INTRODUCTION

Meningiomas comprise up to 15% of adult intracranial tumors. These tumors, usually benign and slow-growing, may affect various anatomic structures in the posterior fossa and specifically the cerebellopontine angle. The term cerebellopontine angle (CPA) meningioma has been used widely to describe meningiomas that share a common location, that is, occupancy of the CPA, although these tumors may have diverse origins with regard to the site of dural attachment, which can be outside the CPA.¹

The first report of a tumor that would now be classed as a CPA meningioma was by Rokitansky² in 1856. Virchow³ later described a psammoma originating from the posterior lip of the acoustic meatus. In 1928, Cushing and Eisenhardt⁴ reported on seven patients with meningiomas “simulating acoustic neuromas,” emphasizing the high surgical risk in dealing with these tumors. Several surgical series of posterior fossa meningiomas involving the CPA have been reported since then. Microsurgical series were reported by Yasargil,⁵ Sekhar and Janetta,⁶ Ojemann,⁷ Al-Mefty,⁸ Haddad and al-Mefty,⁹ Harrison and al-Mefty,¹⁰ Matthies and colleagues,¹¹ Samii and Ammirati,¹² and Samii and colleagues.^13,14

Management options for these tumors include (1) microsurgery, (2) radiosurgery, and (3) expectant observation.

SURGICAL PROCEDURE

Historically, surgical removal has been the definitive treatment of these tumors. The literature abounds in surgical series of neurosurgeons and neuro-otologists describing a multitude of approaches to these meningiomas. The underlying theme is that each tumor should be approached in the context of its anatomic extent and the patient’s clinical deficits. The translabyrinthine and transcochlear approaches do not preserve hearing.

CPA meningiomas may arise from any area of the dura on the posterior surface of the petrous bone (Fig. 42-1A–C). Four general categories of tumor are found, depending on where they arise and their relationship to the VIIth and VIIIth nerve complex:

FIGURE 42-1 A–C, MRI showing typical CPA meningiomas with varying brain stem compression.

The retrosigmoid suboccipital approach is the one most familiar to neurosurgeons and the most widely used. The primary advantage is that it provides a wide-angle of approach through a relatively small craniotomy. This enables a large area to be visualized, and is ideal for both small and large tumors. The need for cerebellar attraction is a disadvantage for larger tumors, and the approach is more problematic for tumors arising primarily anterior to the cranial nerves, as these structures will be at risk during the dissection.

The patient can be positioned in the lateral, three quarters park bench, or sitting position. The lateral position is the most commonly used, but the degree of neck flexion necessary to obtain access to the posterior fossa may result in relative venous obstruction, thereby increasing the pressure within the posterior fossa and resulting in a cerebellum that is bulging on opening the dura.

Operative conditions are much improved in the sitting position, with reduced intracranial pressure and blood not impeding vision of the surgical field. However, it does increase the risk of air embolism to the patient. For the majority of our cases we have used the lateral position with the sitting position reserved for very large patients with short thick necks. A facial nerve monitoring system should be routinely used, and in some centres auditory evoked brain stem monitoring has been utilized for tumors where the VIIIth nerve function is a particular risk.

Cerebrospinal fluid (CSF) drainage via a lumbar drain improves access, particularly in the lateral position, and is opened prior to the dural opening. This is preferable to relying on opening the cisterna magna or other subarachnoid systems after the dural opening as the cerebellum may be bulging causing tight access to the cisterns, particularly if the patient is in the lateral position and the tumor is large.

A slightly “S” shaped retromastoid skin incision 1 cm medial to the mastoid is made extending from above the transverse sinus down to just above the level of the foramen magnum. The muscles and fascia are divided down to the suboccipital region, and the tissues are then reflected from the bone in a subperiosteal dissection, with the bulk of the musculature being reflected anteriorly, so as to reduce the mass of tissue posteriorly, which may impair visualization. The dissection of the soft tissue structures extends laterally to the digastric groove. The bone flap is elevated, with the aid of the high-speed drill and further bone then may be removed superiorly and laterally to expose the transverse and sigmoid sinuses. During elevation of the bone flap, it is necessary to take great care to preserve the integrity of the venous sinuses. A large emissary vein is often seen arising from the sigmoid sinus, which may be skeletonized with the drill. It needs to be dissected free from the bone flap and coagulated before division. On occasion, the dura may be very adherent to the inner table and elevation of the bone flap may cause a tear in the venous sinuses. This is best controlled with a small piece of judiciously placed gel foam, held in place with a cottonoid. The bone flap need not be large, as a wide angle of access can be obtained through even a relatively small opening. In general, the bony opening would be 2.5 cm in a craniocaudal diameter, and approximately 2 cm in medial–lateral diameter. The mastoid air cells are thoroughly waxed if they are opened. Great care should be taken to preserve the dural margins, as it is essential to obtain a water tight dural closure at the end of the procedure. Pericranium may be harvested at this stage.

The dura is opened, being hinged laterally on the sigmoid sinus and superiorly on the transverse sinus. After withdrawal of CSF from the spinal drain, the cerebellum nearly always slackens off well, but further exposure can then be obtained by removing CSF from the arachnoid cisterns.

At this stage, the operating microscope is introduced into the surgical field. Great care should be taken to preserve the arachnoid planes during the dissection, as this helps to protect the adjacent cerebellum and cranial nerves. The margins of the tumor are identified, and the tumor margins are dissected away from the adjacent cerebellum and vascular and neural structures, with care being taken to preserve the arachnoid. Any feeding vessels are coagulated and divided, but great care must be taken to preserve all vascular and neural structures that may be densely adherent to the tumor capsule (Fig. 42-2).

FIGURE 42-2 Retrosigmoid suboccipital exposure showing tumor in the cerebellopontine angle.

After identification of the plane around the tumor margins, the tumor is then entered and its interior debulked with the aid of an ultrasonic aspirator. The thinned out tumor capsule can then gently be dissected away from the adjacent vascular and neural structures. For tumors situated posteriorly to the internal auditory canal, there is nearly always an excellent arachnoid plane, which aids in preservation of the cranial nerves. For tumors extending anteriorly to the VIIth and VIIIth cranial nerves and lower cranial nerves the access is more problematic, but after removal of the bulk of the tumor it is possible to dissect the capsule away from these critical structures.

It is often necessary to divide small bridging veins between the cerebellum and the tentorium or posterior petrous bone. In larger tumors it may occasionally be necessary to divide the superior petrosal vein, draining into the superior petrosal sinus, and it is far better that this is coagulated with diathermy and divided under vision, rather than inadvertently torn during the procedure. Avulsion of this vein from the superior petrosal sinus can result in a substantial venous hemorrhage that needs to be controlled with careful placement of Surgicel and/or Gelfoam held in place with a cottonoid. The veins draining the brain stem must be preserved during the dissection.

In some cases, the tumor may extend into the internal auditory canal, and in these the VIIth and VIIIth cranial nerves are at particular risk. The dura needs to be divided over the posterior rim of the internal auditory meatus, and the bone is drilled away to allow adequate exposure of the tumor within the canal. The VIIth and VIIIth cranial nerves must be identified within the canal before dissection of the tumor. A facial nerve monitoring system is crucial during this stage. Any air cells that are opened must be sealed with bone wax.

In the vast majority of cases, the tumor capsule displaces the cranial nerves, and following debulking of the tumor the capsule can be just gently dissected away from these structures. However, in a small percentage the tumor will engulf the cranial nerves as well as critical vascular structures such as the anterior inferior cerebellar artery. In these dissections, should each be undertaken with great care, and it may be necessary to leave a very small rim of tumor tissue around the structures to preserve their function.

The dural attachment and origin is cleaned of all visible tumor tissue, and is coagulated. It is important to be aware that heat transmission of the bipolar diathermy may result in damage to cranial nerves.

Meticulous hemostasis is essential, and some surgeons prefer the anesthesiologist to perform a Valsalva maneuver, to confirm the adequacy of venous hemostasis.

The dural closure must be water tight, and in most cases this necessitates the use of a dural patch, either artificial or taken from a piece of pericranium. This is sutured in with 4/0 Monocryl. Fibrin sealants may be used to reinforce the dural reconstruction to aid in a watertight closure. The bone flap is replaced, and any bone defect is filled with a cranioplasty construct. A complete bony reconstruction of the defect with a cranioplasty material to fill in any holes left by the craniotomy may minimize development of postoperative headache, a specific complication of this approach. Finally, the wound is closed in layers.

Postoperatively the patient is positioned with the head of bed elevated at 30 degrees and vital signs are monitored closely, preventing episodes of hypo- and hypertension.

Other approaches that have been used include the presigmoid cranial skull-base exposures.^15–21 The presigmoid retrolabyrinthine craniotomy provides only a very restricted angle of approach, and is usually reserved for petroclival tumors or those lying primarily anteriorly to the pons and the mid brain. It is usually used in conjunction with a middle fossa approach with division of the superior petrosal sinus and tentorium to allow adequate access. It is only rarely necessary to use this approach in a true cerebellopontine tumor. The translabyrinthine approach gives good access to the CPA, but hearing is sacrificed. It has the major advantage of minimizing cerebellar retraction, but the access is much more restricted than with the retrosigmoid exposures, and it is difficult to remove tumors with origins extending below the lower cranial nerves. The transcochlear approach is reserved for tumors lying primarily anterior to the brain stem, and that necessitates rerouting of the facial nerve. It is only very rarely used for tumors extending into the CPA.

Meningiomas arising within the inner auditory canal (IAC) are very uncommon, and only 21 documented cases have been identified in the literature, most of them presented as case reports. The clinical and neuroradiologic features of intrameatal meningiomas are very similar to those of vestibular schwannomas, which are much more common in this location.

The clinical presentation of these intracanicular tumors with cranial nerve symptoms such as acoustic (tinnitus and hypacusis), vestibular (dizziness), and facial (paresis or spasm) symptoms later on is very similar to that of vestibular schwannoma, but facial nerve symptoms seem to be more common. A radiologic differentiation from vestibular schwannoma is not always possible. Some intrameatal meningiomas show broad attachment and sometimes a dural tail at the porus. Other radiologic features, such as bone invasion or invasion of middle ear structures, are more frequent in meningiomas. Surgical removal of intrameatal meningiomas should aim at wide excision, including involved dura, to prevent recurrences. The variation in the anatomy of the faciocochlear nerve bundle in relation to the tumor has to be kept in mind, and preservation of these structures should be the goal in every case. Functional preservation of the facial nerve may be achieved at similar rates as that for intrameatal schwannomas, whereas vestibular and auditory functional outcome for the most part will be superior in intrameatal meningiomas.²²

COMPLICATIONS AND NUANCES

The most feared complications from this operation relate to damage to the cerebellum, brain stem and cranial nerves, and may occur intraoperatively due to direct trauma, or as a result of inadvertent damage to a critical vascular structure. A postoperative hematoma may be rapidly fatal, and it is essential that a patient is under vigilant observation, especially with regard to blood pressure management.

Samii and colleagues²³ have shown that the clinical presentation and outcome differ between CPA meningiomas lying anterior or posterior to the IAC.

In our most recent microsurgical series of more than 100 cases House–Brackman grade 1 and 2, facial nerve function could be preserved in the great majority of patients and with much better results for tumors arising posterior to the IAC. Schaller and colleagues²⁴ reported on 6 of 10 patients with normal preoperative facial nerve function retaining House–Brackman grade 1 to 2 after surgery by use of the retrosigmoidal suboccipital approach. Voss and colleagues²⁵ reported 306 facial nerve dysfunction in their series of 40 patients with CPA meningiomas operated on via different approaches. The overall possibility of hearing preservation is far superior for CPA meningioma compared with vestibular schwannomas.

Tumor location seems to affect the clinical outcome in surgery of CPA meningiomas. Schaller and colleagues²⁴ reported that postoperative outcome of facial nerve function was substantially worse in premeatal than in retromeatal tumors. Voss and colleagues²⁵ reported in their series that facial nerve paresis occurred postoperatively in 60% of tumors arising anterior to the IAC, in 50% of tumors arising inferior to the IAC, and in fewer than 15% of tumors arising either posteriorly or superiorly. Batra and colleagues²⁶ emphasized that all 10 patients with retromeatal tumors had House–Brackman grade 1 facial nerve function preoperatively and that all of these patients retained grade 1 function after surgery.

A delayed and often rapid development of a facial paralysis occurs in a small percentage of patients postoperatively, up to 14 days after the surgery. A short course of a steroid is often helpful.

CSF leak after CPA tumor surgery remains a fairly common reported complication. Rates of CSF leak range from 2% to 21.9% although we have had no leak in our last 100 patients with CPA meningiomas in whom we used a retrosigmoid approach. In a recent meta-analysis of CSF leak after vestibular schwannoma surgery, Selesnick and colleagues²⁷ found an average rate of greater than 10% across 25 reported series (total of 5964 patients). This meta-analysis also found that varied prophylactic measures, including lumbar drainage, fibrin glue, hydroxyapatite cement, bone wax, abdominal fat graft, fascia, and ionomeric cement, did not significantly reduce the CSF leak rate below the average for the grouped data. The rates of leaks among translabyrinthine, retrosigmoid, and middle fossa approaches were statistically equal. Leaks via the eustachian tube into the nasopharynx were slightly more common than incisional leaks. Although most reported series focus exclusively on vestibular schwannoma surgery, the approaches used for various CPA tumors have similar issues contributing to CSF leak.

CONCLUSION AND RECOMMENDATIONS

Meningiomas in a CPA can represent a major operative challenge. Assessment of the extent of the tumor with magnetic resonance imaging (MRI) preoperatively helps in planning the surgical approach. Careful positioning of the patient is critical to optimize access to the CPA. Meticulous microneurosurgical technique is necessary to preserve the critical vascular and neural structures that lie in intimate contact with the tumor capsule. Great care should be taken during the dissection to preserve the adjacent arachnoid plane. Postoperatively it is essential to be vigilant in the observation of the patient, as a postoperative hematoma may be rapidly fatal if not diagnosed promptly.

The operative risk depends on the size of the tumor, the extent of brain stem compression, and the ability to preserve the critical cranial nerves and vascular structures.