Abstracts

Rationale: Unique high-frequency oscillations (HFOs), termed fast ripples (FR), have been identified in seizure generating limbic areas of kainic treated rats, and in patients with mesial temporal epilepsy using depth electrodes. Results from these studies have provided evidence supporting the view that fast ripples in the human brain appear to reflect field oscillations composed of hypersynchronous action potentials of pathologically interconnected neuronal clusters related to epileptogenesis. We have previously reported on the use of magnetoencephalography (MEG) to detect HFOs. One critique of that study was that the criteria used for HFO determination were significantly different from the published studies utilizing implanted electrodes. To address that, a study specifically replicating one such methodology was performed. Methods: MEG data was recorded on an epileptic patient during the patient's routine pre-surgical evaluation. Data was recorded on 248 MEG channels at a sample rate of 2034.51 Hz, with a high pass filter 0.2 Hz. One minute of data was recorded with the patient awake, eyes open, and one minute with the patient awake, eyes closed. The data was then transferred to MATLAB for HFO analysis. The data from each sensor was considered separately. The input signal was first filtered using a 30 order Butterworth filter with a bandpass of 151-500 Hz (for fast ripple screening) and separately with a bandpass of 80-150 Hz (for ripple screening). The filtered signal was then rectified to zero voltage. The root mean square of a moving 3 ms window was calculated sequentially over the signal. A positive HFO detection was made if a cut-off of five standard deviations above the mean RMS was exceeded for great than 6 ms. This process was repeated for each of the 248 sensors, and the sensor locations where HFOs were detected were plotted, and the specific HFO frequencies were noted. Results: Using this method, of nine patients analyzed to date, outliers consistent with HFOs have been detected in two. HFO duration ranged from 6-10 ms. Every HFO was detected in sensors over the temporal regions, unilateral in one patient, bilateral in the second. Data collected in all patients studied over a six month period will be presented, along with each patient's underlying pathology, if known. Conclusions: HFOs can be detected using magnetoencephalography in refractory epilepsy patients using signal processing methods similar to those published for analysis of data from implanted electrodes. Whether these detections represent the same phenomenon has yet to be determined.