Chapter 45 Suboccipital Retrosigmoid Surgical Approach for Vestibular Schwannoma (Acoustic Neuroma)

For the surgery of vestibular schwannoma (acoustic neuroma), refinements of microsurgical techniques, combined with improvements in intraoperative monitoring of facial and cochlear nerve function and advances in neuroimaging, have resulted in the shift of focus from reduction of mortality to optimizing facial nerve function,^1–4 hearing preservation,^1,2,5 and preservation of other cranial nerves.⁶ Facial nerve dysfunction is a major concern among patients undergoing unilateral vestibular schwannoma surgery, and loss of useful hearing can be debilitating.⁷ Patients with vestibular schwannoma have many options, including watchful waiting, radiosurgery, fractionated radiation, and surgery through one of several routes: translabyrinthine, middle fossa, or suboccipital retrosigmoid. The ideal treatment strategy depends on several factors, including the age, medical condition, and hearing status of the patient and the anatomy of the tumor as seen on imaging studies. Surgeons generally try to save hearing if the speech discrimination is above 50% and the average pure tone loss is less than 50 dB.^8,9 However, even worse hearing is worth saving in some circumstances, such as when poor hearing is present on the contralateral side or in a patient with neurofibromatosis type 2 (NF2), where bilateral hearing loss could later occur. Moreover, these various treatments are not mutually exclusive; in some patients, a combination of subtotal surgical tumor removal followed by radiosurgery of the residual may prove appropriate.

Preoperatively, whenever appropriate, we have the patient see not only the neurosurgeon but also an otologist and a radiosurgeon or radiation oncologist to explore the various options. If surgery is the chosen option, the various risks and complications are explained, the patient is tested with the neck in the position of surgery to assure that no symptoms are produced from spinal cord compression due to a secondary cervical lesion, and preoperative laboratory studies are performed. An audiogram with speech discrimination and magnetic resonance imaging (MRI) with gadolinium and fine cuts through the posterior fossa are routinely performed. We do not routinely perform a computed tomography (CT) scan or combined CT and magnetic resonance angiogram or CT and magnetic resonance venogram; these are only done if indicated by the initial MRI. If the patient has NF2, cervical MRI with gadolinium is performed to detect possible cervical spinal cord and nerve tumors.

The patient is started on dexamethasone (8 mg twice daily) for 48 hours prior to surgery, along with a gastric proton blocking agent such as pantoprazole (40 mg twice daily) or a related pharmaceutical. The patient is admitted the day of surgery, and the side and site of surgery are confirmed with the patient and marked on the skin. The surgical team reviews the plans for the procedure. The anesthesiologist places an intravenous line, and general anesthesia is administered. Care must be taken not to use paralytic agents that will last into the procedure and interfere with monitoring of facial or other cranial nerve function. An arterial line and embolism-prevention airboots are placed. Antibiotics are administered, and a urinary catheter and nasogastric tube are placed. Dexamethasone (10 mg) and furosemide (10 mg) are administered intravenously. An infusion of mannitol is begun (250 to 500 cc of a 20% solution). If hearing is to be saved and if there is no medical contraindication, nimodipine (60 mg) is given via the nasogastric tube.

The patient is positioned supine with one or two folded soft cotton blankets beneath the ipsilateral shoulder and padding beneath the knees and is secured with one or two belts to allow the table to be rolled if necessary during surgery. For patients with a cervical tumor (as in NF2) or elderly patients with cervical spondylosis or limitations in neck movement, additional blankets are used to further turn the upper body of the patient and thereby require less turning of the neck. However, too much turning of the body can place the shoulder in a position that interferes with surgery. To prevent peripheral neural compression, the surgeon must ensure the contralateral ulnar and fibular head areas will not be under pressure if the bed is rolled and pad them appropriately. The head is gently turned toward the contralateral side. Hair is shaved from about 7 cm behind the ipsilateral ear, and the patient is fixed in a three-pin headrest (Fig. 45-1A). The surgeon must assure that there is adequate space between the chin and the clavicle and that contralateral jugular compression is avoided. Physiologic monitoring equipment is placed. Bipolar facial monitoring electrodes are routinely placed in the orbicularis oculi and orbicularis oris. If trigeminal nerve function is of concern, an additional electrode is placed in the masseter. For hearing monitoring, the ear canal is inspected and cleaned if necessary; an electronic clicker is placed in the ipsilateral ear and the contralateral ear, with electrodes in the scalp to record a brain stem auditory evoked response (BAER) during surgery. If lower cranial nerves are involved, we use an endotracheal tube with an embedded electrode.

FIGURE 45-1 Positioning.

A, The patient is positioned supine, with the head gently turned toward the contralateral side and fixed in a three-pin headrest. A timeout is taken for the surgical team to assure the correct patient, correct site, and correct procedure. The incision and the monitoring apparatus are demonstrated. B, The supine position of the patient is seen from the vertex. The endotracheal tube exits the mouth on the contralateral side to avoid distortion of the ipsilateral lip and cheek. The small TV camera (secured with yellow tape) has a well-illuminated view of the patient’s ipsilateral face beneath a triangular metallic protector that will hold the operative drapes upward.

The self-retaining retractor post is secured to the side of the headrest away from the surgical site. A metal triangle is used to protect the face and allow both the anesthesiologist and the neurophysiologist or audiologist visualization of the airway and of facial movement. The endotracheal tube exits the mouth on the contralateral side to avoid distortion of the ipsilateral lip and cheek. A fiberoptic tube delivers a cool but bright light to the area under the drapes and assures that the small TV camera also under the drapes has a well-illuminated view of the patient’s ipsilateral face (Fig. 45-1B). Stimulation of the facial nerve in the operative field causes a clearly visible “smile” reaction, which can be seen on the external TV monitors. In contrast, electric activity that causes stimulation of the motor branch of the trigeminal nerve in turn causes jaw movements due to activation of the masseter muscle. An audiologist or neurophysiologist monitors facial movements, electromyogram (EMG), and BAER throughout the procedure.

The area behind the operative ear and an area on the ipsilateral abdomen are sterilely prepared and draped for surgery. A linear incision is made approximately 3 cm behind the insertion of the pinna and going from approximately 1 cm above the tip of the pinna to 1 cm below the ear lobe. Superiorly, a 3-cm piece of pericranium is removed and placed in sterile solution during the procedure for later use in closing the dural defect. The muscles are then divided inferiorly, hemostasis is obtained, and two muscle self-retaining retractors are placed. They are angled such that the medial skin area is free from these muscle retractors, because that is where the cerebellar retractors will later be situated (Fig. 45-2A).

FIGURE 45-2 Opening and tumor exposure.

A, The skin, muscle, and bone opening exposing the dura. (Cranial is to the left. Lateral is upward.) The superior limit of the exposure is the inferior edge of the transverse sinus. The lateral limit of the exposure is the medial edge of the sigmoid sinus. The retractors are angled such that the medial skin area is free to make room for the cerebellar retractors. B, The dural opening. CSF has been removed, and the cerebellum falls away from the cerebellopontine angle because of gravity in this supine position. A ½-inch cottonoid patty gives a reference as to the size of the exposure. C, Three 1⁄4-inch retractors are placed on Telfa and a rubber dam is placed on the cerebellum and positioned to spare the very large vein located superiorly (left side of the photo). The tumor is being stimulated with a monopolar facial nerve stimulator.

The transverse sinus and sigmoid sinus generally curve around the asterion. Therefore, a bur hole is placed inferomedial to the asterion. A high-speed drill with irrigation, as well as rongeurs, is used to define the inferior edge of the transverse sinus and the medial edge of the sigmoid sinus. The medial dura is then stripped from the bone using a No. 3 Penfield dissector, and a craniotome is used to remove a free bone flap, which is placed with the pericranial graft for later insertion. The bone edges are waxed, paying particular attention to the areas of the mastoid where nonsecured air cells can be a cause of postoperative cerebrospinal fluid (CSF) leakage.

The incisional area is surrounded with antibiotic-soaked sponges and towels in preparation for dural opening. The dura is opened with a linear vertical incision approximately 2 cm medial to the edge of the sigmoid sinus. The medial dura is left intact to protect the cerebellum. Additional dural incisions are made laterally toward the edge of the venous sinuses. The resulting triangular-shaped dural leaves are sutured to the surrounding tissue with 4-0 Vicryl sutures. An adjustable bar is attached to the retractor bar that previously had been affixed to the headrest, and a long flexible arm and long narrow retractor are attached. The inferior cerebellum is gently elevated with the retractor, and the arachnoid of the cistern is opened sharply to release CSF and provide for cerebellar relaxation. For larger tumors, a ventriculostomy catheter is put into the cistern, secured to the inferomedial dura with a 4-0 Vicryl suture, cut off at the dural edge, left in place for continuous CSF drainage throughout the procedure, and removed during closure. For smaller tumors, this is often unnecessary. At the time of initial dural opening, if the cerebellum is under a lot of pressure, it is often beneficial to open initially the most inferior triangular dural flap only and then release a small amount of CSF to allow the remainder of the dural opening to be performed under more controlled conditions. The surgeon must, however, be cautious not to remove too much CSF when the dura is not fully opened; doing so can be associated with too much cerebellar relaxation, causing a venous tear near the tentorium or in the area of the petrosal sinus. This can be difficult to control and forces a more rapid dural opening than might be desired.

Once the dura is opened and CSF removed, the cerebellum is generally relaxed and, in this position, falls away from the cerebellopontine angle because of gravity (Fig. 45-2B). A piece of moistened rubber dam covered with Telfa is placed on the cerebellum. Three long flexible arms and three long, 1⁄4-inch-wide self-retaining retractors are used. The retractors are placed on the Telfa with the tips on the cerebellum 1 or 2 mm proximal to the tumor edge. Care must be taken to be sure the retractors do not occlude important vascular structures or impinge on or stretch cranial nerves. They should be formed to sit securely on the skin edge to prevent unwanted movement. Occasionally, with a large tumor and an operculum of cerebellum overhanging the tumor, it is safer to resect the lateral cerebellar operculum than to retract it for long periods.

The operating microscope is engaged. At this point, stimulation of the posterior capsule of the tumor is necessary (Fig. 45-2C). I use a monopolar stimulator and first check it at 3 mA on the exposed neck muscles to be sure the stimulator is working and the patient is not pharmaceutically paralyzed. If muscle stimulation occurs, the stimulator is turned down to 1 mA and the posterior capsule is stimulated at all points to exclude a posteriorly placed facial nerve. If no stimulation is encountered, the tumor may be entered. In more than 80% of patients, the facial nerve is anterior or inferior. However, if stimulation occurs, the surgeon must determine whether this is due to transmission to a distant facial nerve or whether the nerve is indeed on the posterior tumor surface. To determine this, the posterior capsule is mapped out at sequentially lower milliampere levels to determine the course of the facial nerve. The surgeon must define an entry point into the tumor that is devoid of facial nerve fibers.

The surgeon must pay close attention to the arachnoid over the tumor. If this can be stripped away without coagulation, it provides not only the best plane for dissection but also a protective barrier for the cranial nerves and their small feeding vessels. After peeling away the arachnoid, a nonstimulating area on the posterior tumor is coagulated with bipolar cautery and opened with microscissors and specimens are sent for pathologic study. The internal portion of the tumor is then decompressed using an ultrasonic aspirator (Fig. 45-3A). Hemostasis is obtained, and the tumor capsule is dissected from the arachnoid plane and rolled inward. Depending on the size of the tumor, this process of internal decompression followed by dissection and inward rolling the capsule may need to be repeated several times. Each time, some of the capsule is also cut away but always leaving a visible portion to make the next round of microdissection easier. The retractors may need to be moved, but the surgeon must always keep them low profile, affixed to the skin, and with the tips on the cerebellum, not the cerebellar peduncle, brain stem, or critical blood vessels.

FIGURE 45-3 Tumor removal.

A, Internal decompression of the tumor is performed with an ultrasonic aspirator in one hand and No. 16 suction in the other. B, The internal auditory canal is being exposed using a high-speed drill in one hand and a continuous suction-irrigator in the other. C, The dura of the internal auditory canal is exposed. Superiorly (to the left) within the canal is a large mastoid air cell. D, The microdissector and No. 16 suction are being used to dissect the tumor off the vestibular–cochlear and facial nerves. The tumor is rolled from medial to lateral. Some bleeding over the facial nerve, as seen in this photo, is best stopped with irrigation.

At some point, the limiting factor becomes the entry of the tumor into the internal auditory canal and its attachments at the medial edge of the canal. It then becomes necessary to open the canal. I usually work with an otologist to do this part. However, in some circumstances, the neurosurgeon may do this portion of the procedure. The superior and inferior lips of the internal auditory canal are defined. If there are open CSF cisterns, they are occluded with moistened gelfoam pledgets to prevent entry of bone dust. However, all patties with strings on them must be removed from the field prior to using a drill. The dura over the canal is coagulated and flapped medially. Using a suction-irrigator and high-speed diamond drill (Fig. 45-3B), the canal is opened to expose the dura of the canal (Fig. 45-3C). Attention is paid to air cells that will later need to be secured (Fig. 45-3C) and to a high jugular bulb if present. If hearing is being spared, the drilling of the canal should be limited to 10 mm or less and must not include the vestibular apparatus. Once drilling is completed, the entire operative field is washed clear of bone dust, the temporary gelfoam pledgets are removed, and the CSF spaces are irrigated with saline. The dura of the canal is then opened sharply.

For a case in which hearing is being spared, we do not necessarily define the facial nerve at this point in the canal; rather, because these are smaller tumors, we proceed to define the facial nerve medially as it enters the brain stem. The tumor can then be rolled medially to laterally, sparing both the facial and the cochlear nerves (Fig. 45-3D). There is often a nice arachnoid plane, and as the tumor is rolled into the canal, parts of the tumor may need to be removed to decrease the bulk. During the dissection of tumor off the facial nerve or cochlear nerves, the surgeon must minimize the use of bipolar cautery. If bleeding starts between the tumor and the facial nerve, it can often be stopped with irrigation or temporary application of Surgicel. Even using bipolar cautery at low levels can cause facial or cochlear nerve damage. Ultimately, the most lateral portion of the tumor can be removed with microdissectors. Ideally, the surgeon desires not only that the facial nerve be anatomically intact but also to stimulate at low milliamperes at the brain stem level and that a good wave V remains on the BAER.

In the case of larger tumors and patients for whom hearing cannot be saved, once the dura of the canal is opened, using the stimulator initially on 1 mA but then gradually lowering to 0.1 mA, the facial nerve is identified in the distal canal as distinct from the tumor and from the other nerves. The vestibular and cochlear nerves are cut, and the tumor is rolled out of the canal from distal to proximal, with the facial nerve preserved under direct view. The tumor is invariably tightly attached at the proximal edge of the internal auditory canal. These attachments must be isolated. Stimulation is used to exclude the presence of the facial nerve; the attachments are then bipolar coagulated and sharply divided. At this point, the surgeon can start to roll the tumor in on its internally decompressed self and away from the facial nerve. The petrosal vein may hinder dissection at the superior part of the tumor. For large tumors, the petrosal vein is usually coagulated and divided. For smaller tumors, this may not be necessary. Occasionally, the petrosal vein is particularly large and may be the only major draining vein in this area. In such instances, it is wise to preserve it if possible to minimize the possibility of cerebellar venous infarction.

If the tumor is being dissected from lateral to medial, the facial nerve most commonly adheres to the tumor just medial to the internal auditory canal. This often limits the lateral to medial dissection, and the surgeon must start dissection in a different direction. If the superior tumor capsule is rolled in on the main bulk of the tumor, the trochlear nerve (cranial nerve IV) becomes visible parallel to the tentorial edge. Deeper and slightly inferiorly, the fibers of the trigeminal nerve (cranial nerve V) are encountered. At times, there appears to be a nice plane between the trigeminal nerve and the tumor; however, do not be tempted by this, as it is often a different plane from the plane between the facial nerve and the tumor and, if followed, may lead to transection of a thinned-out facial nerve tightly attached to the tumor capsule.

In contrast, if the surgeon goes to the inferomedial part of the tumor, the facial nerve and vestibular–cochlear nerves can be seen as separate from the tumor as they enter the brain stem. The vestibular–cochlear nerves do not stimulate and can be divided (provided hearing is not being spared), and the tumor can then be rolled from medial to lateral away from the facial nerve and from inferior to superior away from the glossopharyngeal (cranial nerve IX), vagus (cranial nerve X), and accessory (cranial nerve XI) nerves. Deep in the cavity of the dissection may be the abducens (cranial nerve VI) nerve.

The arachnoid plane must be retained between the facial nerve and the tumor; however, as the surgeon comes to the area near the internal auditory meatus, where the facial nerve can be more tightly attached to the tumor, it may be necessary to sharply cut the tumor away from the facial nerve if blunt dissection is not adequate. Even if the facial nerve is tightly stretched and has the appearance of wet tissue paper such that the surgeon can see through it, the nerve can still be functional. I have found that if it stimulates at 0.3 mA or less at the brain stem level after tumor removal, good facial function will likely ensue.

Although much of the focus is on removing the tumor while sparing the important cranial nerves, blood vessels may also be encountered, including the anterior inferior cerebellar artery, which may loop near the internal auditory canal or be displaced in various other locations, including near the facial nerve. Superiorly, the surgeon may encounter the superior cerebellar artery; deep to the tumor, the larger vertebral–basilar trunk may be seen. These must be preserved and with microdissection can usually be dissected away from the tumor. In cases for hearing preservation, it must be remembered that the internal auditory artery is the only blood supply to the cochlea and must be saved if hearing is to be saved.

Once the tumor is removed and hemostasis is obtained, the internal auditory canal that has been drilled must be secured to prevent CSF leakage postoperatively. A 1-inch incision is made in the skin of the abdomen that had been previously prepared sterilely. A piece of subcutaneous adipose tissue is removed, hemostasis is obtained, and the incision is closed with 3-0 Vicryl sutures in the subcutaneous tissue plus a 4-0 Vicryl subcuticular closure and Steri-Strips. If large air cells are visible where the internal auditory canal had been drilled, some Tisseel or fibrin glue is placed within (Fig. 45-4A) and then packed with one or a few small pieces of adipose tissue covered with bone wax. The drilled surface of the canal is secured with bone wax, and a larger piece or multiple pieces of adipose tissue are then placed over the bone wax. This is covered with one piece of moistened surgicel to secure it to the surrounding dura (Fig. 45-4B). This is then further sealed and secured to the surrounding dura with Tisseel or fibrin glue.

FIGURE 45-4 Closure.

A, The tumor has been removed, but the facial and cochlear nerves have been spared. The large air cell is filled with some Tisseel via a syringe. A few small pieces of adipose tissue removed from a small incision in the abdomen will then be placed within the air cell. B, The drilled bone of the internal auditory canal has been waxed, and the entire canal is filled with a larger piece of adipose tissue, which is then further secured to the surrounding dural with Surgicel and Tisseel. C, The previously removed pericranial tissue is used to repair the dural defect in a watertight manner using a running suture of 4-0 Vicryl. Two central dural tenting sutures are placed. D, The previously removed craniotomy bone plate is reinserted in the superior–lateral portion of the defect and held in place with a superiorly placed miniplate and screws to avoid placing screws into mastoid air cells. Medially, a preformed titanium cranioplasty plate is secured both to the surrounding bone and to the craniotomy bone plate to obtain a firm construct to which the dural tenting sutures are then secured.