gms | German Medical Science

Objective: Intraoperatively, facial nerve (FN) function is assessed with direct electrical stimulation, observation of spontaneous activity and corticobulbar Motor Evoked Potentials (FN-coMEP). MEP-amplitude decrement and increase in motor threshold (MT) serve as warning criteria. A novel threshold criterion for extremity MEP additionally compared MT-increments to the non-operated side (bilateral final-to-baseline motor threshold level, BFB-MT). We applied BFB-MT for FN-coMEP with regard to postoperative FN-function.

Methods: 47 patients (26f; 46 ± 18yrs., 28 vestibular schwannoma (VS), 12 meningeoma, 2 metastasis, 1 medulloblastoma, 1 lipoma, 1 chordoma, 1 epidermoid cyst, 1 teratoma) undergoing posterior fossa tumor surgery were analysed. FN-function was assessed as per House-Brackmann score (HB) pre- and postoperatively at day 1 (1d), 7 (7d), 3 months (3m) and grouped in mild (HB score increase ≤1) or marked deterioration (HB score increase ≥2). FN-coMEP were elicited with anodal transcranial electric stimulation at C4; C3 referenced to Cz, and recorded from bilateral orbicularis oris and mentalis muscles. A BFB-MT difference of >20% (operated vs. non-operated side) was regarded as significant. Increase of MT on the operated side of ≥20mA served as reference criteriion (RefC). Risk factor analysis for postoperative FN deterioration was performed (patient characteristics, tumor size, extent of resection, BFB-MT and RefC)

Results: At 1d, 15% of the patients (7/47) showed mild HB deterioration, 36% (17/47) marked. At 7d, 12 patients showed mild deterioration (25%), 10 marked (19%). 3m follow-up in 40 patients (85%) showed recovery in 31 patients (78%). 5 patients (13%) showed mild and 4 (10%) marked deterioration (HB 3 (1), HB 4 (2); HB 6 (1)), the latter suffered from giant VS (2), recurrent tumor (1) or NF II (1). FN-coMEP changes according to RefC occurred in 10 patients (39.5±17.2mA) and BFB-MT in 11 patients (35.2±20.6mA), whereas mean threshold increase being 6±7.8mA on the unaffected side. 3/4 patients with FN-coMEP loss showed signs of recovery in their HB score at 3m, but didn’t reach preoperative status. BFB-MT and RefC correlated significantly with FN function at 1d,7d and 3m(p < 0.001). The correlation was stronger using the BFB-MT (spearman correlation: 1d: p = 0.683; 7d: p= 0.741; 3m: p = 0.616) compared to the RefC (spearman correlation: 1d: p = 0.620; 7d: p = 0.564; 3m: p = 0.684). The multivariate analysis revealed BFB-MT to be an independent predictor of postoperative FN deterioration at all 3 time points (1d: p = 0.017; 7d: p < 0.001; 3m: p = 0.002) - despite the low incidence of FN deterioration at 3m.

Conclusion: An increase of BFB-MT of more than >20% between the affected vs. unaffected side serves as a better predictive indicator for postoperative FN function than the RefC. Further, it was identified as an independent variable for postoperative FN-function at 1d, 7d and 3m.