Correlation of Facial Nerve Function and Electrical Stimulation During Vestibular Schwannoma Surgery.

OBJECTIVE

The aim of the present study is to identify electrophysiological parameters in facial nerve monitoring that provide the best predictive values in vestibular schwannoma surgery.

STUDY DESIGN

Retrospective study.

SETTING

Tertiary care hospital.

METHODS

In total, 76 patients undergoing translabyrinthine vestibular schwannoma resection were included. Facial nerve monitoring was conducted with free-running electromyography and direct electrical stimulation. Direct electrical stimulation protocol was predicated on a constant-current delivery at the root exit zone before and after tumor excision; specifically, recordings were made with increasing current settings: 0.05, 0.1, and 0.3 mA. The maximum amplitude measured from either of the two muscle groups was used for further correlations. Facial nerve function was graded according to the House-Brackmann (HB) scale before and after tumor excision.

RESULTS

Immediate postoperative facial nerve outcomes were HB I (21%), HB II (40%), HB III (18%), HB IV (9%), and HB V (12%). Long-term facial nerve outcomes were distributed as follows: 66% HB I, 21% HB II, 12% HB III, and 1% HB IV. Results show that patients with a higher amplitude ≥ 1024 μV at orbicularis oris and/or frontalis had a >90% estimated probability of an HB I-II score in the long term. In contrast, patients with a low amplitude of, for example, 128 μV at orbicularis oris had an estimated 53% probability of an HB score of III or IV in the long term.

CONCLUSION

Facial nerve monitoring is an indispensable objective measure during vestibular schwannoma surgery. Higher amplitudes lead to more favorable results for patients. Amplitudes 500 and 1000 μV seem to be useful cutoff points to help guide patient expectations with respectively >84% and >90% estimated probability of an HB I-II score in the long term.