By Alkawadri et al.

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Fast, Very Fast, Ultrafast, and Even Faster: How High Frequency Should We Be Recording on Intracranial EEG?

Intracranial EEG presents two major challenges: 1) the risks increase in rate as a function of duration of recording (1) and 2) the chance of sustained seizure freedom after surgery is only 30 to 80 percent depending on the epileptic region and resected sites , suggesting that current methods of localization are not optimal. Hence, identifying reliable biomarkers of epileptogenicity in the interictal phase without the need to record seizures (that is, shorter evaluations) is of great interest. In addition, increasing the specificity of localization and identifying markers of ongoing epileptogenesis or dormant sites may prove beneficial in improving long-term outcomes.

There has been a growing interest in the utility of interictal high-frequency oscillations HFOs (80–500 Hz, classically) for localization of the epileptic focus (4). Several challenges arose because similar oscillations have been associated with specific tasks or occur naturally during sleep (5), and no signal parameter can reliably distinguish between physiologic and epileptogenic subtypes in a given individual (5, 6). Hence, interictal HFOs, although useful, are not highly specific and do not replace current standards. Ironically, it appears, to date, that the most effective factor that increases specificity of HFOs for detection of pathology is their co-occurrence with other known markers of pathology, such as spikes (7, 8).

With the advancing technology of EEG acquisition systems, higher sampling rates, and greater computational power, it is now possible to record very high-frequency oscillations (VH-FOs; oscillations >500 Hz). Some recent important (yet limited) studies suggest that VHFOs are more specific for the epileptic regions (9). In this commentary, we will discuss two recent studies on the utility of interictal VHFOs and HFOs for localization of epileptic foci in extraoperative (10) and intraoperative (11) settings.