Facial Nerve Function in a Series of 494 Vestibular Schwannomas: Improvement by Technological Amendments?

Background and Objective As a consequence of conservative strategies such as watchful waiting or radiosurgical management, surgeons are faced mainly with medium to large vestibular schwannomas. Preservation of facial nerve function still remains the most important goal. This study investigated the possible improvement in functional outcome by technological amendments in imaging and neuromonitoring.

Methodology Out of a consecutive, prospectively collected series of 600 patients with vestibular schwannoma surgery (33% small without any brainstem contact, 67% large with broad brainstem contact or compression), 494 with sufficient follow-up of at least 12 months were analyzed for facial nerve function by House Brackmann scale before, early and long-term after tumor resection. Outcome was correlated with applied techniques such as preoperative imaging by gadolinium-enhanced CISS MRI and with applied monitoring techniques such as conventional electromyography (EMG) monitoring and, in more recent cases, combination with additional motor-evoked potentials (MEP) of the facial nerve. Surgeons were informed intraoperatively on MEP amplitude reductions to 60% of the baseline as relevant and to 40% as critical. Outcome at 7 to 10 days was summarized as useful in case of complete eye closure HB 1 to 3, and as nonuseful in missing or incomplete eye closure HB 4 to 6.

Results In the most recently applied MRI technique with additional Gd-Ciss, in 200 cases, the facial nerve could be identified in about two-thirds of these at its origin at the brainstem and at its anterior displacement. Within the whole series, early useful function HB 1 to 3 was present in 389 (79%) of patients and increased to 95% within the first year of follow-up. While sole EMG monitoring resulted in good early function in 174 of 232 patients (75%), those 262 patients monitored with additional facial MEP presented a significant increase to 82% early HB 1 to 3 (214 of 262; p < 0.01%). Patients' age, gender, or the sequence of surgical maneuvers were no relevant factors. Strong tumor adherence with the brainstem was a negative indicator. Positive predictors were normal preoperative facial function and small tumor size.

Conclusion Early facial nerve outcome may be improved by a dual monitoring technique of motor cranial nerves. In addition to EMG monitoring, mainly useful for nerve localization and mechanic reactivity of fiber groups, MEP, by transcranial activation, gives feedback on the integrity of the whole facial motor tract at any time, independent of the surgical exposure of the nerves and therefore independent of tumor size.

The increase in information, on the site and the function of the facial nerve, constitutes a major support for the surgeon who may anticipate the surgical field and who receives valuable directives for the continuation or change of the surgical procedure.

By this multimodality monitoring technique, the rate of useful early function is increased significantly from 75 to 82%. Complete eye closure 1 week after surgery is the best predictor for long-term normal facial nerve function and good quality of life.

No conflict of interest has been declared by the author(s).