36.1 Introduction

The modern retrosigmoid (RS) approach for resection of vestibular schwannomas (VSs) is the result of an evolution from the classical suboccipital approach initially used for VS debulking/resection. The surgical pioneers of the mid to late 1800s and early 1900s who endeavored to save the lives of patients with advanced VSs were focused on avoiding operative mortality and facial paralysis.s. Literatur ^,​ s. Literatur ^,​ s. Literatur While Krause advocated for a unilateral suboccipital approach, Cushing favored bilateral suboccipital exposure (Fig. 36‑1 ). Following the description of unilateral hearing loss as the most common first symptoms of VS by Cushing,s. Literatur awareness of the characteristic symptoms and diagnostic advances allowed subsequent surgeons to address smaller tumors, shifting focus from avoidance of death to preservation of function. Smaller tumor size at the time of diagnosis increased likelihood of facial nerve survival. By the 1970s, the prospect of hearing preservation with complete tumor removal was being discussed and reported in the literature.s. Literatur ^,​ s. Literatur ^,​ s. Literatur Initially, the unilateral suboccipital/transmeatal approach was favored for hearing preservation. However, William House demonstrated good results with the middle fossa (MF) approach.s. Literatur In the subsequent decades, the MF approach was reported to be more successful for hearing preservations. Literatur ^,​ s. Literatur ^,​ s. Literatur until reports of equally successful hearing preservation were reported for the RS approach.s. Literatur ^,​ s. Literatur ^,

Currently, equally good hearing preservation results are achieved by surgical centers using MF and RS approaches by experienced teams adept at their chosen techniques. Each approach has its advantages and disadvantages. The MF approach allows complete exposure of the superior compartment of the internal auditory canal (IAC). However, the lateral aspect of the inferior compartment is obscured by the position of the facial nerve, the surgical angle, and the falciform crest.s. Literatur ^,​ s. Literatur The RS approach provides an early view of the more medial aspect of the tumor and the course of the internal auditory artery with minimal facial nerve manipulation during tumor removal. However, the distal 2 to 3 mm of the IAC are not directly visible.s. Literatur Other differences in the approaches pertain to rates of temporary and permanent facial nerve paresis/paralysis, cerebrospinal fluid (CSF) leak rates, and chronic postoperative headache. The MF approach is reported to have a slightly higher rate of transient and permanent facial paresis. This is due to the facial nerve traveling over the superior aspect of the tumor and necessary mobilization to achieve tumor removal.s. Literatur ^,​ s. Literatur The RS approach is associated with a higher rate of lingering postoperative headaches that range from 5 to 16%.s. Literatur ^,​ s. Literatur

Regardless of surgical approach, intraoperative monitoring of hearing is of vital importance. A variety of studies have demonstrated the advantage of direct monitoring of the cochlear nerve action potential over auditory brainstem response (ABR) monitoring in achieving higher rates and quality of postoperative hearing preservation.s. Literatur ^,​ s. Literatur ^,​ s. Literatur Further descriptions of eighth nerve monitoring techniques during VS microsurgery can be found in Chapter 28.

36.2 Patient Selection

Patients with tumors of any size up to 25 mm are candidates for RS approach for VS resection with attempted hearing preservation. The success rate for hearing preservation is inversely proportional to increased tumor size. The best rates of hearing preservation are achieved in patients with good preoperative hearing and tumors of ≤ 10 mm.s. Literatur ^,​ s. Literatur ^,​ s. Literatur As tumor size increases, the rate of hearing preservation decreases fairly rapidly. Patients with tumors larger than 25 mm rarely have hearing preserved following tumor removal.s. Literatur However, in the authors’ practice, a well-informed patient with a tumor ≥ 25 mm who demands an attempt at hearing preservation is accommodated with the understanding that tumor recurrence and the facial nerve are placed at some small increased risk for the effort of hearing preservation.s. Literatur

Some reports indicate that involvement of the fundus by tumor is an indicator of reduced success in hearing preservation.s. Literatur Other authors find that overall tumor size is more important than fundal involvement.s. Literatur Other factors such as quality of hearing present at the time of surgery may suggest better or worse prognosis for hearing preservation. The authors do not advocate preoperative ABR or video nystagmography to aid in the decision-making process for hearing preservation since such information is not as important as tumor size and patient desires. The absence of preoperative ABR waveforms does not preclude intraoperative monitoring with direct eighth nerve monitoring (DENM) since a cochlear nerve action potential is commonly obtained even in patients with absent ABR.s. Literatur

All patients evaluated for VS are counseled regarding the options of: observation with serial magnetic resonance imaging (MRI); stereotactic radiosurgery/radiotherapy; and microsurgical resection. If hearing preservation is a priority in a patient with a small tumor, either observation or microsurgical resection may yield better long-term results. Radiation treatment is associated with a loss of useful hearing within 5 to 10 years of treatment in greater than 75% of cases.s. Literatur ^,​ s. Literatur For patients with small tumors and word recognition scores (WRS) of 100%, observation results in preservation of good quality hearing in 88% over the decade following diagnosis. However, only 55% of patients with even modest initial WRS decline maintained good hearing during observation.s. Literatur For select tumors ≤ 10 mm, the likelihood of hearing preservation with microsurgical removal may be as high as 80 to 85% when performed by an experienced team.s. Literatur ^,​ s. Literatur ^,​ s. Literatur The interested reader is directed to Chapter 53 for a review of hearing preservation outcomes according to surgical approach, Chapter 51 for observation, and Chapter 52 for radiosurgery.

36.3 Surgical Technique

The patient is positioned on the operating table in the supine position. After induction of general anesthesia and placement of facial and cochlear nerve monitoring leads, the patient is positioned for surgery. Some surgeons place a shoulder roll under the ipsilateral shoulder but the authors find that this is counterproductive to achieving access to the retroauricular scalp. Park-bench position is advocated by some surgeons but is awkward and time consuming to perform, and may increase the risk of pressure sores and transient neuropathy. Lumbar CSF drainage is not employed. The patient’s head is placed in pin fixation and turned toward the contralateral shoulder. The head is brought above the plane of the ipsilateral shoulder, if possible, and then nodded toward the contralateral shoulder (Fig. 36‑2 ). This rotates the suboccipital skull away from the shoulder, facilitating surgical access to the postauricular scalp. Care must be taken not to overrotate the head, which can impair cerebral venous outflow. Furthermore, overrotation can cause stretch injury to the brachial plexus. Finally, space must remain between the mandible and contralateral clavicle to avoid creating pressure sores.

A small area of hair is clipped from behind the ear to allow placement of a 6- to 7-cm incision line roughly two fingerbreadths behind the auricular crease. The mastoid tip is usually the lower limit of the incision. A tangent along the upper margin of the ear canal approximates the plane of the transverse sinus and usually demarcates the upper one-third of the incision from the lower two-thirds (Fig. 36‑3 ). Local anesthetic consisting of 1 or 2% xylocaine with 1:100,000 epinephrine is injected into the skin and subcutaneous tissues primarily for hemostatic purposes. Preincision intravenous medications include dexamethasone 10 mg, mannitol 75 g, and cefazolin 2 g.

Facial nerve and ABR monitoring electrodes are placed on the patient and connected to their respective devices. The electrode and ear speaker setup for ABR is used for DENM as well. The skin is prepared antiseptically. The authors prefer DuraPrep which, when dry, leaves the skin sticky, facilitating drape adhesion. The skin and underlying muscle are incised down to the calvarium, keeping the trajectory perpendicular to the skin. Periosteal elevation is directed mostly anterior to expose the posterior aspect of the mastoid process. The posterior aspect of the digastric groove is visualized and serves as a guide to the posterior edge of the sigmoid sinus (Fig. 36‑4 ). The 2-cm craniectomy is undertaken in a position near where the sigmoid–transverse junction is expected. The anterior limit of bone removal is the posterior aspect of the sigmoid sinus. Superiorly the limit of bone removal is the inferior aspect of the transverse sinus (Fig. 36‑5 ). Any mastoid air cells that are opened during the craniectomy are carefully occluded with bone wax to reduce the chance of CSF leak.

The dura is opened in a cruciate fashion and the dural flaps held aside with tacking sutures. The cerebellum is next elevated from the floor of the posterior cranial fossa to access the basal cistern and CSF is released. This results in cerebellar relaxation and facilitates posterior retraction of the cerebellum. The cerebellum is initially held in place with a retractor, but as the case proceeds, the retractor often becomes unnecessary as the cerebellum remains relaxed. The anatomy of the cerebellopontine angle (CPA) is next inspected to identify the lower cranial nerve (glossopharyngeal, vagus, spinal accessory) entering their pars nervosa. The trigeminal nerve is typically anterosuperior to the cochleovestibular nerve/facial nerve bundle and/or tumor. If no tumor is present in the CPA, the facial nerve is usually found anterior to the cochleovestibular nerve. If tumor is present in the angle, the facial nerve stimulating probe is used to stimulate the posterior surface of the tumor to ensure the facial nerve is not traveling on this side of the tumor.

Usually, ABR is used during CPA tumor debulking; however, if the tumor in the angle is small, placement of a cochlear nerve electrode for DENM during this activity is feasible. The capsule of the tumor in the CPA is then opened sharply and internal debulking of the tumor is then undertaken. As the capsule becomes more mobile and can be collapsed into the space created by debulking, identification of the cochlear and facial nerves is undertaken. Once significant debulking of the CPA portion of the tumor is performed, attention turns to the posterior surface of the petrous bone. The location of the operculum of the endolymphatic sac (ELS) is often well seen as a dimple on the surface of the posterior petrous bone. The double-layer dura of the ELS is typically whiter than the adjacent single-layer dura. An inverted U-shaped incision is made in the dura centered on the IAC with the lateral limit of the incision being the operculum. This dural flap is then elevated and excised. Direct visualization of where the ELS enters the bone at the operculum is achieved and bony dissection of the IAC is undertaken. If the DENM electrode was placed during CPA tumor debulking, it is removed during drilling to avoid potential entanglement with the drill.

The CPA cisterns above and below the cochleovestibular nerve are filled with papaverine-impregnated BICOL collagen sponges to reduce dispersion of bone dust. Using constant suction–irrigation and a 4-mm diamond burr, the bone on the posterior surface of the petrous bone is removed in a semicircle pattern until the dura of the posterior aspect of the IAC can be seen through thin bone. The location of the operculum is kept in sight during this dissection to avoid labyrinthine injury. Once the dura of the posterior surface of the IAC is clearly seen, corridors of bone removal are developed superior and inferior to the IAC. As bone removal around the IAC proceeds from medial to lateral, burr size is changed to accommodate the anatomic limitations of the spaces being worked. To determine the lateral limit of bony dissection, the course of the ELS and duct are carefully exposed and followed for a short distance toward the labyrinth (Fig. 36‑6 , Fig. 36‑7 ). Staying medial to this important landmark will prevent labyrinthine injury.s. Literatur The goal for the extent of bone removal is to achieve 270-degree (or greater) circumferential IAC exposure to within 2 to 3 mm of the fundus.

In addition to using the ELS and duct as guides to the lateral limit of bone removal, specially designed IAC dissectors (available from Grace Medical, Memphis, TN) are employed to separate the dura from the bony IAC and extradurally estimate the remaining amount of unexposed IAC. This direct measurement is very useful at determining the adequacy of exposure relative to the tumor’s degree of fundal extension. The markings on the dissectors are millimeter markers allowing fairly precise determination of remaining unexposed IAC. The falciform crest is routinely palpated and dissection commonly taken to within 2 mm of the inferior compartment, and within 3 mm of the superior compartment of the IAC (Fig. 36‑8 ). Once the bony work has been finished, preparation is made to open the IAC dura and remove tumor from the canal.

Direct cochlear nerve monitoring is always employed during the removal of the IAC portion of the tumor. A specially designed electrode (available from AD Tech Medical Instrument Corporation, Racine, WI) provides stable positioning of the electrode that is not displaced by CSF pulsations. DENM provides the most rapid feedback regarding cochlear and cochlear nerve status of all the hearing monitoring modalities. Waveform amplitudes are one to two orders of magnitude larger than ABR, allowing capture of a cochlear nerve action potential with 10 to 20 sweeps (1–2 s). The earliest changes in DENM that may signal cochlear/cochlear nerve stress is a prolongation of action potential latency with decline in amplitude occurring later. Amplitude changes without latency prolongation may be due to blood in between the electrode contact surface and the cochlear nerve.

Once the cochlear nerve electrode is placed, the dura of the IAC is sharply opened to form superior and inferior rectangular flaps. The dural flaps are laid into the superior and inferior bony corridors created during bony IAC dissection. Dissection progresses from medial to lateral to diminish any sheering force on the cochlear nerve at the habenula perforata. Additionally, the course of the internal auditory artery (also known as the labyrinthine artery) is kept in mind. Should the artery travel directly on the tumor surface, dissection may result in arterial spasm which will often respond to suspension of dissection and application of papaverine. As in the CPA, the IAC tumor is centrally debulked once the facial and cochlear nerves are identified. The superior and inferior vestibular nerves are purposefully sacrificed as these are the nerves of origin in the vast majority of these tumors. Additionally, prior experience with trying to preserve these nerves during VS removal was found to result in a higher incidence of prolonged postoperative imbalance problems.

As the tumor dissection approaches the fundus of the canal, the IAC dissectors are most useful in separating tumor from the facial and cochlear nerves. Additionally the spoon shape of the dissecting tip facilitates delivery of tumor from the portion of the fundus that is not directly visible (Fig. 36‑9). Endoscopic inspection of the canal fundus may be employed following completion of tumor removal if the surgeon desires. In most cases, it is clear by the shape/contour of the tumor delivered from the fundus whether residual tumor may remain. A careful systematic dissection of the distal IAC is undertaken to ensure no tumor remains.

Once tumor removal has been completed, the cochlear and facial nerves are transiently covered with papaverine-impregnated Gelfoam or BICOL. All bony surfaces that were drilled during the IAC exposure are carefully coated with bone wax pressed into place with a micro-Cottonoid to reduce the risk of CSF leak. The papaverine-impregnated Gelfoam or BICOL is removed and methylprednisolone acetate (Depo-Medrol)-impregnated Gelfoam/BICOL is placed on the IAC nerves for a few moments. The latter is removed and then copious irrigation of the CPA is undertaken to remove any bone dust or blood. Careful survey to ensure hemostasis has been achieved is performed before initiating dural closure.

The dura is closed with a combination of interrupted and continuous 4–0 braided nylon (Neurolon) suture. Layered microfibrillar collagen hemostat (Avitene) which is then saturated with venous blood is used to fill the small craniectomy defect. This serves to seal the dural suture-line and will be replaced by scar in the healing process. The muscular fascia layers are approximated with interrupted 2–0 braided absorbable sutures. The same suture is used in an inverted, interrupted fashion to approximate the dermal layer of the skin. Staples are then used to approximate the epidermis. The skin is then cleansed and dried. A small quantity of antibiotic ointment is applied along the staple line and a nonstick gauze, adhesive island dressing is placed over the closed incision. The patient is then taken out of pin fixation and allowed to awaken from general anesthetic.