Introduction

Effective surgical management of drug-resistant epilepsy depends on precise epileptogenic zone delineation. While clinicians typically rely on preictal and ictal stereoelectroencephalography (SEEG) data, interpreting these signals remains challenging. Recent research suggests that examining interictal high-frequency oscillations (HFOs) provides a more reliable complementary approach. Specifically, mapping region-specific HFO rates helps distinguish pathological activity from normal physiological brain signals.

The Role of HFO Rate Ratios

High-frequency oscillations are electrical signals exceeding 80 Hz. Although they are hallmarks of the epileptogenic zone (EZ), they also appear in healthy cortex areas. Consequently, surgeons struggle to identify which HFOs indicate disease. To solve this, researchers retrospectively constructed a normative database from patients who achieved seizure freedom. They compared pathological HFO rates against these normative benchmarks to calculate a specific rate ratio. This method allows for a more objective identification of the seizure-generating tissue.

Improving Epileptogenic Zone Delineation with HFO Ratios

The study analyzed 63 patients and over 7,500 SEEG recordings to determine the most effective thresholds. Results showed that the HFO rate ratio method accurately localized the EZ in 92% of seizure-free patients. Specifically, the optimal threshold for ripples was 5.8 times the normative value. Furthermore, for fast ripples, the threshold was 2.7 times the normative rate. These thresholds yielded sensitivity and specificity rates exceeding 85%. Moreover, the method identified pathological activity even in patients with persistent seizures, indicating its robust diagnostic potential.

Clinical Impact on Surgical Outcomes

Differentiating between physiological and pathological interictal activity is vital for personalized surgical planning. By using normative benchmarks, clinicians can better predict which brain regions require resection. Therefore, this approach reduces the risk of removing healthy tissue while maximizing the chances of post-operative seizure freedom. Future prospective validation will likely solidify this methodology as a standard in neurosurgical practice.

Frequently Asked Questions

What are high-frequency oscillations (HFOs) in SEEG?

HFOs are rapid brain signals above 80 Hz recorded during intracranial monitoring. They are divided into ripples (80-250 Hz) and fast ripples (over 250 Hz), acting as biomarkers for the epileptogenic zone.

How does the HFO rate ratio help in epileptogenic zone delineation?

The ratio compares a patient\'s local HFO rates to a database of healthy cortical activity. If the rate exceeds a specific threshold (e.g., 5.8 for ripples), the area is highly likely to be part of the epileptogenic zone.

Why is interictal activity important for epilepsy surgery?

Interictal activity occurs between seizures. Analyzing these signals allows clinicians to map the brain without waiting for a seizure to occur, making the diagnostic process faster and potentially safer.

Disclaimer: This content is for informational and educational purposes only. It does not constitute medical advice or establish a doctor-patient relationship. Always seek the advice of a qualified healthcare provider for any medical condition. Refer to the latest local and national guidelines for clinical practice.

References

1. Parasuram H et al. Mapping physiological high-frequency oscillation rates of the cerebral cortex for improved epileptogenic zone delineation in stereoelectroencephalography. J Neurosurg. 2026 Feb 27. doi: 10.3171/2025.9.JNS25602. PMID: 41759173.

2. Frauscher B, et al. High-frequency oscillations: The state of the art and future perspectives. Epilepsia. 2017;58(8):1316-1329.

3. Zijlmans M, et al. High-frequency oscillations as a biomarker for epilepsy. Expert Rev Neurother. 2012;12(10):1231-43.