Abstracts

Rationale: High frequency oscillations (HFO, ripple: 80-250 Hz, fast ripple (FR) 250-500 Hz) are EEG markers for the localization of epileptic tissue and epileptic disease activity. In animal models FR only occur in those animals developing chronic epilepsy after status epilepticus and FR occurrence has been described to correlate with seizure frequency. In the kainate - model of mesial temporal lobe epilepsy seizure strength is classified by using the Racine scale. We hypothesize that HFO occurrence around seizures mirrors seizure-strength in these animals during the chronic phase of epilepsy. Methods: Adult male Wistar rats were injected with kainic acid into the right hippocampus to induce status epilepticus, this was followed by an implantation of two bipolar depth-electrodes each in the ipsi- and contralateral hippocampus. Local field potential (LFP) recordings were then performed on a 24 h basis with a continuous video-EEG monitoring system. Seizures were selected from the period of chronic epilepsy and scored according to the modified Racine scale (R1: Mouth and facial movement; R2: Head nodding; R3: Forelimb clonus; R4: Rearing with forelimb clonus; R5: Rearing and falling with forelimb clonus; R6: running and jumping) by two independent reviewers. HFOs and their duration were marked for a 30s preictal and postictal period as well as the complete ictal period. The time occupied by HFOs (HFO time per minute) was calculated to account for varying length of HFOs during the different period. Time per minute occupied by ripples and FRs were statistically compared for the different periods as well as different Racine stages (R 1-4 vs. R 5-6, p<0.05). Results: A total of 96 EEG segments from 16 seizures in two rats were analyzed. Five seizures were classified as generalized (Racine 5&6). Time covered by ripple and FR was significantly higher in the ictal than pre- and postictal period (p< 0.001), Figure 1). It was also significantly higher in the pre- than in the postictal period (ripple: p=0.007, FR: p=0.05). These observations were seen independent of the Racine scale. Time occupied by FR during the ictal period were significantly higher in R 5-6 than R 1-4 seizures (p=0.02, Figure 2). In the postictal period time occupied by FR was significantly lower in R 5&6 than in R 1-4 (p=0.03). Conclusions: The present results show that HFOs increase during the ictal period and shows a consecutive postictal suppression. The ictal increase as well as postictal depression of FR mirror the strength of seizures and are stronger during generalized seizures. This observation is in line with previous studies suggesting that FR can mirror disease activity in epileptic animals. Analysis of more seizures from further animals is needed to secure these results and analysis of human intracranial data from seizures with and without secondary generalization might be interesting.