 Meningiomas constitute up to 40 percent of all primary brain tumors if cases diagnosed at autopsy are included. Nearly 10 percent of all meningiomas arise in the posterior fossa.
The site of dural attachment within the posterior fossa dictates which neurovascular structures will be involved first by a meningioma; thus, it determines the initial symptomatology, the differential diagnosis, and the ease of surgical extirpation. Using postmortem studies, Castellano and Ruggiero classified posterior fossa meningiomas into five groups, according to their dural attachment: (1) cerebellar convexity (10 percent), (2) tentorium (18 to 30 percent), (3) posterior surface of the petrous bone (cerebellopontine angle) (42 percent), (4) clivus (11 percent), and (5) foramen magnum (4 to 9 percent). Based on intraoperative observations, Yasargil and colleagues classified basal posterior fossa meningiomas into clival, petroclival, sphenopetroclival, and cerebellopontine angle groups. Other authors have added more categories: intraventricular meningiomas, peritorcular meningiomas, meningiomas arising in the jugular foramen, and meningiomas of the posterior fossa without dural attachment. Some meningiomas, such as the petroclival or sphenopetroclival, involve more than one site. Petroclival Meningiomas Petroclival meningiomas are defined as those meningiomas arising from the superior two-thirds of the clivus and the petrous bone medial to the fifth cranial nerve. Tumors that also involve Meckel's cave have been termed sphenopetroclival by Yasargil et al. Owing to their rarity, location, and relentless progression toward fatality if left untreated, clival and petroclival meningiomas remain the most formidable of all meningiomas. Prior to 1970, only one successful total removal was reported. Operative mortality exceeded 50 percent. Thus, these tumors were generally deemed inoperable. The previously dismal surgical outcome has been largely overcome by the advent of new, refined approaches, microsurgical techniques, intraoperative monitoring, advanced imaging, and modern anesthesia and postoperative care. The average age at presentation is the mid-forties and there is a strong female preponderance. The onset of symptoms is insidious, with a mean duration before diagnosis of 4 years. Symptomatology is secondary to several factors, the first being involvement of cranial nerves: V (70 percent), VIII (65 percent), VII (60 percent), VI (40 percent), and III (30 percent). Deficits of cranial nerves III, IV, and VI are uncommon despite their frequent and intimate involvement with these tumors. Other factors affecting symptomatology include cerebellar compression, brain stem compression with involvement of the corticospinal tract (60 percent) (somatosensory deficits are rare), and increased intracranial pressure (70 percent). Rarely, a petroclival meningioma may present as basilar arterial insufficiency. The test of choice is magnetic resonance imaging (MRI) with gadolinium enhancement. Computed tomography (CT) scans, however, do reveal the amount of bony destruction to a better degree. Three-vessel angiography or high-quality MRA is necessary. The basilar artery is characteristically displaced posteriorly and to one side, and the posterior cerebral and superior cerebellar arteries are elevated on the ipsilateral side. However, if a large portion of the tumor involves the tentorial hiatus, the two arteries may be splayed. These tumors are supplied by branches of the intrapetrous and intracavernous segments of the internal carotid artery as well as by branches of the external carotid artery through the ascending pharyngeal and middle meningeal arteries. The venous phase of angiography demonstrates the transverse sinuses, the side of the dominant sinus, and whether the two transverse sinuses communicate at the torcular Herophili. It also demonstrates the position and size of the vein of Labbe. This information is crucial for surgical planning. Cerebellopontine Angle Meningiomas The cerebellopontine angle (CPA) is bounded superiorly by the tentorium; inferiorly, it is continuous with the space lateral to the medulla. The medial limit is the pons, and the lateral boundary is formed by the petrous pyramid. The CPA space is bounded posteriorly by the cerebellum. The CPA cistern contains cranial nerves V, VII, and VIII; the anterior inferior cerebellar artery; and the superior petrosal vein. CPA meningiomas arise lateral to the trigeminal nerve, in contradistinction to petroclival meningiomas, which involve the upper two-thirds of the clivus and the petrous bone medial to cranial nerve V. CPA meningiomas are, in turn, subdivided into a medial group arising lateral to the trigeminal nerve but medial to cranial nerves VII and VIII, and a lateral group arising lateral to these nerves. Symptoms and signs of tumors localized to the CPA include (I) loss of hearing, tinnitus, and vestibular dysfunction secondary to involvement of the cochleovestibular nerve: (2) headaches; (3) cerebellar dysfunction; (4) facial pain or numbness secondary to involvement of the trigeminal nerve; (5) long tract signs arising from involvement of the corticospinal tract; and (6) symptoms and signs attributab!e to increased intracranial pressure secondary to hydrocephalus. Meningiomas of the CPA tend to involve cranial nerves V and VII more often than acoustic neuromas (vestibular schwannomas) do; conversely, auditory and vestibular dysfunction are less common. Meningiomas also may involve the lower cranial nerves, but such involvement is less likely with an acoustic neuroma. It is, however, extremely difficult to differentiate clinically between a CPA meningioma and an acoustic neuroma, given that most patients with CPA meningiomas actually present with hearing loss. It is interesting to note that 6.6 percent of the patients with CPA meningioma reported by Yasargil et a1. presented with subarachnoid hemorrhage. This probably represents a referral bias. The main differential diagnosis of a CPA tumor is (I) acoustic neuroma (80 percent); (2) meningioma (9 percent); and (3) other tumors, such as epidermoids, schwannomas and neurofibromas of other cranial nerves, arachnoid cysts, lipomas, metastatic tumors, and choroid plexus papillomas. The percentage of posterior fossa meningiomas occurring in the CPA has been stated to range from 30 to 50 percent. The preoperative distinction between meningiomas and acoustic neuromas is important for technical and prognostic reasons. Another differentiating feature is that an acoustic neuroma is usually centered over the internal acoustic meatus, whereas CPA meningiomas are eccentric. The enhancement after gadolinium administration of a tail of dura adjacent to the main tumor has been termed the meningeal sign. This sign is highly specific for meningiomas. The tail is thickest near the tumor and tapers off. Rarely, an acoustic neuroma may display a pseudomeningeal sign. This is a thin, short dural tail present on a single aspect of the tumor. caused by bone marrow adjacent to the porus acusticus. This pseudomeningeal sign is present on T1-weighted images before and after gadolinium enhancement. unlike the true meningeal sign that is present only after the administration of gadolinium. Both acoustic neuroma and CPA meningioma have a heterogeneous appearance on the T1-weighted MRI. This heterogeneity is due to cystic degeneration in acoustic neuroma and to calcifications in CPA meningioma. On T2-weighted scans, the cystic areas will appear hyperintense, whereas the calcified areas remain hypointense. Thus, a PCA tumor that has regions of low intensity on T1- and T2-weighted MRI probably is a calcified meningioma. An angiogram or a magnetic resonance angiogram may be of help in ruling out a giant calcified aneurysm. Differentiating between an acoustic neuroma and a CPA meningioma preoperatively helps the surgeon choose the best approach and minimize the surgical risks. The facial nerve typically is located on the anterior or anterosuperior surface of an acoustic neuroma. This relationship is lost in the case of a pea meningioma. The facial nerve may be displaced in any direction or may be engulfed by tumor, thus making its preservation more difficult. Unlike acoustic neuromas, PCA meningiomas have a propensity to track along posterior fossa nerves so that they involve Meckel's cave or the jugular foramen. The surgeon should be prepared for this eventuality, lest he or she find the approach suboptimal for achieving complete resection. Hearing preservation has been reported to be more successful after PCA meningioma removal than after acoustic neuroma remova1. Dramatic postoperative hearing improvements have also been reported in CPA meningiomas. If a meningioma is suspected. one should be very cautious before resorting to an approach that results in decreased hearing (such as the translabyrinthine or the transcochlear approach). CPA meningiomas are approached differently, depending on their location vis-à-vis cranial nerves VII and VIII (medial vs. lateral CPA meningiomas). The surgical approach to a lateral CPA meningioma is through a suboccipital craniectomy. The patient is placed in the supine position with the shoulder elevated and the head rotated to the opposite side. Other positions used are the "park bench" position, with the affected side up. or a semi sitting position. The edges of the craniectomy should fully expose the transverse and sigmoid sinuses (by drilling of the mastoid). There is no need to open the foramen magnum. The dura is opened with a curvilinear incision along the course of the transverse-sigmoid sinus, and the cisterna magna is opened to release cerebrospinal fluid. The cerebellum. falling back under the pull of gravity, is held medially and resection of the tumor is begun. Neurophysiological monitoring is an aid to preservation of facial and acoustic function. Care must be taken not to injure the facial and cochleovestibular nerves by traction on the adherent tumor. The tumor is first devascularized by coagulation of its blood supply along the petrous pyramid, and then it is debulked and resected. The anterior inferior cerebellar artery may loop to the region of the porus acusticus to give off an internal auditory branch and then return to supply the brain stem. Therefore, all arteries in the vicinity of the tumor should be preserved to prevent an ischemic insult to cranial nerve VIII (internal auditory branch) or infarction of the brain stem. Small medial CPA meningiomas are approached via an extended middle fossa approach: Large meningiomas in this location are better managed via a petrosal approach. Both approaches allow the surgeon to remove supratentorial tumor extensions. Other approaches to the CPA include the translabyrinthine, the transcochlear, and the subtemporal approaches. The translabyrinthine approach is good for small tumors but inevitably results in loss of hearing. The chief advantage is the fact that the facial nerve is identified early and is followed medially toward the tumor. It also obviates the need for cerebellar retraction. The subtemporal route has fallen into disfavour because of the degree of the temporal lobe retraction and the risk of injury to the vein of Labbe. Furthermore, the cranial nerves are not optimally visualized by this technique. Cerebellar Convexity Meningiomas Cerebellar convexity meningiomas constitute 1 to 2 % of all meningiomas and 10 to 20 percent of posterior fossa meningiomas. They are frequently adjacent to the transverse sinus , sigmoid sinus and torcular Herophili. They may attach to the cerebellar dura, the petrous dura, or the tentorium. Patients may present with symptoms and signs characteristic of a posterior fossa lesions: headache, cerebellar dysfunction, and symptoms and signs of increased intracranial pressure. Some of these tumors may become large before they produce symptoms. These tumors appear as well-circumscribed, usually isodense to mildly hyperdense masses on CT and may contain areas of calcification. They enhance homogeneously after infusion with a contrast agent. MRI usually reveals a tumor that is isointense (60%) or mildly hypointense (30%) on T1-weighted images. and isointense (50 %) or moderately hyperintense (40%) on T2-weighted images. Meningiomas usually enhance intensely and uniformly after gadolinium injection. MRA and MRV are useful in determining whether there is involvement of the venous sinuses. The preoperative angiogram delineates the arterial supply of the meningioma and may allow preoperative embolization of the feeders. The venous phase should be studied to determine the side of the dominant transverse sinus, the involvement and patency of the venous sinuses, and whether the transverse sinuses communicate at the torcular Herophili. Cerebellar convexity meningiomas are approached through a suboccipital craniectomy. They can be divided into three groups: medial, lateral. and superior. The preference of surgical approach for all three types is the three-quarters prone position. The medial type has a dural attachment that does not extend more than 3 cm lateral to the midline. A linear skin incision and a bilateral suboccipital craniectomy are performed. The falx cerebelli or the occipital sinus may be involved by the tumor; if so, these may be sacrificed with impunity. For the lateral cerebellar convexity meningiomas, a paramedian linear or an S-shaped incision is made. The craniectomy extends laterally to the sigmoid sinus and superiorly to the transverse sinus. For the superior type, a linear or reverse U incision is used. If the transverse sinus is to be included in the surgical procedure, a supratentorial craniotomy is added. The sinus can be resected safely if its lumen is completely occluded by tumor. If the sinus is only partially involved, the decision of whether to attempt complete resection rests on several factors related to the dural sinuses and venous anatomy: whether the involved sinus is dominant and whether both transverse sinuses communicate at the torcular. If the transverse sinus is to be resected. careful attention must be paid to preserving the cortical venous drainage, particularly of the ipsilateral vein of Labbe. The sinus can be reconstructed with a pericranial or venous graft. When operating in the posterior fossa, it is always good practice to mark the projected site of an occipital ventriculostomy (7 cm above the inion and 3 cm off the midline). If a complication occurs and ventricular decompression is needed. the surgeon places a single burr hole at the marked point and inserts the ventricular catheter. No large series of cerebellar convexity meningiomas has been reported. However, with the advent of microsurgical technique, complete resection and good postoperative outcome have been the rule. Obviously, there are risks of complications and of recurrence if the tumor has encroached on the venous system. Meningiomas of the Fourth Ventricle Intraventricular meningiomas account for 1.5 percent of all intracranial meningiomas; however, they account for 15 percent of all childhood and adolescent meningiomas. In 1986 Criscuolo and Symon reviewed 400 intraventricular meningiomas and found that 80 percent occurred in the lateral ventricles, 15 percent in the third ventricle, and 5 percent in the fourth ventricle. The female/male ratio in this category was 1:1, and the mean age at diagnosis was 25 years. Abraham and Chandy classified posterior fossa meningiomas without dural attachment into three categories: (I) those originating from the choroid plexus of the fourth ventricle and developing solely within the ventricle, (2) those of the inferior tela choroidea, developing partly in the cerebellar hemisphere and vermis, and (3) those within the cisterna magna and having no attachment to the dura mater. Fewer than 30 cases of fourth ventricular meningiomas have been reported. Most patients present with increased intracranial pressure secondary to hydrocephalus. Surgical removal is through a midline suboccipital craniectomy. The vermis is opened through a vertical incision. Early coagulation of the feeding vessels arising from the choroid plexus will decrease the vascularity of the tumor and ease the resection process. Care must be taken not to manipulate the floor of the fourth ventricle to avoid injuring the neurovegetative centers of the medulla. Foramen Magnum Meningiomas Meningiomas may arise along the perimeter of the foramen magnum. Tumors involving the anterior lip usually arise from the lower third of the clivus and extend downward; they are thus referred to as craniospinal meningiomas. Those arising posteriorly or posterolaterally are at the level of the spinal cord and extend superiorly. They are termed spinocranial meningiomas. The differential diagnosis of a foramen magnum meningioma consists of other extramedullary intradural tumors, such as neurinomas (less common than meningiomas with a 1:3 ratio), dermoids, teratomas, lipomas, and cavernous angiomas. Other pathological entities with a similar presentation include intramedullary tumors, syringomyelia, metastases, chordomas, platybasia, Chiari malformation, and multiple sclerosis. The clinical presentation of foramen magnum lesions is protean. There might be a paucity of signs within a context of a flurry of symptoms. In 1984 Meyer reviewed 102 cases of benign extramedullary tumors of the foramen magnum and stated that 40 percent of the patients had a normal neurological examination at first evaluation. Therefore, one should maintain a high degree of suspicion in diagnosis of these tumors and recall that the medical literature is replete with such cases being misdiagnosed as hysteria or psychosomatic illness. Patients with foramen magnum meningiomas may complain of occipital headaches and neck pain. cold or burning dysesthesias, Lhermitte's phenomenon, and weakness. Cranial nerve disturbances are rare except for those of cranial nerve XI, which may be involved in up to 30 percent of the patients. Horner's syndrome, late respiratory distress, and sphincteric disturbances are rare. Symptoms and signs that should make one suspicious of a foramen magnum meningioma include (I) unilateral neck pain aggravated by coughing or straining, especially if the pain is localized to the C2 dermatome; (2) Lhermitte's phenomenon in the absence of other evidence of multiple sclerosis; (3) involvement of the spinal accessory nerve (cranial nerve XI); (4) cold dysesthesias: (5) progression of motor and sensory deficits starting in one arm and spreading to the other limbs: and (6) atrophy of the intrinsic hand muscles (which occurs in up to 50 percent of patients). the exact pathogenesis of which is not clear. The most widely accepted theory to explain the atrophy in the hands holds that obstruction of venous outflow at the level of the foramen magnum causes subsequent spinal cord infarction and necrosis at lower levels. Patients may also demonstrate "piano-playing fingers." When patients close their eyes and hold their arms outstretched, many will have slow athetosis-like movements of their arms, hands, and particularly fingers, a phenomenon that has been reported in up to one-third of patients with extramedullary foramen magnum tumors. This is secondary to disturbances in position sense. These patients may also demonstrate astereognosis. Up to 50 percent of patients have a relapsing and remitting pattern of symptoms. The irregular pattern of symptoms and signs prompted Castellano and Ruggiero to state that' 'Diagnosis is very difficult due to the capricious anamnesis and peculiar findings on neurological examination. This is especially important in light of the fact that spastic quadriparesis often develops and that the earlier the diagnosis, the better the chance for a cure and for reversing the symptoms. The mainstay of radiological diagnosis is gadolinium-enhanced MRI. Plain MRI may not reveal a small meningioma, and CT scans of the area are unsatisfactory because of bony artefacts. Angiography should be considered in all cases of suspected meningioma to determine the vascularity and vascular supply of the tumor. to define the position of the major blood vessels and their possible encasement, and to rule out a giant partially calcified aneurysm. The blood supply to foramen magnum meningiomas is usually from the anterior and posterior meningeal branches of the vertebral arteries and the meningeal branches of the ascending pharyngeal and occipital arteries. Meningiomas are commonly located anterolaterally in relation to the spinal cord. A few will be strictly anterior, and up to 20 percent arise posteriorly or posterolaterally. Approaches to the foramen magnum area can be divided into posterior, lateral, and anterior approaches. Anterior approaches have been used primarily for extradural lesions. The main drawbacks of these approaches (except for the transcervical) are that they all violate contaminated spaces and most do not provide adequate lateral exposure. Anterior approaches include the transoral, labiomandibular. labioglossomandibular. and anterior retropharyngeal (transcervical) approach. The posterior approach is appropriate for most meningiomas of the foramen magnum. If the tumor is located posteriorly or posterolaterally, a suboccipital craniectomy with C I and C2 laminectomies may suffice. The transcondylar approach is preferred for tumors located laterally and anteriorly. Meningiomas of the Jugular Foramen The jugular foramen measures I5 x 10 mm. Cranial nerves IX, X, and XI and the inferior petrosal sinus occupy the anteromedial aspect of the foramen, and the internal jugular vein occupies the posterior aspect. The right jugular foramen is usually larger than the left. The differential diagnosis of a tumor involving the jugular foramen consists of glomus jugulare tumors (paragangliomas) schwannomas (neurinomas), and meningiomas. The meningiomas are divided into two categories, those arising in the jugular foramen and those involving it by direct extension. Meningiomas of the posterior fossa involve the jugular foramen more often by extension from another site of origin. Intrinsic meningiomas of the temporal bone arise from four sites: the internal auditory canal. the region of the geniculate ganglion, the sulcus of the greater and lesser superficial petrosal nerves, and the jugular foramen. Arachnoid cells and granulations are regularly found at these locations. However, intrinsic meningiomas of the jugular foramen are exceedingly rare. Patients with meningiomas involving the jugular foramen may present with a combination of cranial nerve involvement (IX. X. and XI-Vernet's syndrome; IX, X. XI. and XII- Collet's syndrome). They may also complain of pulsatile tinnitus even though this finding is more typical of paragangliomas. The preoperative diagnosis of a meningioma of the jugular foramen can be made on the basis of CT. MRI and angiography. CT scans reveal bony destruction, sclerosis, or hyperostosis. The smooth regular contour of eroded bone seen in a paraganglioma is absent. On MRI the salt and pepper appearance or serpentine flow voids seen in glomus tumors are not seen. On external carotid angiography both meningiomas and paragangliomas are hypervascular. However. the meningioma blush tends to last longer, whereas a rapid early venous drainage is seen in paragangliomas. These lesions can be approached through a combined infratemporal posterior fossa approach in which the patient is placed supine with the head elevated and turned away from the side of the lesion with the ipsilateral shoulder resting on a bolster. A C-shaped incision is made behind the ear and extended up to the temporal area and down along a natural skin crease in the neck. The external ear canal is everted and closed as a blind sac. The skin flap. including the auricle, is reflected anteriorly. The sternocliedomastoid muscle insertion is detached from the mastoid and reflected inferiorly. The neurovascular structures in the neck are dissected and exposed, including the common, internal, and external carotid arteries with the branches of the external carotid artery; the jugular vein: and cranial nerves IX through XII. A radical mastoidectomy exposes the sigmoid sinus down to the jugular bulb and is continued with a posterior fossa craniectomy. With the aid of a high-speed drill, the facial nerve is skeletonized from the stylomastoid foramen to the geniculate ganglion if necessary. It is then removed from the fallopian canal, transposed. and secured anteriorly. The internal carotid artery and the jugular vein are followed toward the base of the skull. To obtain exposure. the posterior belly of the digastric and the stylohyoid muscles are transected, and the styloid process is removed. If necessary, the ascending mandibular ramus is dislocated anteriorly. If the sigmoid sinus and jugular bulb are invaded by tumor, they are ligated and the tumor resected en bloc with the venous structures. If the latter structures are salvageable, the dura is opened both anterior and posterior to the sigmoid sinus. Using both the intradural and the infratemporal routes, the tumor is then removed piecemeal. The tumor may be extremely adherent to the lower cranial nerves; dissecting the tumor off these nerves may lead to serious postoperative morbidity secondary to aspiration pneumonia. In this context, intensive postoperative pulmonary care is of cardinal importance.
| 
