20 Falcotentorial Meningiomas
Introduction
Falcotentorial meningiomas are rare; ~60 cases have been reported.1–25 These tumors occur at the junction of the dural folds of the tentorium and falx cerebri, either anteriorly at the junction of the vein of Galen with the straight sinus, or all along the length of the junction of the falx cerebri/cerebelli and tentorium in which the straight sinus, torcular, and arachnoid granulations are found. Large lesions involving the falcotentorial junction (FTJ) can be challenging to remove, leaving patients with transient or permanent neurological deficits, such as cortical blindness. Complications may arise during the ligation or manipulation of the sagittal sinus, bridging veins, vein of Galen, and torcula.
Symptoms and Signs
Most patients present with headaches (with or without nausea and vomiting) and visual disturbance.19 Incontinence, personality changes, gait ataxia, dizziness, and mild cognitive impairments have also been reported.3,13,16,26 Some of these complaints may be secondary to associated obstructive hydrocephalus.
The most common finding on physical exam has been papilledema, a sign of increased intracranial pressure.3,13,16,26 Patients may also have cranial nerve dysfunction (such as facial droop or homonymous hemianopsia). Some series have reported that patients report progressively worsening symptoms for an average of 29 months (ranging from 6 months to 4 years) before seeking medical attention.19
Preoperative Planning
We recommend that patients be evaluated using magnetic resonance imaging (MRI) and cerebral angiography. For MRI, patients undergo T1-weighted imaging, T2-weighted imaging, and contrast enhancement with gadolinium. Magnetic resonance venograms can also be obtained and represented as a three-dimensional (3-D) object at surgery on image-guided systems. Cerebral angiography is usually performed in all cases to evaluate the arterial supply of the tumor, and the venous phase can be used as the gold standard to assess the patency of the straight sinus, transverse sinuses, and torcula. We also recommend that patients have a preoperative Humphrey visual field examination and funduscopic and acuity examination documented by a neuroophthalmologist. Patients and their families are counseled that postoperatively there will be a few days of cortical blindness with a bilateral occipital transtentorial approach and a hemianopsia with a unilateral occipital transtentorial approach. In both situations the blindness and field defects have spontaneously recovered in all the patients most recently reported in our series.19
Placement of a lumbar subarachnoid drain for drainage of cerebrospinal fluid during the case may be considered in large lesions. Postoperatively the drain is left in place, clamped off for the first 1 to 2 days so as not to potentiate a postoperative hematoma and to prevent the occurrence of an occult cerebrospinal fluid (CSF) leak through the durotomy made by the needle used to insert the drain. Intraoperatively, adjuvant treatments include administration of steroids and mannitol as well as hyper-ventilation, all of which reduce postoperative edema and intraoperative intracranial pressures.
Radiographic Evaluation: Magnetic Resonance Imaging and Angiography
Most tumors in this region are found to have marked homogeneous enhancement upon administration of gadolinium. On T1-weighted images the main characteristic is isointensity to the brain parenchyma. On T2-weighted images, these lesions can be slightly hyperintense or hypointense, the latter predicting a fibrous or partly calcified nature of the tumor. Hydrocephalus can be present, and it has been reported.19
Preoperative cerebral angiography reveals that falcotentorial meningiomas usually derive their vascular supply from the internal carotid artery, external carotid artery, or the abnormal branches of the posterior cerebral artery. Supply from the internal carotid artery includes meningohypophyseal branches, branches off the inferolateral trunk, and the anterior choroidal artery. Posterior cerebral artery supply is derived from the me-dial and lateral posterior choroidal arteries. The external carotid supply for many of these tumors comes mainly from branches of the middle meningeal artery and falcine artery via the ethmoidal and ophthalmic arteries. If the predominant supply to the tumor is from a muscular branch of the left vertebral artery in the atlantal segment, this can be ligated with exposure of the region of the foramen magnum. In a majority of cases reported in the literature, either the vein of Galen or the straight sinus was occluded.3 The transverse sinus can be partially occluded depending on the location of the tumor, usually in its proximal portions to the lateral edge of the tentorial component of the tumor.
Embolization is thought to be unsuccessful in most cases because the feeding vessels are either too small or inaccessible. Despite this, the results of the angiography can be key to determining preoperatively the status of the venous sinuses and therefore how they could be safely managed during tumor resection. Knowledge about the blood supply is useful in planning the surgery and understanding the vascular supply and anatomy of the tumor. This allows the surgeon to direct initial attempts at identifying and interrupting the arterial supply before tumor debulking.
Surgical Approach
In our experience, when the tumor is smaller (< 3 cm) and involves predominantly one side of the falx or tentorium, a unilateral occipital craniotomy with the retracted right occipital lobe in a gravity-dependent position can be used3,16 ( Fig. 20.1 ). In most cases a left hemianopsia seems to be tolerated better than a right hemianopsia, even transiently, due the fact that we scan from left to right for reading. For patients in the prone position, in addition to routine mannitol and steroids, we would consider placing a lumbar subarachnoid drain for smaller tumors and an external ventricular drain for those with hydrocephalus or some degree of posterior fossa mass effect. Drainage of CSF will reduce intracranial pressures and make the large dural openings on the convexity less troublesome.
Patient Positioning
Patient positioning (i.e., prone versus semisitting) depends on the surgeon’s preference and the patient’s body habitus. We recommend the prone or three-quarter-prone position because in our experience it is safe and has a lower risk of air embolism. In this position, the patient’s neck is extended on the chest and the head flexed on the neck. We elongate the neck as much as we can by placing the neck in extension when the patient is prone and then flexing the head on the neck so that a finger passes between the chin and the chest to avoid rubbing between the lower portion of the jaw and the sternum.20 In obese patients, the semisitting position is a viable alternative because the prone position makes it difficult to adequately ventilate the patient without causing extreme airway pressures leading to elevated transmitted venous pressures. Obese patients or patients in the semisitting position should be preoperatively evaluated for a patent foramen ovale to potentially avoid intraoperative air emboli. A thorough discussion with the anesthesia team will most times lead to the utilization of an armored endotracheal tube to avoid kinking or obstruction associated with positioning or prolonged operative time. We also recommend that an external ventricular drain be routinely placed via the parietooccipital trajectory with the aid of the surgical navigation system.
Craniotomy: Unilateral versus Bilateral Occipital Transtentorial/Transfalcine Approach
There have been several reports describing different surgical approaches for falcotentorial meningiomas. These include the infratentorial supracerebellar,10,27,28 the biparietooccipital craniotomy in the sea lion position,21 and a combined supra-/infratentorial transsinus approach as described by Sekhar et al and Ziyal et al.29,30 Furthermore, Okami and colleagues have described the occipital transtentorial and the combined midline occipital and suboccipital approach.16 The majority of these reports include patients without falcotentorial meningiomas and include patients with pineal meningiomas, teratomas, and pineal cysts, among others.10,27,28,29,30 Isolated case reports from the past make it difficult to assess the efficacy of a particular type of approach.16,29 In studies with more than two patients, gross total resection (GTR) was achieved in 25 to 50% regardless of surgical approach.10,16,28,30 Complications, including vision loss and cranial nerve injuries, ranged from 20 to 50% in series that had more than two patients.16,27,30
For large tumors, a large U-shaped, inferiorly based incision is marked out to allow for a supra- and infratentorial craniotomy ( Fig. 20.2 ). When the tumors are large and involve both sides of the tentorium, as well as the supra- and infratentorial spaces, a bilateral occipital transtentorial/transfalcine (BOTT) approach is recommended (8/9 cases in our most recent series).19 We prefer this combined bilateral BOTT craniotomy technique, exposing the supra- and infratentorial compartments, for larger tumors, which have most often occluded the straight sinus.
In tumors with a significant infratentorial component, we prefer opening of the foramen magnum to reduce the resistance against inferior retraction of the cerebellar hemispheres. The bone flap over the torcular and transverse sinuses is removed in two components: the supratentorial bone flap is removed first and then the infratentorial bone flap is removed only after the dura over the torcular and transverse sinuses is dissected under direct vision ( Fig. 20.2 ).
The advantages of the occipital transtentorial approach are that (1) it allows for good visualization of the internal cerebral veins as well as the posterior and lateral midbrain, (2) it has a low risk of air embolism, and (3) it provides a relatively wide exposure of the lesion. The wide exposure achieved with a torcular craniotomy that extends far out laterally reduces compression of the occipital lobes against the dural openings, especially in long cases. The availability of exposure above and below the tentorium also provides the surgeon with more options intraoperatively and allows one to alter the plan of attack when troublesome bleeding is encountered.
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