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.

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