Surgical Anatomy

The tentorium cerebelli is a dural duplication that covers the upper surface of the cerebellum and supports the occipital lobes, thus separating the supra- from the infratentorial intracranial compartment. It is attached posteriorly to the transverse ridges on the inner surfaces of the occipital bone and encloses the transverse sinuses at these sites. Anterolaterally it is attached to the superior ridge of the petrous portion of the temporal bone, enclosing the superior petrous sinus on both sides. It continues anteriorly to be attached to the posterior and anterior clinoid processes. Its medial free concave border bounds an oval opening (incisura tentorii) for transmission of the midbrain. The falx cerebelli is a small triangular dural process that arises in the midline from underneath the tentorium. It indents between the two cerebellar hemispheres. The upper surface of the tentorium connects to the posterior end of the falx cerebri in the midline, enclosing the straight sinus along the line of junction. The straight sinus receives the vein of Galen and the inferior sagittal sinus at the tentorial apex and runs posteriorly to meet the transverse sinuses from both sides and the superior sagittal sinus from above at the torcular Hero-phili. The tentorial incisura is divided into an anterior incisural space located in front of the brain stem, a middle incisural space situated lateral to the brain stem, and a posterior incisural space located behind the brain stem.4 Anatomical structures located in relation to the tentorial incisura may be compromised by tentorial meningiomas and may be at risk during surgery. Preservation of these structures is usually facilitated at first surgery by arachnoid dissection layers forming the lateral walls of the chiasmatic, crural, ambient, and interpeduncular cisterns and is important to obtain a good surgical result. At the anterior incisural space, the most important neurovascular structures in close proximity include the oculomotor nerve, the basal vein, the posterior communicating artery, the anterior choroidal artery, the P1 and proximal P2 segments of the posterior cerebral artery, and the superior cerebellar artery. Less often, the optic nerve and the optic chiasm may be involved. The middle incisural space is bounded medially by the cerebral peduncle and upper pons. This narrow space contains the crural cistern, located between the uncus and cerebral peduncle anteriorly, and the ambient cistern situated between the midbrain and parahippocampal gyrus posteriorly. The trochlear nerve, the anterior choroidal artery, the P2 segment of the posterior cerebral artery, the superior cerebellar artery, and the basilar vein constitute the neurovascular contents of the middle incisural space. Particular attention should be paid to preserve the tiny trochlear nerve located just beneath the tentorium because it may be damaged in the subtemporal or petrosal approach. The ambient cistern continues posteriorly into the quadrigeminal cistern, which is the main cistern of the posterior incisural space. This space forms the pineal region and is related to anterior falcotentorial meningiomas. It contains the trunks and branches of the posterior cerebral and superior cerebellar arteries, and the vein of Galen, which receives the paired internal cerebral and basal veins. The trochlear nerve exits from below the inferior colliculi, curves around the dorsal midbrain, and enters the ambient cistern in the middle incisural space.

The tentorium receives its arterial supply from the basal tentorial artery (artery of Bernasconi-Cassinari) originating from the meningohypophyseal trunk, the marginal tentorial artery arising from the inferolateral trunk of the intracavernous carotid, and tentorial branches originating from the superior cerebellar and posterior cerebral arteries.4 A peculiarity of the tentorium is the presence of interdural venous sinuses, which may become particularly prominent when major venous channels are occluded by tumor. Brisk bleeding from these venous lakes may be anticipated during incision of the tentorium in the occipital transtentorial approach. This bleeding can be controlled by bipolar coagulation. In the authors’ experience, no neurological sequelae have been caused by their occlusion. The surgeon should study the venous drainage pattern of the temporal lobe and the point of termination of the subtemporal veins into the venous sinuses when planning for a subtemporal or petrosal approach.5

Classification

Classification of tentorial meningiomas, as any classification for meningiomas, has inherent limitations due to their inconsistent pattern and extent of dural attachment, direction of growth, and potential to transgress or obviously invade different anatomical structures (e.g., dura, bone, brain, vascular channels, and cranial nerves). This concern was already expressed by Cushing and Eisenhardt in their discussion of meningiomas of the cerebellar chamber.6 The surgical approach should be tailored for each individual patient with these limitations in mind. Nonetheless, classification of these tumors is useful to systematize the most appropriate surgical approaches and to discuss problems and potential complications inherent to each tumor subgroup. Tentorial meningiomas may be confined to the infra- or supratentorial space or may extend both infra- and supratentori-ally ( Fig. 18.1 ). Several classification schemes have been proposed; however, the one introduced by Yaşargil7 is the most accurate in regard to surgical anatomy: (1) meningiomas arising from the free tentorial notch (i.e., inner ring meningiomas, anterior T1, middle T2, and posterior T3), (2) meningiomas originating from the intermediate tentorial surface (T4), (3) meningiomas involving the torcular Herophili (T5), (4) meningiomas arising from the lateral outer tentorial ring (posterior T6, anterior T7), and (5) falcotentorial meningiomas (T8). In our retrospective study we have modified this classification because often we could not accurately differentiate between T1 and T2 and T6 and T7 tumors from preoperative neuroimaging.8 Furthermore, we have considered T3 and T8 tumors as one group of falcotentorial meningiomas. For practical purposes we therefore prefer a modified Yaşargil classification scheme comprising the following five tumor subgroups ( Fig. 18.2 ):

1. 
   

   T1–T2 (medial “incisural” meningioma)

   
   
2. 
   

   T4 (paramedian “intermediate” meningioma)

   
   
3. 
   

   T5 (peritorcular “torcular” meningioma)

   
   
4. 
   

   T6–T7 (lateral tentorial meningioma)

   
   
5. 
   

   T3–T8 (falcotentorial meningioma)

Clinical Presentation

Clinical symptoms and signs depend on the location and size of the tumor ( Table 18.1 ). Tumor-related complaints are often present for long periods, usually months or even years, although rarely patients can present with acute symptoms of increased intracranial pressure caused by obstructive hydrocephalus. Patients harboring a T6–T7 infratentorial meningioma, the most often encountered subtype, commonly present with headache, dizziness, and gait unsteadiness8,9 ( Fig. 18.3 A–E ). Clinical examination usually reveals a gait ataxia and occasionally impairment of the vestibulocochlear nerve. Hearing loss may be caused by direct involvement of the eighth cranial nerve (CN), or it may be the result of distortion of the central auditory pathways, such as the lateral lemniscus or the inferior colliculi.10,11 Marked improvement of hypacusis or even restoration of normal hearing after tumor removal has been observed by several investigators.8,10 It may be difficult to differentiate between infratentorial T6–T7 and posterior petrosal meningiomas on clinical and radiological grounds; indeed, lateral tentorial meningiomas may intraoperatively prove to have an additional origin from the suprameatal posterior petrosal surface.8,12 Supratentorial meningiomas, particularly those closely related to the medial temporal lobe, may present with seizures. T1–T2 meningiomas may intimately involve the brain stem and the fifth CN. Accordingly, patients may present with hemiparesis, trigeminal neuralgia, and facial numbness.13–15 Patients with T3–T8 meningiomas often present with headache ( Fig. 18.4 A–D ). Mental changes are reported in up to 46% of patients and a gait ataxia in 43 to 62% of cases. A homonymous hemianopsia is present in 20 to 46% of these patients.16–18

Diagnostic Workup and Preoperative Considerations

A small tentorial meningioma may be discovered incidentally on magnetic resonance imaging (MRI) in an elderly patient who is asymptomatic or presents with minor unspecific symptoms, such as headache and dizziness. In these cases a wait-and-see policy may be adopted and follow-up clinical and radiological examinations scheduled. The tumor may remain quiescent for many years, and the initial unspecific symptoms may resolve spontaneously. If follow-up examinations demonstrate progressive tumor growth or a deterioration of symptoms, surgery may be indicated. Radiosurgery may alternatively be considered in selected patients who are not good surgical candidates due to advanced age or significant comorbidity. Triplanar contrast-enhanced T1-weighted MRI gives the most accurate information for planning the surgical approach. The dural attachment zone and extent of the tumor, displacement of the brain stem, displacement or engulfment of vertebrobasilar arteries, invasion of the cavernous sinus, and patency of the straight, transverse, and sigmoid sinuses can be sufficiently studied. The interface between the tumor and the brain stem is well depicted on T2-weighted MRI. A hyperintensive signal within the brain parenchyma indicates disruption of the blood–brain barrier and difficulties in resecting the tumor from the brain stem may be anticipated. It is often safer to leave a tumor remnant in these situations. The drainage pattern of the subtemporal venous complex and patency of venous sinuses are demonstrated with good-quality magnetic resonance venography (MRV). This investigation has largely replaced invasive catheter angiography. Computed tomographic (CT) scanning is less important for the preoperative workup of tentorial meningiomas due to its inaccuracy in demonstrating posterior fossa pathologies and due to the fact that involvement of bone is not a prominent feature in these tumors. The rare presence of tumor calcification can give some clue to tumor consistency. Even the most sophisticated radiological investigations available today have important shortcomings that should be recognized. A nonvisible venous sinus on MRV or catheter angiography may prove to be patent during surgery. Tentorial sinuses, which may be a source of brisk bleeding, are usually not visualized preoperatively.19 Reliable information on functional significance of venous sinuses and major veins is usually lacking. Finally, the exact relationship of the tumor to neighboring cranial nerves (displacement, infiltration) can only be fully appreciated during surgery.