Neurophysiological monitoring of cranial nerves III and VI in endoscopic skull base surgery: scoping review and comparative study

Abstract

Intraoperative neurophysiological monitoring (IONM) of extraocular cranial nerves (EOCN) during skull base surgery has expanded to reduce post-operative neurological deficits. However, multiple techniques exist, and comparative evidence supporting their routine use in endoscopic endonasal approaches (EEA) remains limited. The aim of the study is to evaluate the reliability, signal quality, and safety of different IONM techniques for CN III and VI during EEA, integrating a scoping literature review with a single surgeon’s clinical experience. A scoping review identified studies reporting EOCN monitoring during EEA. In parallel, a retrospective analysis was conducted in patients undergoing extended EEA with multimodal IONM. Monitoring modalities included free-run electromyography (fEMG), triggered electromyography (tEMG), electrooculography (EOG), and corticobulbar motor evoked potentials (CbMEPs). Direct muscle recordings were obtained using hook-wire electrodes or modified 90°-bent subdermal needle electrodes, while indirect monitoring relied on EOG. Signal quality, reproducibility, feasibility, and procedure-related complications were assessed. Eleven studies involving 476 patients were included in the scoping review. In the clinical series, 51 patients underwent multimodal IONM. Direct EMG recordings provided stable and specific signals, whereas EOG showed a high artefactual component, limiting reliable nerve identification. CbMEPs yielded clear and reproducible responses in selected cases. Modified 90° bent dual subdermal needle electrode demonstrated superior signal quality and low impedance compared with hook-wire electrodes, which were associated with a higher rate of minor complications. Multimodal IONM combining CbMEPs, tEMG, and fEMG is feasible and effective during EEA. Modified 90° bent dual subdermal needle electrode appear to offer the most reliable and safe monitoring strategy for EOCN.