Occurrence of scalp-fast oscillations among patients with different spiking rate and their role as epileptogenicity marker

Abstract

We aim to analyze the fast oscillations in the scalp EEG of focal epilepsy patients with low-to-high rates of interictal epileptiform discharges (IEDs), in order to determine how this neurophysiological feature influences fast oscillation occurrence and their significance as markers of the seizure onset zone (SOZ). Thirty-two patients were studied, subdivided in four categories based on IED frequency: groups A, B and C respectively with high, intermediate and low IED rate, and group D with no IED. Thirty minutes of slow-wave sleep EEG, low-pass filtered at 300Hz and sampled at 1000Hz, were reviewed. IEDs and fast oscillations (gamma activity, 40-80Hz; and ripples, >80Hz) were marked. Each channel was classified as inside or outside the irritative zone and the SOZ. We calculated the number and rates of IEDs and fast oscillation, their co-occurrence, their frequency in the irritative zone and SOZ, and the specificity, sensitivity and accuracy to determine the SOZ in the overall population and separately for each group. We analyzed 984 channels. Group A (high IED rate) showed the highest fast oscillation rate (gamma: 0.37±0.73; ripples: 0.17±0.26), followed by group B (gamma: 0.08±0.06; ripples: 0.07±0.05), group C (gamma: 0.06±0.06; ripples: 0.04±0.01), and finally group D, with very low values (gamma: 0.03±0; ripples: 0.03±0). IEDs co-occurred with gamma in 9.5% and with ripples in 3.2%; and gamma and ripples co-occurred with IEDs in 46.2% and 44.4%, respectively. The fast oscillations were more frequent inside than outside the irritative zone and the SOZ (p<0.001). Compared to the IEDs, the fast oscillations were less sensitive (sensitivity: IEDs 78%, gamma 66% and ripples 48%) but more specific (specificity: IEDs 50%, gamma 76% and ripples 83%) and accurate (accuracy: IEDs 54%, gamma 74% and ripples 77%) in identifying the SOZ; the same results were reproduced for the different groups separately. This study confirms that fast oscillations can be recorded from the scalp EEG. Gamma activity and ripples are more frequent in patients with frequent IEDs and, in general, inside the irritative zone. However, compared to IEDs, gamma and ripples are less sensitive but more specific and accurate in identifying the SOZ, and this remains in patients with low fast oscillation rates. These findings suggest that IEDs and fast oscillations could share some common neuronal network, but gamma activity and ripples are a better biomarker of epileptogenicity.