Stereo-electroencephalography (sEEG) provides a critical window into the brain's activity for patients with drug-resistant epilepsy. However, the quality of these recordings heavily depends on the choice of a reference point. Traditionally, surgeons used white matter (WM) electrode contacts as the standard. Recent clinical research now highlights the subgaleal electrode reference as a safer and more precise alternative for obtaining localizable data.

Advantages of the Subgaleal Electrode Reference

White matter references often introduce limitations such as a limited pool of available electrodes and a laterality bias. Moreover, white matter is not always electrically neutral, which can distort delicate neural signals. In contrast, the subgaleal (SG) space at the parietal midline offers a neutral extracranial location. Consequently, using an SG reference reduces cross-channel correlations and preserves significantly more power at higher frequencies. This technical improvement ensures that clinicians receive a clearer picture of seizure dynamics without the noise associated with intracranial references.

Clinical Impact and Patient Safety

A study involving 14 patients compared the effectiveness of SG, WM, and gray matter (GM) references. The researchers measured the seizure onset zone selectivity index (SSI) to evaluate how well each reference localized seizure activity. Notably, the SG reference yielded a higher SSI than both WM and GM options. This higher selectivity allows for more accurate mapping of the epileptogenic zone, which is vital for surgical planning. Furthermore, the placement of these electrodes proved entirely safe. No participants reported adverse effects from the SG electrode installation. Therefore, the SG reference acts as a reliable alternative that enhances signal quality while maintaining a high safety profile.

Key Findings for Neurologists

The transition to a midline subgaleal reference does not interfere with the clinical investigation. Instead, it improves the signal-to-noise ratio and simplifies the selection process for reference points. Since many WM contacts carry residual activity from nearby gray matter, the SG reference provides the neutrality required for high-fidelity research and clinical monitoring. Surgeons can easily place these electrodes during the standard sEEG procedure, making it a practical upgrade for modern epilepsy centers.

Frequently Asked Questions

What is the seizure onset zone selectivity index (SSI)?

The SSI is a metric used to quantify the ability of a specific electrode reference to isolate and localize the point where a seizure begins. A higher SSI indicates better signal quality and more precise localization of the epileptogenic zone.

Are subgaleal electrodes safe for long-term monitoring?

Yes, clinical data from recent studies shows that placing subgaleal electrodes at the midline is a safe procedure. Participants in these studies reported no adverse effects or complications during the monitoring period.

Why is white matter considered a suboptimal reference?

White matter was traditionally thought to be electrically neutral. However, research indicates it often contains volume-conducted activity from nearby gray matter and can exhibit a laterality bias depending on where the electrodes are placed.

Disclaimer: This content is for informational and educational purposes only. It does not constitute medical advice or a professional relationship. Refer to the latest local and national guidelines for clinical practice.

References

Devara D et al. Subgaleal versus white matter reference for stereo-electroencephalography recordings: comparison of clinical impact and signal quality. J Neurosurg. 2026 May 01. doi: 10.3171/2025.12.JNS251675. PMID: 42066368.

Mercier MR, et al. A glimpse on white matter signal. CerCo/Elsevier. 2017.

Cole AJ et al. Intracranial EEG Validation of Single-Channel Subgaleal EEG for Seizure Identification. J Clin Neurophysiol. 2022.