Abstracts

Rationale: We hypothesized that local cortico-cortical evoked potentials (CCEPs) after paired-pulse stimulation differ between epileptogenic and less epileptogenic cortices, and provide a new measure for cortical excitability.Methods: Local CCEPs after paired-pulse stimulation were recorded intra-operatively using 4-contact subdural electrodes with 5 mm inter-electrode distance in 4 patients with intractable focal epilepsy. The epilepsy diagnosis included left frontal lobe epilepsy in one, left. temporo-occipital lobe epilepsy in one, and right temporal lobe epilepsy in two patients. The paired-pulse CCEPs were measured at the cortex near the ictal onset zone (iCCEP: n=8), and at the cortex not associated with ictal EEG changes (nCCEP: n=12). Alternating electrical pulse stimuli were delivered through two of the four contacts, and cortical evoked potentials were recorded from the other 2 electrodes. A constant-current square wave pulse of 0.3 ms duration was delivered at a frequency of 1.5Hz, and 20 responses were averaged. Stimulus intensity was fixed at 5 mA, and interstimulus interval (ISI) was graded between 5 ms and 150 ms. Differences of the latency between the first response (the first pulse to the first negative peak: L1) and the second response (the second pulse to the second negative peak: L2) were compared between iCCEP and nCCEP.Results: The second peak was clearly separated from the first peak when the ISI was between 20 and 50 ms. The L1 and L2 latencies were 24.33 ± 8.5 ms and 21.69 ± 10.2 ms, respectively. The latency difference (L2 - L1) was longer in iCCEP (3.15 ± 4.8 ms) than in nCCEP (-6.5 ± 3.1 ms) (P=0.0004).Conclusions: Difference of the latency between the first and second peaks after paired-pulse CCEPs was longer near the ictal onset zone. Our results suggest that cortical inhibition was prolonged near the epileptogenic zone. Paired-pulse local CCEPs may be useful as an adjunctive measure of cortical excitability in patients undergoing epilepsy surgery.