High-frequency oscillations (HFOs), termed ripples (80–200 Hz) and fast ripples (FRs, >200/250 Hz), were recorded initially in mesial temporal lobe epilepsy by intracranial electroencephalography (EEG) using microelectrodes (Bragin et al., 1999, 2002). FRs are thought to be a more specific surrogate marker of the seizure-onset zone (SOZ) than ripples (Bragin et al., 1999; Staba et al., 2002). Subsequently HFOs were also recorded by macroelectrodes (Jirsch et al., 2006; Urrestarazu et al., 2006), and they were seen in the neocortex (Worrell et al., 2004; Jacobs et al., 2009a). The resection of the brain regions containing HFOs, especially FRs, correlated with good seizure outcome using depth macroelectrode recording in adult patients (Jacobs et al., 2009b) and intraoperative electrocorticography (ECoG) in pediatric patients (Wu et al., 2010). However, these studies used relatively small numbers of contacts with limited coverage of the brain, which may not be optimal for pediatric patients with more extensive epileptic network than adults. In addition, extensive surgical resection, as is often required for pediatric patients, may overestimate the link between HFOs and seizure outcome; it may increase the chance of success by inclusion of both brain regions with HFOs and the epileptogenic zone in the resection, even when they are independent.
Utilizing automated HFO detection and multivariate analysis to remove confounding effects, we tested the following hypotheses with chronic extraoperative intracranial EEG monitoring with relatively extensive brain coverage in pediatric epilepsy patients: (1) the more complete resection of the brain regions containing high-rate HFOs is associated with the better seizure outcome regardless of the size of surgical resection, and (2) FRs are more predictive of seizure outcome than ripples. We also investigated the relationship between the brain regions with ripples, FRs, and the SOZ.