Enhanced Precision in Personalized Intraoperative Monitoring of Facial Nerves: A Comparative Analysis of Functional...

Facial nerve preservation is critical in skull base surgery. Transcranial facial motor-evoked potential (tfMEP) monitoring can be limited by interindividual primary motor cortex (M1) facial cortex variability and intraoperative perturbations. We evaluated a functional MRI (fMRI)-guided, neuronavigation-assisted montage to personalize stimulating electrode placement and reduce thresholds during cerebellopontine angle tumor surgery.

In a prospective cohort (n = 20), tfMEPs were recorded simultaneously using 2 montages: (1) fMRI-guided electrode directly over the patient-specific M1 facial hotspot identified by preoperative task fMRI and (2) conventional C3/C4. Train-of-five stimulation elicited compound muscle action potentials from orbicularis oculi/oris. The primary end point was stimulation threshold (mA) to elicit >50 µV, measured at predefined stages (precraniotomy through closure). Secondary analyses assessed threshold changes after durotomy/cerebrospinal fluid (CSF) drainage and postoperative House-Brackmann (HB) grades.

fMRI-guided placement yielded lower baseline thresholds than conventional within patients (mean 65.5 vs 73.5 mA; Δ = -8.9 mA; P < .0001). The advantage persisted at closure (85.3 vs 96.3 mA; P < .0001). After durotomy/CSF drainage, thresholds increased modestly with both montages (+3.3 vs +4.0 mA; between-group P = .65). Monitoring feasibility was 100%, providing continuous data early in the case when direct nerve stimulation was not yet possible. At the latest follow-up, 17/20 (85%) had HB I and 3/20 (15%) HB II; no patient had permanent moderate/severe palsy.

Neuronavigation-assisted, fMRI-guided tfMEP reliably targets the individual M1 facial area and significantly lowers stimulation thresholds vs conventional C3/C4, without increasing susceptibility to threshold drift after CSF loss. This simple, adoptable workflow may enhance intraoperative decision making for facial nerve preservation. Larger multicenter studies are warranted to validate outcome prediction and refine threshold-based alarm criteria.