PERITORCULAR MENINGIOMAS

Introduction
Surgical treatment in meningiomas.
Histological aspects in meningiomas.
Supratentorial meningiomas in general.
Parasagittal meningiomas.
Falx meningiomas.
Convexital meningiomas
Olfactory groove meningiomas.
Tuberculum sellae meningiomas.
Sphenoid wing meningiomas.
Optic sheath meningiomas.
Middle fossa meningiomas.
Cavernous sinus meningiomas.
Intraventricular meningiomas.
Malignant meningiomas.
Peritorcular meningiomas.
Tentorial meningiomas.
Infratentorial meningiomas.
Meningiomas outside the nervous system.

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Peritorcular meningiomas, although rare, have a special place in neurosurgical history. As noted by Cushing and Eisenhardt, the tendency of these tumors to produce visual field defects facilitated their preoperative localization, and such precise localization encouraged neurosurgical pioneers to attempt to remove them. Birdsall and Weir in 1887 reported the first effort at removal of a peritorcular meningioma; their case was one of the earliest intracranial operations for tumor, and it ended in fatal postoperative hemorrhage. One of the first successful operations for brain tumor was the removal of a parasagittal and peritorcular meningioma, as reported by Oppenheim and Krause in 1906. The first case in Dandy's initial description of ventriculography was a peritorcular meningioma.
Cushing and Eisenhardt first differentiated the peritorcular meningiomas as a distinct entity. They identified 12 tumors among 77 parasagittal meningiomas that, because of involvement with the tentorium and lateral sinuses as well as the superior sagittal sinus and falx, presented the surgeon with difficulties of exposure and removal that increased the hazards of surgery. Torcular meningiomas have, as part of their dural base, dura forming the torcular; i.e., they arise from, invade, or are attached to a wall of the torcular itself. Peritorcular meningiomas arise from the dura of the posterior falx cerebri, posteromedial tentorium cerebelli, or superoposterior falx cerebelli and, by virtue of compression or invasion of the superior sagittal, straight, transverse, and/or occipital sinuses, threaten dural venous sinus flow at the torcular Herophili. The relationship of peritorcular meningiomas to the posterior dural venous sinuses is paramount in the definition of this entity because of the profound implications of the tumor to venous sinus anatomic relationships for the clinical presentation, diagnosis, treatment, and outcome of patients with these tumors.


Pathologic Anatomy

The torcular Herophili. the site of confluence of the superior sagittal, straight. occipital, and both transverse sinuses, varies greatly from individual to individual. Rarely is there symmetrical intersection of these five sinuses at a central point beneath the internal occipital protuberance. Rather. peritorcular venous channels are usually asymmetrical and septate. The distal superior sagittal sinus often has a double lumen. The right transverse sinus, which frequently carries most of the superior sagittal sinus outflow, is often larger than the left transverse sinus. which frequently carries most of the straight sinus flow. Occasionally. one transverse sinus may be congenitally atretic or even absent; distal outflow may be occluded as a result of sigmoid sinus or jugular vein occlusion by prior tympanic disease. The extent of communication of the superior sagittal flow with each transverse sinus and of one transverse sinus with the other is often critical to the clinical pathophysiology and surgical resectability of peritorcular meningiomas.
Peritorcular meningiomas arise from arachnoidal cap cells in the region of the torcular Herophili. Cushing and Eisenhardt noted a predominance of angioblastic meningiomas in their group of peritorcular meningiomas; of the 12 tumors, 6 were angioblastic, 5 of which showed malignant histologic and clinical features. Three of 12 tumors were of the fibroblastic "whorl" type, 1 was psammomatous, 1 was fibrotic, and 1 was mesothelial. Such a skewed distribution of histologic subtypes has not been noted in the peritorcular tumors of other series of meningiomas. Interesting variants of peritorcular meningiomas include those associated with extensive hyperostosis in the region of the internal occipital protuberance (e.g., Cushing's case 8, in which there was hyperostosis from the lambda to the foramen magnum) and those with en-plaque extension of tumor along the venous sinuses (e.g., also Cushing's case 8, in which tumor extended along the transverse sinus to involve three of the four peritorcular quadrants). The dura is a poor barrier to tumor extension; there are numerous examples of tumors perforating the distal falx or posterior tentorium to reach an adjacent quadrant. Although meningiomas of benign histology may have such extension, it occurs with greater frequency among tumors with malignant histologic characteristics.
Except in cases of malignant histology, tumor growth is slow. Tumor expansion progressively displaces the adjacent occipital lobe or cerebellar hemisphere and compresses proximate sinuses.
The clinical syndromes produced thus reflect occipital lobe or cerebellar dysfunction or manifest elevated intracranial venous pressure. The latter may produce generalized intracranial hypertension, which may be augmented by the mass of the tumor itself. and either focal or generalized cortical dysfunction. The development of clinical symptoms is usually insidious, and the tumor may become very large before symptoms become clinically evident. An exception to this tendency is the tumor that arises in the wall of a dural sinus and extends inwardly rather than outwardly; in such a case, florid clinical symptoms can be produced by a small tumor obliterating a venous sinus lumen.

Clinical Presentations

The rarity of meningiomas in this region and the imprecision with which the term peritorcular meningioma has been used make esti­mation of a true incidence difficult. In addition to the 12 peritor­cular meningiomas Cushing and Eisenhardt found in their series of 77 parasagittal meningiomas,4 three series of posterior fossa me­ningiomas combined contained 4.9 percent (4 of 82) peritorcular tumors.8.9.ll Three series of tentorial meningiomas combined contained 13 percent (14 of 109) peritorcular tumors".6.12 and if the relative incidences of parasagittal, tentorial, and posterior fossa tumors are 10 percent's percent, and 10 percent, respectively, of

all intracranial meningiomas, peritorcular tumors represent about I percent of intracranial meningiomas. Because of this low inci­dence, peritorcular tumors have not been distinguished from other meningiomas in reported series other than that of Cushing and Eisenhardt.

In that series, seven patients were male and five were female4 The average age at presentation was 35.5 years. These patients uniformly had large tumors causing severe neurological deficits.

The median interval between symptom onset and diagnosis was I year; the mean interval was 1.5 years. Predictably, the presenting symptoms and signs reflected either occipital or cerebellar com­pression or intracranial hypertension secondary to venous outflow obstruction (Table 90-1). All patients had papilledema; at least half also had homonymous field cuts suggestive of an occipital contri­bution to their visual loss and the supratentorial presence of tumor. Headache was global in half of the patients who noted it; this most likely reflects increased intracranial pressure. Occipital and suboccipital pain, probably resulting from deformation of surrounding dura, was described by the other half. Neck pain and stiffness may have indicated incipient tonsilar herniation. Cerebellar signs, indicative of infratentorial tumor extension, included nystagmus, dysmetria, hypotonia. and ataxia. Notably. there was no case of acute neurological deterioration that might result from sudden thrombosis of a partially occluded dominant sinus. Presumably, the slow growth of these tumors permits the development of sufficient collateral flow to preclude such a catastrophe.

Diagnostic Studies

Gadolinium-enhanced magnetic resonance (MR) imaging and multiprojection subtraction angiography are the most valuable neurodiagnostic tools for assessing peritorcular meningiomas. Computed tomography greatly facilitated the preoperative diagnosis of these tumors and still is valuable in defining bone detail, especially if there is hyperostosis and intratumoral calcification.

Multiplanar MR imaging, however, offers far superior anatomic detail of the tumor and its relation to both surrounding brain and adjacent venous sinuses. On T1-weighted images, most meningiomas are isointense with brain. Large parenchymal or tumor vessels are apparent as cylindrical areas of signal void. There is some increase in relative intensity of tumor on T2-weighted images, but this is usually less marked than is the case with neurofibromas. Intratumoral calcification appears as a signal void on MR T1- and T2-weighted scans. Peritumoral edema shifts from hypo­intense on T1-weighted images to hyperintense with T2 weighting. Gadolinium enhances the contrast between tumor and brain. The distinctiveness of the margin between tumor capsule and cortex may correlate with the ease of maintenance of a plane of surgical dissection and with benign rather than invasive malignant histology.

The multiplanar capacities of MR permit depiction of the tumor's relation to the sinuses and tentorium. valuable information in preoperative planning. The midsagittal and medial parasagittal views show the relation of the tumor to the falx cerebri and the falx cerebelli, and the coronal view clearly demonstrates the relation of the tumor to the transverse sinuses.Tumor extension is evident as thickening of dural leaves that becomes more intense with gadolinium; enhancement without thickening represents only dural hyperaemia along the course of the tumor's blood supply. The MR image also helps predict the status of the sinuses. A patent sinus has a signal void characteristic of flowing blood; this is especially evident on T2-weighted scans, in which contrast with the higher intensity of CSF is maximized. A partially occluded sinus is heterogeneous in intensity; regions of signal void corresponding to normal flow are interrupted by regions of increased intensity indicative of stasis. A uniformly intense sinus suggests occlusion; occasionally, however, a sinus with very slow flow appears bright and falsely appears to be completely occluded.

Equally essential to preoperative planning is multiplanar angiography. The diagnosis of meningioma is confirmed by the appearance of the tumor's dense capillary blush in the late venous phase. Displacement of normal blood vessels or their involvement with tumor is shown by angiography. The feeding arteries usually the middle meningeal and occipital branches from the ex­ternal carotid artery, meningeal branches of the vertebral artery, and the tentorial branches of the cavernous internal carotid artery should be identified and, as often as is possible, embolized. An effort should be made to embolize vessels within the tumor itself. Large feeding arteries should be filled with embolic material and occluded more proximally by a detachable balloon after embolization is completed.

Even more critical is precise delineation of the peritorcular venous anatomy. The pattern of flow in each sinus should be carefully studied. Sinus occlusion is manifest by absence of filling and collateral flow of blood away from the torcular Herophili. The details of site and cause of venous sinus obstruction are best seen on retrograde dural sinus venography. Direct endovascular cannulation of the transverse sinus also permits trials of sinus obliteration; clinical tolerance of sinus occlusion and intraluminal pressure proximal to the blockage are assessed during balloon inflation. Such information is invaluable to the surgeon planning operative excision of a peritorcular meningioma

Treatment

The goal of treatment of patients with torcular meningiomas is the relief of neurological manifestations and the prevention of further tumor growth. This is best accomplished by total surgical resection. Efforts at total removal, however, must be tempered by the following: (I) the patient's age, medical condition, and type of neurological deficit; (2) the probability that a catastrophic outcome will follow occlusion of a sinus on which the brain is reliant; and (3) the favourable long-term results achievable by subtotal resection followed by radiation therapy. Clearly a more aggressive surgical stance is warranted for the otherwise healthy young female patient with a large tumor causing progressive focal neurological deficits than for the elderly male patient with an apparently slowly growing tumor discovered incidentally. Temporizing surgical measures such as optic nerve sheath fenestration, designed to relieve visual deficits arising from elevated intracranial venous pressure, or ventriculoperitoneal shunting of obstructive hydrocephalus may be indicated in cases where a patient's medical condition or the tumor's anatomic geometry precludes tumor resection by craniotomy. Such procedures, if they relieve neurological deficits, may also permit delay of an operation on a tumor incompletely occluding a major sinus until further tumor growth completely occludes the sinus such that it can be divided and resected. Ventriculoperitoneal shunting is not indicated if removal of tumor is anticipated; intraoperative ventricular drainage of an expanded ventricle facili­tates occipital lobe retraction in supratentorial approaches to peritorcular tumors, and tumor removal may relieve the aqueductal and fourth ventricular compression responsible for the hydrocephalus.

The choice of operative approach depends on the tumor's size and location, the nature of sinus involvement, and the goals of surgery. The extent of peritorcular exposure needed depends on the number of peritorcular quadrants containing tumor and the tumor's geometry. A unilateral occipital craniotomy or suboccipital craniectomy may suffice for tumors occupying only a single quadrant. Exclusively supratentorial tumors that penetrate the falx cerebri but not the tentorium may be exposed by a bilateral occipital craniotomy. In the case of large bilateral supratentorial exten­sions, the risk of inducing cortical blindness from bilateral calcarine injury may be reduced by monitoring visual evoked potentials or staging sequential unilateral procedures. Similarly, exclusively infratentorial tumors perforating the falx cerebelli can be removed through a bilateral suboccipital craniectomy. For unilateral tumors extending through the tentorium, a combined supratentorial and infratentorial approach is often justified. In many cases, however, the portion of the tumor extending across the tentorium can be removed through a unilateral occipital craniotomy or suboccipital craniectomy alone. The supratentorial approach has the advantage of the extensive exposure gained by wide lateral retraction of the occipital lobe; few cortical veins drain medially from the occipital lobe, and those that exist can be sacrificed without risk of neurological deficit. The infratentorial approach, however, averts the risk of retraction-induced injury to the visual cortex. In either case, wide transtentorial exposure can be obtained by dividing the tentorium from the incisura to the anterior margin of the transverse sinus. In the removal of tumors that arise directly from the torcular wall, exposure of all four quadrants of the peritorcular region is advisable because it permits control of all venous inflow and outflow to the region.

The position of the patient is a matter of surgeon's preference. The sitting, Concorde. and 45-degree prone oblique positions all afford adequate access in unilateral approaches. Either the sitting or Concorde position is preferred for bilateral exposures. A horse­shoe incision with its apex at lambda and its base between the mastoid processes is excellent for bilateral combined exposures. It may be narrowed and/or shortened if less exposure is required. Bone removal is usually accomplished by free occipital flap craniotomy and/or suboccipital craniectomy. Piecemeal rongeuring of bone is the safest method of removal when traversing the superior sagittal sinus, transverse sinuses, or the torcular Herophili itself. Dural adhesion to bone that results from hyperostosis or age greatly increases the risk of tearing a feeding artery or unroofing a venous sinus. The danger of the former can be reduced by performing a strip craniectomy at the margins of the bone flap and interrupting dural vessels before removal of the bone flap. The incidence of the latter can be reduced by performing a unilateral free-flap craniotomy to the edge of the superior sagittal sinus and torcular Herophili on one side and then, under direct vision, freeing the superficial surface of the sinuses from overlying bone before removing the contralateral craniotomy plate. When dural adhesion to bone is unlikely because the patient is young, the tumor small, and hyperostosis absent, a single craniotomy flap can be used to expose all four quadrants of the peritorcular region. Infiltrated or hyperostotic bone should be removed and discarded. Dura should be incised at a distance of at least 2 cm from the tumor's margin. It should be hinged so as to maximize the view of peritorcular tumor. Drainage of CSF supratentorialy by ventriculostomy of the lateral ventricle or infratentorially by opening the cisterna magna facilitates subsequent retraction. Tumor distant from the venous sinuses should be separated from occipital or cerebellar cortex by microsurgical dissection and truncated.

The likelihood of achieving the goal of total resection depends on the nature of sinus involvement by the tumor; only rarely is tumor resection precluded by tumor attachment to eloquent cortex or critical arteries. The portion of a venous sinus completely occluded by tumor can be resected safely. When removing an occluded superior sagittal sinus, special care must be taken to preserve the anterior and lateral collaterals carrying the hemispheric flow. An involved transverse sinus can be divided and resected if preoperative venography has shown that (I) the sinus is occluded by tumor or is congenitally atretic, (2) its proximal portion communicates completely with a patent contralateral transverse sinus of adequate caliber, or (3) the superior sagittal sinus and straight sinus are fully confluent with a patent contralateral transverse sinus of adequate caliber. Interruption of a transverse sinus should be proximal to the inflow of the vein of Labbe. If preoperative venography has not clearly demonstrated the adequacy of the contralateral sinus, trial occlusion of the involved sinus by temporary intraoperative clamping may help assess the compensatory capacity of the contralateral sinus. The occipital sinus is of little importance unless, as a hypertrophied collateral of an obstructed transverse sinus, it carries substantial superior sagittal or straight sinus outflow to the jugular bulb.

When the venous anatomy dictates preservation of the flow through a sinus involved with tumor, the possibility of removal of the tumor from the sinus depends on the extent of sinus involvement. If the tumor merely abuts or is attached to the sinus by arachnoidal adhesions, it can be peeled from the sinus wall. The external sinus surface should then be coagulated with the bipolar cautery. Laser fulguration of the sinus wall should be avoided, because it may cause endothelial damage that subsequently induces catastrophic sinus thrombosis. When the tumor is frankly invasive of one wall of a critical sinus, tumor external to the sinus should be truncated. The invaded wall and intrasinus extension of the tumor can often be removed, and the resultant defect closed by direct suture or with a patch graft. This is best done by sequentially opening and closing small segments of the sinus as the tumor is progressively removed from within the sinus. The walls of the partially opened sinus can be grasped by vascular forceps or small clamps such that they can be released to permit removal of the tumor or approximated to allow closure. Transient opening of the sinus in such a controlled fashion permits removal of an intrasinus tumor with relatively little blood loss.

Attempts to remove a tumor involving more than one wall of the sinus are much more hazardous. Sinus flow must be interrupted during the resection of two walls. Construction of a patch graft that will maintain patency when sinus venous pressure is low is difficult. There have been isolated reports of tumor removal and patch graft repair of the more proximal portions of the superior sagittal sinus using a diversionary shunt. An analogous approach using a superior sagittal sinus to transverse sinus shunt during isolation and repair of a tumor-invaded torcular Herophili is conceptually intriguing but extremely hazardous. Removal of such a tumor might necessitate shunting flow from the superior sagittal, straight, and occipital sinuses and replacement of the torcular Herophili by a four-limbed prosthesis.

The risks concomitant with interruption or diversion of venous sinus flow and reconstruction of the torcular Herophili in an attempt to achieve complete resection of a peritorcular tumor are unwarranted. Although incomplete resection of meningiomas is frequently followed by recurrent tumor growth, excellent results have accrued from subtotal resection of tumor external to the sinus and irradiation of the residual lesion.

Fractionated external cobalt beam irradiation (50 to 60 Gy) of residual tumor is recommended. Stereotactic radiosurgery using a proton beam, gamma knife, or linear accelerator is an alternative for tumors smaller than 10 cm. Intraoperative marking of tumor margins by nonmagnetic metal clips facilitates planning of radiation therapy. Chemotherapy should be considered only if the tumor has malignant histologic characteristics. Postoperative follow-up review should consist of a periodic neurological examination and gadolinium-enhanced MR imaging.