Abstracts

Rationale: Localizing the epileptogenic zone in patients undergoing video-EEG monitoring with intracranial electrodes can be challenging because seizures may be infrequent, impacting on the time constraints to complete the monitoring while ensuring accurate localization. Reliable secondary interictal markers may aid in epileptogenic area identification and/or improve localization accuracy.Methods: Three epileptic patients undergoing invasive EEG monitoring with depth electrodes were continuously recorded at 2 kHz sampling rate. Simultaneously, we staged the patients sleep with conventional polysomnography methods. EEG signals were recorded from the most mesial contacts of the hippocampal depth electrodes, and detected using a custom-built software algorithm. Hippocampal ripples were defined as 20-180 Hz oscillations occurring within a sharp wave ripple complex that originated from the hippocampus. We identified ripples by first applying a low pass filter (<180 Hz). The 180 Hz filter was selected to rigorously exclude any possibility of detecting a pathologic process, based on previous data showing the majority of pathologic high frequency oscillations (HFO’s) occur above 180 Hz. The data was rectified by taking its absolute value and applying a Hilbert transformation. Next, we applied a high pass filter (>20 Hz) and rectified the resultant signal in the same manner. A z-score was then applied to the data. Any signal with a z-score value greater than 5 was identified as a ripple candidate. Valid ripples were then selected only if their durations were between 10- 100 msec and temporally spaced at least 250 msec apart from one another. Hippocampal ripples meeting these criteria were then counted for each 30 second epoch from 7pm-7am. The ripple rates were then averaged for each respective sleep stage.Results: In two patients that subsequently underwent anterior temporal lobectomy, the average ripple rate from the epileptogenic hippocampus during SWS was 6.05/epoch, compared to 2.2 /epoch in the non-epileptic side. During wakefulness, the average ripple rate was 1.9/epoch and did not differ between epileptic and non-epileptic side (Figure 1). In SWS, the ratio of ripples in SWS between the epileptic and non-epileptic hippocampus was 3.07:1 in patient 1, and 2.89:1 in patient 2. The 3rd patient did not progress to surgery due to bilateral seizure onset. In that patient the right to left hippocampal ripple ratio during SWS was 1.15:1. (Table 1)Conclusions: Our preliminary analysis suggests that patients with mesial temporal lobe epilepsy may have increased normal frequency ripples only during SWS in the affected hippocampus. Ripples in SWS may be a biomarker for localization of the epileptogenic zone. This finding may help aid in localization of seizure onset in patients undergoing intracranial recordings for epilepsy.