Abstracts

Rationale: Epileptic high frequency oscillations (HFOs: 80-500 Hz) may be used to guide neurosurgeons during epilepsy surgery to identify epileptogenic tissue. We studied the effect of the anesthetic agent propofol on the occurrence of HFOs in the intra-operative electrocorticogram (ECoG). Methods: We selected patients undergoing surgery for temporal lobe epilepsy with a standardized electrode grid placement. Intra-operative ECoG was recorded at 2048 Hz following cessation of propofol. The number and distribution of interictal spikes, ripples (80-250 Hz) and fast ripples (FRs: 250-500 Hz) were analyzed. The amounts of events on mesiotemporal channels and lateral neocortical channels were compared between patients with a suspected mesiotemporal and lateral epileptogenic area (t-test) and HFOs were compared to the irritative zone (correlation between amounts of events per channel) to provide evidence for the epileptic nature of the HFOs. Then, the amount of events within the first minute and the last minute were compared to each other and the change in events over the whole epochs were analyzed using correlation analyses of ten epochs during the emergence periods (Spearman rank test). It was studied whether the duration of HFOs changed over time. The change in events within presumed epileptogenic area was compared to the change outside this area. Periods of burst-suppression and continuous background activity were compared (t-tests). Results: 12 patients were included, 5 with suspected mesiotemporal epileptogenic area and three with suspected lateral epileptogenic area (and 4 other). Spikes, ripples and FRs were related to the epileptogenic areas. Figure 1 shows that ripples and FRs increased during emergence from propofol anesthesia (mean number of ripples: first minute to last minute: 61.5 to 73.0, R=0.46 p<0.01; FRs: 3.1 to 5.7, R=0.30, p<0.01) and spikes remained unchanged (80.1 to 79.9, R=-0.05, p=0.59). There was a decrease in number of channels with spikes (R=-0.18, p=0.05), no change in ripples (R=-0.13, p=0.16) or FRs (R=0.11, p=0.45). There was no change in the durations of HFOs. The amount of HFOs in the presumed epileptogenic areas did not change more than the amount outside the presumed epileptogenic area, while spikes paradoxically decreased more within the suspected epileptogenic area. Six patients showing burst-suppression had lower rates of ripples than six other patients with continuous background activity (p=0.02). No significant difference was found between burst-suppression and continuous background activity in four patients, but there was a trend towards showing more ripples during continuous background activity (p=0.16). Conclusions: Propofol, known for its anti-epileptic effects, reduces the number of epileptic HFOs, but has no effect on spikes. This enforces the hypothesis that in epilepsy HFOs mirror the disease activity and HFOs might be useful to monitor anti-epileptic drug treatment. It is feasible to record HFOs during surgery, but propofol infusion should be interrupted some minutes to improve the detection.