Researchers at University Medical Center Utrecht, The Netherlands and other centers review the pathophysiology, clinical relevance, identification, and interpretation of high-frequency oscillations (HFOs, >80Hz) in the epileptic EEG. HFOs are further classified in ripples (80-250Hz), fast ripples (250-600Hz), and very-HFOs (>1,000Hz). HFOs are observed between seizures, at seizure onset, and during seizures. Interictal HFOs occur during slow-wave sleep. The intracranial EEG is sampled at =/> 2,000Hz. In patients with focal epilepsy who may benefit from surgery, removal of brain tissue generating HFOs results in better outcome than removal of the seizure onset zone. Interictal HFOs are reliable markers of the seizure onset zone, and better than epileptic spikes. HFOs occur in different types of epilepsy, mesiotemporal with hippocampal sclerosis and also in extratemporal epilepsies with tumors, focal cortical dysplasia, and nodular heterotopia. HFOs are sometimes recorded in nonlesional epilepsies. Evaluation of seizures with HFO recordings >80Hz may improve the pre-surgical workup and outcome and reduce the necessity for invasive monitoring. 
COMMENT. Methods used to determine the extent of epileptic foci of brain tissue before and during cortical resection include EEG telemetry, ECoG, MRI, histological, and immunohistochemical. Tissue markers of epileptic foci include mitochondrial “hypermetabolic” neurons and a-B-crystallin (Pediatr Neurol Briefs 2009;23(11);81-82) [2, 3, 4]. The Annals review provides a comprehensive account of the utility of HSOs as a biomarker of epileptogenesis.
HSOs are studied primarily in mesiotemporal epilepsies. In a series of 30 consecutive pediatric patients at UCLA undergoing surgery for refractory epilepsy due mainly to extratemporal lesions, ECoGs were recorded at 2,000Hz and visually inspected for fast ripples (FR 250-500Hz). FR episodes were identified in ECoGs from 24 patients (80%); FR-containing cortex was removed in 19 and all became seizure-free. FR-containing cortex was found outside of abnormalities defined by MRI and FDG-PET in 6 children. The authors conclude that interictal fast ripples are an excellent surrogate marker of epileptogenesis. .