Introduction

Torcular meningiomas arise from, invade, or are attached to a wall of the torcular Herophili, the site of confluence of the superior sagittal, straight, occipital, and both transverse sinuses ( Fig. 19.1 ). Dura forming the torcular Herophili comprises part of their primary dural base. Peritorcular meningiomas, in contrast, arise primarily from the dura of the posterior falx cerebri, posterome-dial tentorium cerebelli, superoposterior falx cerebelli, or adjacent occipital or suboccipital convexity; although they may alter the venous flow through the torcular, they do not involve the torcular wall itself. Torcular and peritorcular meningiomas, hereafter referred to jointly as 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.1 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.2 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 1906.3 The first case in Dandy’s initial description of ventriculography was a peritorcular meningioma.4

The imprecision with which the term peritorcular meningioma has been used and the rarity of meningiomas in the torcular region make estimation of their true incidence difficult. Cushing and Eisenhardt’s 12 peritorcular meningiomas constituted 16% of their 77 parasagittal tumors.1 Additionally, three series of posterior fossa meningiomas included four (5%) of 82 peritorcular meningiomas,5–7 and three series of tentorial meningiomas contained 14 (13%) of 109 peritorcular meningiomas.8–10 If the relative incidences of parasagittal, posterior fossa, and tentorial tumors are 10%, 10%, and 5%, respectively, of all intracranial meningiomas, then peritorcular tumors represent ~1% of intracranial meningiomas.11

Pathology

Peritorcular meningiomas arise from arachnoidal cap cells in the region of the torcular Herophili. All subtypes of meningioma may develop. Cushing and Eisenhardt noted a predominance of angioblastic meningiomas in their group of peritorcular meningiomas; of the 12 tumors, six were angioblastic, five of which showed malignant histologic and clinical features. Three of 12 tumors were of the fibroblastic “whorl” type, one was psammomatous, one was fibrotic, and one 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).1 Although meningiomas of benign histology may extend through dural barriers, this extension occurs with greater frequency among tumors with malignant histologic characteristics.

Although arachnoid cap cells giving rise to peritorcular meningiomas exist in each quadrant of occipital and sub-occipital dura about the intersection of the superior sagittal, occipital, and each transverse sinus, they are relatively abundant along the posterior margin of the tentorium.8 The dura is a poor barrier to tumor extension; tumors frequently penetrate the distal falx or posterior tentorium to reach an adjacent torcular quadrant. By compressing or invading the superior sagittal, straight, transverse, and/or occipital sinuses, peritorcular meningiomas threaten dural venous sinus flow at the torcular Herophili. The anatomical relationship of peritorcular meningiomas to the posterior dural venous sinuses is paramount because of its profound implications for the clinical presentation, diagnosis, treatment, and outcome of patients with these tumors.

Clinical Presentation

Because of their low incidence, peritorcular tumors have not been distinguished from other meningiomas in reported series other than that of Cushing and Eisenhardt.1

In that series, seven patients were male and five were female.1 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 1 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 19.1 ). 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 tonsillar 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, except in cases of malignant histology, permits the development of sufficient collateral flow to preclude such a catastrophe.

Modern imaging has permitted documentation of progressive tumor expansion that displaces the adjacent occipital lobe or cerebellar hemisphere and compresses peritorcular sinuses. Dysfunction of the occipital lobe (visual field loss and seizures) or cerebellum (ataxia and weakness) may result from direct compression by tumor or from regional venous congestion. Venous congestion may be widespread and result in symptoms of generalized intracranial hypertension (headache, vomiting, and seizures). The development of these 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 ( Fig. 19.2 ). Rational selection of cases for surgery requires a detailed understanding of the anatomy of the lesion, consideration of the indications and contraindications of surgery for each individual patient, and awareness of the alternatives to complete surgical resection.

Diagnostic Imaging

The most valuable neurodiagnostic tools for assessing the anatomy of peritorcular meningiomas are gadolinium-enhanced magnetic resonance imaging (MRI), magnetic resonance angiography/venography (MRA/MRV), and multiprojection subtraction angiography.

Most meningiomas are isointense with brain on T1-weighted images. There is some increase in relative intensity of tumor to brain on T2-weighted images. Large parenchymal or tumor vessels are apparent as cylindrical areas of signal void. Intratumoral calcification appears as irregularly shaped signal void on T1- and T2-weighted MRI scans. Peritumoral edema shifts from hypointense on T1-weighted images to hyperintense with T2 weighting. Gadolinium enhances the contrast between tumor and brain ( Figs. 19.3 and 19.4 ). 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.

Multiplanar MRI offers anatomical detail of the tumor and its relation to adjacent venous sinuses and the tentorium; this information is extremely valuable in preoperative planning. The coronal view ( Fig. 19.3A ) clearly shows the relation of the tumor to the tentorium, and the midsagittal view shows the relation of the tumor to the falx cerebri and falx cerebelli ( Fig. 19.3B ). Supratentorial and infratentorial portions of the tumor are readily distinguished. Tumor extension along the falces and the tentorium is evidenced by thickening of dural leaves, which appears more intense with administration of gadolinium.

Gadolinium-enhanced MRI not only identifies sinus walls involved with the tumor but also helps to predict the status of the sinus lumina. A patent sinus has a signal void characteristic of flowing blood: this is especially evident on T2-weighted scans, which maximize contrast with the higher intensity of cerebrospinal fluid (CSF). A partially occluded sinus is heterogeneous in intensity; regions of signal void corresponding to normal flow are interrupted by regions of increased intensity corresponding to stasis. A uniformly intense sinus suggests complete occlusion: a sinus with very slow flow, however, occasionally appears bright and resembles a completely occluded sinus. MRA/MRV demonstrates the full course of all phases of vascular supply and can identify points of compromise of venous flow without the invasiveness of standard angiography.

The relation of normal arteries and veins to the tumor and the dural location of the blood supply to the tumor are best seen on multiplanar angiography. The feeding arteries (usually the middle meningeal and occipital branches from the external carotid artery, the meningeal branches of the vertebral artery, and the tentorial branches of the cavernous internal carotid artery) can be identified and possibly embolized, as can vessels within the tumor itself. Large feeding arteries can be filled with embolic material and occluded more proximally by a detachable balloon after embolization is completed.

Angiography also allows delineation of the peritorcular venous anatomy and of the pattern of flow in each sinus. The specific configuration of the torcular Herophili in an individual patient is often critical not only to a tumor’s clinical pathophysiology but also to its resectability ( Fig. 19.4 ). Rarely do the sinuses intersect symmetrically at a central point beneath the internal occipital protuberance.12 Rather, peritorcular venous channels are usually asymmetrical and septate. The right transverse sinus frequently carries most of the superior sagittal sinus out-flow and may be larger than the left transverse sinus, which frequently carries most of the straight sinus flow. The distal superior sagittal sinus has a double lumen in many cases. Occasionally, one transverse sinus is congenitally atretic or even absent, or distal outflow is obstructed because tympanic disease has occluded the sigmoid sinus or jugular vein.

Sinus occlusion manifests as absence of sinus filling and flow of blood from the torcular into dural, cerebral, or cerebellar collaterals. The details of the site and cause of venous sinus obstruction are best seen on retrograde dural sinus venography. Direct endovascular cannulation of the transverse sinus also allows assessment of tolerance of sinus obliteration; neurological function and intraluminal pressure proximal to the blockage should be measured during periods of balloon inflation. The extent of communication of the superior sagittal flow with each transverse sinus and of one transverse sinus with the other determines the safety of sinus occlusion and thus the surgical resectability of a peritorcular meningioma.