Mapping physiological high-frequency oscillation rates of the cerebral cortex for improved epileptogenic zone...

Given the challenges of interpreting preictal- and ictal-phase stereoelectroencephalography (SEEG), a complementary approach of examining interictal activity could be of value. High-frequency oscillations (HFOs), recorded at frequencies greater than 80 Hz in SEEG, are often pronounced in but not unique to the epileptogenic zone (EZ) compared with normal cortex. The aim of this study was to assess a novel HFO rate ratio to identify the EZ using region-specific interictal HFO rates, and to estimate the optimal threshold for the HFO rate ratio for both ripples and fast ripples to localize the EZ.

Patients with epilepsy who underwent resective surgery following SEEG at a single institution (January 2017-December 2022) and had available postoperative MRI with at least 12 months of follow-up were included. A region-specific normative HFO database was retrospectively constructed using interictal physiological HFO detected from nonresected brain areas of 37 patients who remained seizure free after epilepsy surgery. The optimal HFO rate ratio was estimated by finding the ratio that best matched the resection volume that defines the EZ in seizure-free patients. Then an optimal HFO rate ratio was used to detect region-specific pathological HFOs.

Sixty-three patients (32 female, mean age 25 years) with 7512 bipolar SEEG recordings who underwent surgery for epilepsy were analyzed. The HFO rate ratio method accurately localized the EZ in 92% of patients with seizure freedom and identified pathological HFO rates in 80% of patients with persistent seizures, both within (19%) and outside (35%) the resected areas. The optimal threshold for the ripple HFO rate ratio to accurately localize the EZ was 5.8 times the normative value and 2.7 times that of fast ripples, with sensitivity and specificity greater than 85%.

These findings provide evidence that differentiating physiological and pathological interictal HFO rates are helpful in localizing the EZ, with potential utility in patient-specific surgical planning. Further prospective validation of this methodology is warranted.