Surgical Management of Posterior Fossa Meningiomas

Introduction and Classification

Meningiomas in the posterior fossa are a heterogeneous group of lesions in which anatomic location in relationship to the bony and neurovascular structures is key to predict their complexity during resection and to choose the most favorable approach. Many meningiomas also extend but are not primarily located in the posterior fossa. Depending on location and size, the symptoms that can be attributed to posterior fossa meningiomas can be due to dysfunction of cranial nerves (CNs) III to XII, brainstem compression, cerebellar compression, or obstructive hydrocephalus.

The authors classify the posterior fossa meningiomas in the following categories, classified depending on their anatomic location with very different surgical implications. The tumors often pertain to more than one of the following groups:

• Ventral meningiomas

  • A.

    Clival meningiomas

  • B.

    Petroclival meningiomas

• Lateral meningiomas (cerebellopontine angle [CPA] meningiomas)

  • A.

    Anterior to the internal auditory canal

  • B.

    Centered on the internal auditory canal

  • C.

    Posterior to the internal auditory canal

• Posterior meningiomas (occipital squama)

• Foramen magnum meningiomas

• Tentorial meningiomas

Foramen magnum meningiomas and clival meningiomas are explained in detail in their respective chapters and thus will be briefly described in this chapter. Nonoperative management follows the same principles as meningiomas in other locations in the skull base. Radiosurgery, fractionated radiotherapy, and proton beam therapy may be appropriate as adjuvant therapies after incomplete resection or as a primary treatment.

Surgical Anatomy of the Posterior Fossa

The posterior fossa is a three-dimensional space that opens toward the supratentorial space through the tentorial hiatus above, and toward the spinal canal through the foramen magnum below. The roof of the posterior fossa is formed by the tentorium, and the remaining walls are formed by bone. The majority of the osseous surface of the posterior fossa is formed by the occipital bone and is completed by the petrous temporal bone laterally and a small portion of the body of the sphenoid anteriorly and superiorly ( Fig. 30.1 ).

The occipital bone consists of three parts: basilar, squamous, and paired jugular parts. The basilar part is located anteriorly in midline and forms the inferior two-thirds of the clivus, and the squamous part forms the majority of the posterior part of the posterior fossa. The paired jugular parts connect the basilar with the squamous part and form the inferior aspect of the jugular foramen. The jugular part of the occipital bone also includes the occipital condyles and the hypoglossal canals.

Two parts of the temporal bone are related to the posterior fossa: the posterior part of the petrous bone and the mastoid part. The petrous part of the temporal bone forms a pyramid with its vertex (the petrous apex) pointing medially. Its posterior surface, almost vertical, attaches the tentorium at its superior border and articulates with the occipital bone at its inferior border. This junction forms the petroclival synchondrosis anteriorly and opens to form the jugular foramen posteriorly. Thus the jugular foramen is formed by the petrous temporal bone superiorly and the occipital bone inferiorly. The mastoid part of the temporal bone articulates with the squamous part of the occipital bone posteriorly. The only foramen of the petrous temporal bone in the posterior surface is the internal acoustic opening, which is located approximately at the same coronal plane as the anterior aspect of the jugular foramen. The anterior aspect of the jugular foramen is the pars nervosa, which contains CNs IX, X, and XI.

The posterior aspect of the posterior fossa is formed almost entirely by the squamous part of the occipital bone.

Neural and Vascular Relationships

The neural contents of the posterior fossa are the brainstem anteriorly, the cerebellum posteriorly, CN V to XII arising below the tentorium, and CN IV at the level of the tentorium. All these CNs except CN VI are related to very specific osseous structures, which are essential when planning surgery. CN V is related to the petrous apex, in which it forms the trigeminal impression before entering the middle fossa and Meckel cave. CN VII and VIII are related to the internal auditory canal (IAC); CN IX, X, and XI are related to the pars nervosa or anterior aspect of the jugular foramen; and CN XII enters the hypoglossal canal just above the occipital condyle. The vertebral arteries are related to the jugular part of the occipital bone and the clivus, and the basilar artery to the clivus. The posterior fossa similarly can be divided into three vascular territories with the posterior inferior cerebellar arteryrelated to the lower CNs, the anterior inferior cerebellar artery to CNs VII to VIII, and the superior cerebellar artery to CN V ( Fig. 30.2 ).

Thorough knowledge of the venous anatomy is essential in posterior fossa surgery. The transverse sinus runs through an imaginary line drawn from the inion to the zygomatic root, and the sigmoid sinus can be estimated just medial to the mastoid notch, also known as the digastric groove. The petrous temporal bone is surrounded by a venous “border” circumferentially: the sigmoid sinus forms the posterior aspect; the superior petrosal sinus makes up the superior margin, runs along the tentorial attachment to the petrous temporal bone, and is the venous connection between the cavernous sinus and transverse-sigmoid sinus junction. Finally, the inferior petrosal sinus runs along the petroclival synchondrosis to form the inferior border, connecting the cavernous sinus to the jugular bulb. The basilar plexus is not a discrete structure and is located between both layers of the dura mater at the level of the clivus. They all drain directly or indirectly to the jugular bulbs and then into the jugular veins.

Preoperative Studies

Thorough study of the preoperative magnetic resonance imaging (MRI) is of paramount importance to choose the most effective and safe surgical approach. T2-weighted images are useful in evaluating for surrounding brain edema and possible presence or lack of an arachnoid plane. Flow voids can also reveal location, displacement, and encasement of vascular structures and vascularization of the tumor.

Computed tomography (CT) is useful in assessing the tumor calcification and exact location as related to the osseous structures and bone anatomy of the skull base. In particular, CT can reveal possible bony hyperostosis, which is also a good diagnostic indication that the surgeon is dealing with a meningioma and not another neoplastic pathology. CT angiogram can be important in evaluating the location and involvement of arteries, and a CT venogram can show details of venous anatomy and patency, especially the major venous sinuses as detailed earlier.

Especially in cases of large and giant tumors, cerebral angiography can evaluate the arterial supply of the tumor and assess the vertebrobasilar circulation, disposition, and venous anatomy in more detail. It can also allow for preoperative embolization in certain cases. If presigmoid approaches are considered, it is important to study the venous anatomy of the ipsilateral transverse, sigmoid sinuses, and jugular bulb. A large dominant ipsilateral sinus and a high jugular bulb can significantly affect the exposure in presigmoid approaches. When a combined middle fossa and presigmoid approach with transection of the tentorium is being considered, it is very important to understand the size and location of the posterior temporal veins, including the vein of Labbe. If there are dominant draining veins entering the tentorium before draining into the transverse-sigmoid junction, this approach will need to be reconsidered to reduce the risk of possible venous infarct in the temporal lobe if the veins are taken or injured during the approach. If the tumor affects the transverse or sigmoid sinuses, a decision has to be made whether the tumor is resected entirely and the sinus repaired or grafted, versus leaving the portion of tumor in the sinus and potentially treat with stereotactic radiosurgery. In general, it is our practice to favor the latter approach for tumors with direct sinus involvement.

CN function should be carefully clinically evaluated preoperatively and the possible deficits derived from the approach or aggressive tumor removal discussed with the patient. Neuro-ophthalmologic evaluation is important if the patient complains of diplopia. To characterize the deficit, a Lancaster red-green test can quantify it for future reference. We recommend a preoperative audiogram for all posterior fossa meningiomas except those arising posteriorly from the occipital squama, or for clival meningiomas if a transnasal endoscopic approach is undertaken. Vagal nerve function can be evaluated in the office with indirect laryngoscopy, a more formal video swallow or flexible endoscopic evaluation of swallowing. Accessory nerve and hypoglossal nerve function can be assessed by examining the bulk and movement of the sternocleidomastoid and trapezius and tongue, respectively. Electromyography (EMG) can also be helpful if it is not entirely clear if the patient is having deficits in these pure motor nerves.

Intraoperative Monitoring

Intraoperative neuromonitoring is critically important for these cases, in most instances. The modality of intraoperative monitoring depends on the case and may include:

• CN V, VII, X, and XI can be directly monitored with EMG. Indications include the cases in which the tumors involve the nerves, the CNs are in close proximity to the tumor, or the approach can affect them (e.g., facial nerve in the transtemporal bone approaches).

• Brain stem auditory evoked responses are used to monitor cochlear nerve function and brainstem function. Direct cochlear nerve action potentials can also be used to monitor function of nerve VIII as well.

• Changes in somatosensory evoked potentials or motor evoked potentials of upper and lower extremities can indicate changes in brainstem or cortical function due to ischemia or retraction.

• Electroencephalogram slowing can be an indication of ischemia, and this is especially useful in cases where vascular bypasses or prolonged temporary vascular occlusion is planned.