Differentiation of specific ripple patterns helps to identify epileptogenic areas for surgical procedures

• Removal of areas generating fast ripples and areas of seizure onset correlate with a seizure-free postsurgical outcome.
• Ripple oscillations can either occur from a flat or oscillatory background activity.
• Only removal of areas with ripples occurring in flat background activity is correlated with a seizure free outcome.

ObjectiveHigh frequency oscillations (HFOs) at 80–500 Hz are promising markers of epileptic areas. Several retrospective studies reported that surgical removal of areas generating HFOs was associated with a good seizure outcome. Recent reports suggested that ripple (80–200 Hz) HFO patterns co-existed with different background EEG activities. We hypothesized that the coexisting background EEG pattern may distinguish physiological from epileptic ripples.MethodsRates of HFOs were analyzed in intracranial EEG recordings of 22 patients. Additionally, ripple patterns were classified for each channel depending either as coexisting with a flat or oscillatory background activity. A multi-variate analysis was performed to determine whether removal of areas showing the above EEG markers correlated with seizure outcome.ResultsRemoval of areas generating high rates of ‘fast ripples (>200 Hz)’ and ‘ripples on a flat background activity’ showed a significant correlation with a seizure-free outcome. In contrast, removal of high rates of ‘ripples’ or ‘ripple patterns in a continuously oscillating background’ was not significantly associated with seizure outcome.ConclusionRipples occurring in an oscillatory background activity may be suggestive of physiological activity, while those on a flat background reflect epileptic activity.SignificanceConsideration of coexisting background patterns may improve the delineation of the epileptogenic areas using ripple oscillations.