTech Systems Inc., Coquitlam, BC, Canada) was used for recording MEG data at a sample rate of 4,000 Hz. Three-dimensional Magnetization-Prepared Rapid Acquisition Gradient Echo (MP_RAGE) sequences were obtained for all subjects with a 3T scanner (Siemens Medical Solutions, Malvern, PA). HFOs were analyzed with Morlet wavelet transform and dynamic coherence sources were estimated with wavelet-based beamformer. Conventional spikes were visually identified and then localized with dipole modeling. Results: HFOs up to 900 Hz were identified in all patients. Compared to neuromagnetic signals in the normal brain, the HFOs in the epileptic brain had different characteristic in terms of spatial localization, frequency power and coherence. All patients showed one or more focal increase of HFOs which were not found in the normal brain. Rates and durations of HFOs were significantly higher around the lesions revealed by structural magnetic resonance imaging (MRI) than outside. HFOs were found in spiking as well as nonspiking regions in the epileptic brain. Volumetric source estimation showed that spontaneous neuromagnetic activity in the healthy child’s brain had a consistent spatial and temporal cohesiveness. The normal cohesiveness of the neuromagnetic activation was disturbed in the epileptic brain. Furthermore, the maturational changes of the brain’s default mode were significantly altered in the epileptic brain. Conclusions: The results have demonstrated that the HFOs generated by the normal and epileptic brains were significantly different in terms of spatial and frequency coherences. The focal strong abnormal HFOs probably linked to seizure onset areas and/or epileptogenic zones. The cerebral mechanism of the dysfunction of cohesiveness of interactivities between brain regions remains unclear. We consider it was probably related to the comorbidity and/or development delay in pediatric epilepsy.