Modern electroencephalographic (EEG) technology contributed to the appreciation that the EEG signal outside the classical Berger frequency band contains important information. In epilepsy, research of the past decade focused particularly on interictal high-frequency oscillations (HFOs) > 80 Hz. The first large application of HFOs was in the context of epilepsy surgery. This is now followed by other applications such as assessment of epilepsy severity and monitoring of antiepileptic therapy. This article reviews the evidence on the clinical use of HFOs in epilepsy with an emphasis on the latest developments. It highlights the growing literature on the association between HFOs and postsurgical seizure outcome. A recent meta-analysis confirmed a higher resection ratio for HFOs in seizure-free versus non-seizure-free patients. Residual HFOs in the postoperative electrocorticogram were shown to predict epilepsy surgery outcome better than preoperative HFO rates. The review further discusses the different attempts to separate physiological from epileptic HFOs, as this might increase the specificity of HFOs. As an example, analysis of sleep microstructure demonstrated a different coupling between HFOs inside and outside the epileptogenic zone. Moreover, there is increasing evidence that HFOs are useful to measure disease activity and assess treatment response using noninvasive EEG and magnetoencephalography. This approach is particularly promising in children, because they show high scalp HFO rates. HFO rates in West syndrome decrease after adrenocorticotropic hormone treatment. Presence of HFOs at the time of rolandic spikes correlates with seizure frequency. The time-consuming visual assessment of HFOs, which prevented their clinical application in the past, is now overcome by validated computer-assisted algorithms. HFO research has considerably advanced over the past decade, and use of noninvasive methods will make HFOs accessible to large numbers of patients. Prospective multicenter trials are awaited to gather information over long recording periods in large patient samples.

Department of Child Neurology Okayama University Graduate School of Medicine Dentistry and Pharmaceutical Sciences Okayama University Hospital Kita ku Okayama Japan

Department of Medicine and Center for Neuroscience Studies Queen's University Kingston Ontario Canada

Department of Neurology and Neurosurgery Brain Center Rudolf Magnus University Medical Center Utrecht Utrecht The Netherlands

Department of Neurology Christian Doppler Medical Center and Center for Cognitive Neuroscience Paracelsus Medical University Salzburg Salzburg Austria

Department of Neurology Grenoble Alpes University Hospital and Grenoble Alpes University Grenoble France

Department of Neuropediatrics and Muscular Diseases University Medical Center Freiburg Freiburg Germany

Department of Neurosurgery University Hospital Erlangen Erlangen Germany

Departments of Neurology Neurobiology and Psychiatry Brain Research Institute University of California Los Angeles Los Angeles California U S A

Departments of Pediatrics and Neurology Detroit Medical Center Children's Hospital of Michigan Wayne State University Detroit Michigan U S A

Division of Neurology Department of Pediatrics Hospital for Sick Children Toronto Ontario Canada

Division of Pediatric Neurology Mattel Children's Hospital at UCLA Los Angeles California U S A

International Clinical Research Center St Anne's University Hospital Brno Czech Republic

Montreal Neurological Institute and Hospital McGill University Montreal Quebec Canada

National Institute of Health and Medical Research Institute of Neurosciences of Systems Aix Marseille University Marseille France

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