32.1 Introduction

A progressive evolution in diagnostic testing and microsurgical techniques has made it possible to reduce the perioperative mortality rate of vestibular schwannoma (VS) microsurgery to less than 1%, especially at high-volume centers.s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur This, in part, has led to a shift in the surgeon’s attention to the preservation of cranial nerve function, in particular that of the facial nerve (FN) and the cochlear nerve.s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur Due to the intimate association between the FN and VS capsule, surgery for this tumor is fraught with risk of FN injury. Although the anatomy of the FN is fairly consistent in the temporal bone and the cerebellopontine angle (CPA), the tumor itself may distort or displace the nerve, which may be neither evident by way of preoperative FN deficits nor discernible on preoperative imaging. In fact, the FN is very resistant to tumor stretch and compression, and more often than not, preoperative FN function is found preserved even in most giant VS. The surgeon must be prepared to deal with difficult situations wherein the nerve is thinned and stretched, is enveloped, or is found positioned between the surgeon and the tumor. Less commonly, there are intraoperative situations that may lead to interruption of FN continuity, either intentionally or otherwise. Once a nerve is interrupted, reconstruction should be performed immediately in order to obtain the best results, either by means of a primary end-to-end coaptation or by a cable nerve graft interposition.s. Literatur In this chapter, we will discuss various pre- and intraoperative situations with regard to the FN and how to manage them.

32.2 Clinical Onset of Facial Nerve Paralysis

As discussed previously, the FN can withstand stretching imposed by tumor growth to a great deal and hence leave the patient without clinically apparent deterioration of function. The FN, as most motor nerves, is quite resistant to tumor infiltration and nerve dysfunction typically occurs very late in the course of tumor growth. Axon and Ramsdens. Literatur found that House–Brackmann grade I FN function can be maintained with only 10% of motor neurons functioning. Neuronal degeneration and axonal demyelination is counterbalanced with the collateral sprouting and hypertrophy of the innervated muscle fibers. Furthermore, the absence of epineurium around the CPA and internal auditory canal (IAC) segments of the FN allows effacement of the nerve fibers and spreading of the nerve fascicles over large tumors.s. Literatur ^,​ s. Literatur In slow-growing pathologies such as VS and meningiomas, FN fibers are stretched slowly over many months leading to splaying of fibers over the tumor capsule (Fig. 32‑1 ).s. Literatur Conversely, early-onset FN paralysis in cases of smaller IAC or CPA tumors should signal clinicians to a suspect FN schwannoma, cavernous hemangioma, or, much less commonly, a malignant lesion. The incidence of FN dysfunction associated with untreated VS is generally less than 10% and correlates with tumor size. FN paralysis is often preceded by facial twitch or spasm.

32.3 Intraoperative Facial Nerve Monitoring

Intraoperative FN monitoring has established itself as an integral part of skull base surgery. Intraoperative electromyography (EMG) monitoring helps identify and map the FN accurately, thereby enhancing preservation. This is particularly important when tumors or anatomical variations place the nerve at a higher risk.s. Literatur At the Gruppo Otologico, patients are routinely monitored using the Nerve Integrity Monitor (NIM-Neuro 3.0) manufactured by Medtronic Xomed (Jacksonville, FL). This is a two-channel electromyographic system used to detect FN function. The monitor displays the two channels simultaneously for detecting the activity of orbicularis oculi muscle and orbicularis oris muscle. To avoid interference and artifact, this machine is supplied with a “muting probe,” which is connected around the output cable of equipment, such as bipolar cautery or other external devices that generate interfering signals. Electrical stimulation or surgical manipulation of the FN results in evoked EMG responses that provide immediate feedback to the surgeon via loudspeaker and an oscilloscope. Intraoperative FN monitoring assists with: (1) early localization of the FN, (2) definitive identification of the FN, (3) minimization of trauma during dissection, and (4) confirmation of the functional integrity of the nerve during and after tumor removal.

After tumor removal is complete, postoperative FN function can be appraised with electrical stimulation at the proximal and distal sites, usually at the root exit zone of the nerve at the brainstem and at the fundus of the IAC, respectively. The main parameters used in the evaluation of postoperative FN function are the threshold of proximal stimulation and the amplitude of the evoked response. Good function is expected when proximal stimulation between 0.05 and 0.1 mA elicits an FN response. It is important to examine the amplitude of this response and compare proximal and distal stimulation results. Amplitudes higher than 200 mV are usually indicative of good postoperative function. The presence of EMG potentials as bursts and trains during the last steps of tumor removal is another good prognostic factor. Elevation of the threshold to 0.3 mA or above is usually correlated with poor postoperative nerve function, especially if associated with low amplitude. High-amplitude bomber-type trains, especially when they arise suddenly, represent a poor prognostic factor. Finally, a “silent FN” during the last stage of tumor removal, associated with absence of response to the stimulation, is correlated with poor postoperative nerve function. Further discussion regarding intraoperative FN monitoring can be found in Chapter 27.

32.4 Preoperative Decision Making with Respect to the Facial Nerve

Though it is impossible to discuss the entire logic of preoperative decision making in this chapter, we will briefly outline our general approach and strategy. In the case of small tumors (grade I and II)s. Literatur in patients of any age, we prefer to initially follow the tumor with a wait-and-scan approach,s. Literatur except in a few circumstances. In the case of grade I and II tumors that have to be operated upon either due to vertigo, or evidence of fast growth on wait-and-scan, or patient preference, the choice of surgical approach will largely depend on hearing status. In cases where preoperative hearing is poor, the enlarged translabyrinthine approach (ETLA) is employed, which is the safest procedure for the FN preservation in our experience, even in larger tumors. In cases with good preoperative hearing, the retrosigmoid and middle cranial fossa approaches are preferred, with the middle fossa approach carrying a higher risk of FN paresis at least in the early postoperative period.s. Literatur ^,​ s. Literatur Surgery is the preferred choice in tumors greater than grade III. In patients older than 65 years with large tumors, if the patient is fit for surgery, we may perform sub- or near-total resection in an attempt to preserve good FN function. In older patients with debilitation or surgical contraindications, or in cases of regrowth after sub- or near-total resection, we prefer to refer for radiotherapy.s. Literatur ^,​ s. Literatur

Apart from the options discussed above, a variety of other factors including patient preference, occupation, experience of the operating team, tumor location, cerebral venous anatomy, and degree of brainstem compression are to be taken into consideration while deciding how best to treat the tumor and manage the FN.s. Literatur