Is it time to replace epileptic spikes with fast ripples?

Is it time to replace epileptic spikes with fast ripples? In well over a century of performing epilepsy surgery, only 60%–65% of patients are, on average, seizure-free postoperatively. 1 Despite multiple advances in neuroimaging and electrophysiologic techniques for localizing the epileptogenic zone and determining its boundaries, defining the exact amount of neuronal tissue required to resect in order to achieve postoper-ative seizure freedom remains challenging. The quest for a reliable biomarker of the epileptogenic zone continues. In addition to long-term intracranial EEG recordings to identify the seizure onset zone, cortical resections are also tailored in the operating room by including areas with abundant epileptic spikes and seizure-like interictal activity (ictiform patterns) recorded with corticography. 2 Intraoperative localization based on interictal EEG activity would be much more cost-effective than chronic recording to capture spontaneous seizures. In this issue of Neurology ® , van 't Klooster et al. 3 retrospectively studied high-frequency oscillations (HFO) as a biomarker of the epileptogenic zone during intraoperative electrocorticography. They reviewed the occurrence of HFOs after resection of an epileptogenic lesion, mainly glial tumors and cor-tical malformations, in 54 young patients. HFOs included ripples, defined as oscillations between 75 Hz and 250 Hz, and fast ripples (FR), defined as oscillations between 250 Hz and 500 Hz. Ripples are normal electrographic events recorded from hip-pocampus and may reflect summated inhibitory post-synaptic potentials that synchronize neurons over wide areas and facilitate information transfer. 4–6 FR, on the other hand, are pathologic events, believed to reflect summated action potentials of synchronously bursting neurons indicative of epileptogenic tissue capable of generating spontaneous seizures. 4–6 This distinction is not absolute, however, as ripple frequency HFOs in dentate gyrus are pathologic, and presumably other ripple frequency oscillations also reflect pathologic action potentials of synchronously bursting neurons, while FR frequency oscillations can be normal in neocortex. 4–6 HFOs are distinct events that are short in duration , stand out from the background, and consist of more than 4 oscillations. When reporting on HFOs, it is important to clearly define their frequency, as done in this study. 3 HFOs occur at the same time or separate from other epileptiform activity such as sharp waves and epileptic spikes. 5 van 't Klooster et al. clearly distinguished FR from ripples with and without associated spikes. They found that the presence of FR after removal of the presumed epileptogenic lesion correlated with a worse seizure outcome after surgery. …