Subgaleal versus white matter reference for stereo-electroencephalography recordings: comparison of clinical impact and signal quality.

Abstract

Stereo-electroencephalography (sEEG) is a common method for clinical epilepsy monitoring and provides unique opportunities for intracranial research in humans. Optimal selection of reference electrodes is essential for obtaining high-quality localizable data. White matter (WM) electrode contacts are commonly used as references; however, this reference scheme presents several limitations that may influence the data, including a limited selection of electrodes in the WM, nonneutral activity in the WM, and a laterality bias. Here, the authors detail the use of a midline subgaleal (SG) electrode as an alternative reference for sEEG recordings. An SG reference was used for 14 patients with drug-resistant epilepsy undergoing intracranial monitoring. Following the placement of sEEG electrodes, one 8-contact sEEG electrode (n = 2) or 4-contact strip electrode (n = 12) was placed in the SG space at the parietal midline. In a subset of 4 participants, we obtained awake, resting baseline recordings (5-minute duration) using different references, allowing us to compare signals recorded with an SG, WM, and gray matter (GM) reference. The authors compared the number of interictal spikes (IISs) detected by measuring the seizure onset zone selectivity index (SSI), cross-channel correlations, and power spectral density properties across these baseline recordings. No adverse effect of the SG electrode placement was reported in any participant. Recordings using an SG reference have a higher SSI compared with a WM or GM reference. Neural signals obtained with an SG reference have lower cross-channel correlations compared with the other two references and preserve more power at higher frequencies than a WM or GM reference. Extracranial placement of an SG electrode allows for a neutral midline reference. The authors' findings demonstrate that an SG reference is a safe alternative to the standard WM reference, improving the signal-to-noise ratio and not interfering with the clinical investigation.