High-frequency oscillations are brief EEG events above the conventional EEG frequency range. Pathological HFOs are being studied as biomarkers of epileptogenic tissue and possible guides to epilepsy surgery, but they are not yet a universal clinical standard.
What are HFOs?
HFOs are commonly divided into ripples around 80–250 Hz and fast ripples around 250–500 Hz.
They are most often studied with intracranial EEG because the skull attenuates high-frequency activity.
Not every HFO is pathological
Physiological HFOs occur in healthy brain networks, particularly those involved in memory.
The research challenge is distinguishing physiological activity from HFOs generated by epileptogenic tissue.
Why surgeons became interested
Several studies found that removal of regions generating pathological HFOs was associated with better postoperative seizure outcomes.
This suggested HFOs might identify epileptogenic tissue more precisely than conventional spikes or even seizure-onset contacts in some patients.
The evidence is promising but inconsistent
A 2023 meta-analysis found an association between HFO resection and better seizure outcome but also substantial methodological heterogeneity.
Recording settings, definitions, sleep stage, electrode type and automated detection methods vary widely.
New combinations may improve specificity
Recent studies have examined fast ripples and spikes occurring on ripples as potentially more specific biomarkers than spikes or ripples alone.
A 2025 study found that fast-ripple and spike-on-ripple resection measures predicted outcome better than conventional spikes in its cohort.
Technical demands are substantial
HFO analysis requires high sampling rates, low-noise recordings and careful artefact rejection.
These requirements make routine implementation harder than ordinary EEG interpretation.
A biomarker under active development
HFOs should currently be viewed as an advanced adjunct, not a stand-alone instruction about what tissue to remove.
Prospective multicentre validation and standardised methods are still needed.
Related reading
Sources and further reading
- High-frequency oscillations as a new biomarker in epilepsy
- Association between HFO removal and epilepsy surgery outcome — meta-analysis
- Spikes on ripples and fast ripples as biomarkers — 2025
Evidence checked: September 2026.