Abstracts

Rationale: It has been previously described that the major amount of HFOs (ripples, R and fast ripples, FR) is displayed by the mesial temporal lobe contacts. Moreover, we noticed, in some mesial temporal contacts, a peculiar pattern characterized by a continuous/semi-continuous R activity. This makes difficult both to mark the single oscillations, to define in these contacts the background epochs (BKG) and, therefore, to use the automatic HFOs detectors that use the BKG epochs as reference. We studied, in a population of 24 patients implanted consecutively in the mesial temporal structures, the inter-spike BKG activity, evaluating the presence of distinct groups of limbic oscillations and the correlations with different electro-clinical and neuro-radiological variables. Methods: 24 patients implanted in the mesial temporal lobe structures were studied. Depth electrodes were directed orthogonally in the anterior (Amygdale), mid (Hippocampus, Hp), and posterior locations (Para-Hippocampus). Wakefulness 5 minutes and slow-wave sleep 10 minutes epochs were acquired (500 Hz low-pass hardware filter, sampling rate of 2000 Hz). We define as BKG the inter-spike Stereo-EEG segments preceding and following each spike with at least 1 sec pre- and post-spike separation. We collected epochs totaling 30 sec per channel. We visually classified the BKG epochs selected in Continuous/Semi-Continuous (C\SC), Irregular and Sporadic (depending on the length of the oscillations and on the presence or not of a clear cut separation between the transient elements, fig.1) and also visually marked the R and the FR elements. Results: 96 mesial temporal lobe channels were evaluated. The C/SC pattern was found in 34 channels in sleep (35%) and 29 channels in wake (30%). Statistical analyses disclosed a significant concordance between the distribution of the different BKG patterns in sleep and wakefulness (p<0.001). Significant relationships were found between the C\SC pattern and the localization of the contacts in Hp regions (p<0.002) and the seizure-onset channels (SOZ) (p<0.002) (fig.2). Conversely, no significant correlation was found between this BKG pattern and the channels in the lesional zones and the ones with the higher spike rates. Significant relationships were finally found between the higher R rates and the localization of the channels in the Hp and the presence of a C/SC pattern.