SURGICAL APPROACHES TO TENTORIAL MENINGIOMAS

The extensive tentorial surface facing the two intracranial compartments affords attachment to 2 to 9 percent of intracranial meningiomas. Tumors of the clivus, petrous region, cavernous sinus, and Meckel's cave can also be partly based on the tentorium. Depending on their site of origin, direction of growth, and size, tentorial meningiomas differ in their mode of presentation, surgical difficulties, and morbidity. In general, the medial ones, adjacent to the brain stem, can prove to be a significant challenge, especially when they involve the cavernous sinus and petroclival area or the pineal region. Precise microsurgical technique and thorough understanding of the pathological anatomy is essential for successful excision of these tumors. Tentorial meningiomas can be broadly categorized into lateral, medial, and falcotentorial groups. These in turn may project into the supra- or infratentorial compartments or both.

The surgical approach is planned according to magnetic resonance imaging (MRI), with contrast enhancement, performed in thin sections along the coronal and axial planes. For a purely tentorial tumor, a computed tomography (CT) scan is not essential. The major advantage of MRI is the exquisite anatomical depiction of the tumor relative to the brain stem and vasculature.

Arterial displacement and narrowing are best evaluated by MRA, MRV is equally important. The vein of Labbe, the collateral venous drainage, and the sigmoid and transverse sinuses and their communication at the torcular are noted. Venous sinus involvement is better appreciated with MRI. The specific source of blood supply depends on the tumor's attachment, but the majority of these tumors are supplied by the meningohypophyseal (MHT) branches of the cavernous segment of the internal carotid artery (ICA) as well as by branches of the external carotid artery. Occasionally the blood supply may also be derived from the vertebrobasilar system.

Some tumors can be extremely vascular, and the debulking and dissection of the capsule from critical structures can be significantly hampered by troublesome bleeding. Preoperative embolization can be an important step in the management of such tumors. Superselective catheterization and embolization of the external carotid branches can be accomplished easily. However because of its acute angle of origin, the MHT is much more difficult to cannulate. Absolute alcohol or hypertonic glucose can be instilled through the ICA, but it has the danger of injuring the carotid artery itself and is not recommended by some authors. Temporary cranial nerve palsies are not uncommon after this procedure.

The site of attachment and direction of growth of the tumor (supratentorial or infratentorial, or both) determines the operative approach. In addition to a wide access, the approach must permit early interruption of the tumor blood supply. The peculiar configuration of the tentorium, with its medial edge sloping sharply upward, frequently results in supratentorial or combined approaches giving better exposure than purely infratentorial approaches, especially for medially located tumors.

Brain retraction is a major cause of morbidity and mortality in all operations at the base of the brain. Judicious bony resection should be preferred to brain retraction. On occasion, however, resection of a relatively silent area of the brain, such as the temporal tip or the inferior temporal gyrus, may be elected in preference to brain retraction. Close attention must be paid to the vein of Labbe because undue tension and rupture may lead to serious consequences. Appropriate arachnoidal cisterns must be opened widely to allow cerebrospinal fluid drainage and facilitate and optimize the brain retraction. Proper fluid management with the use of osmotic diuretics during the initial phases of the exposure is helpful, but during the operation, colloids are preferred for fluid replacement.

Tumor capsule resection is carried out only after adequate intracapsular debulking has been done. Adjuncts to tumor debulking like the CO2 laser and the ultrasonic aspirator may be helpful occasionally. Sharp dissection is generally preferred for dissecting the vascular and neural structures from the tumor. This is less injurious than blunt dissection; and if an arterial injury is produced, primary repair may be possible in contrast to repair of an irregular laceration produced by traction.

Venous sinus involvement must be dealt with after due consideration is given to the extent of alternate drainage. The transverse or sigmoid sinus may be divided provided there is an adequate outflow for the vein of Labbe. Partial excision of the sinus wall may be carried out with appropriate reconstruction with a vein or dural patch.

Intraoperative electrophysiological monitoring of auditory brain stem and somatosensory evoked responses is useful to limit brain retraction and brain stem manipulation. Stimulation of the facial nerve may be helpful in its identification and dissection. Because these modalities are also sensitive to many extraneous factors, such as anesthesia and blood pressure, a dedicated and experienced neurophysiologist is needed to obtain meaningful information.

Tumors projecting from the lateral and middle portion of the tentorium into the supratentorial compartment are best approached subtemporally. Once a low craniotomy has been performed-centered over the lesion, uncovering part of the transverse sinus, and flush with the middle fossa floor-the important structure is the vein of Labbe. Dissection of the arachnoid around this vein will free it up from the temporal lobe, permitting its retraction; if required, a portion of the inferior temporal gyrus can be resected. The key is to avoid prolonged retraction of the posterior temporal lobe, especially of the dominant hemisphere. The dural site of attachment should be resected as much as possible.

For the laterally situated tumor projecting inferiorly, the infratentorial approaches are most suitable. Sigmoid, transverse, or straight sinus involvement is of critical importance in radical excision. Although division of the sigmoid or transverse sinus leads to few adverse consequences, it is not entirely benign. Hence, good cross-communication with the other side should be documented, or a clamping test with intraluminal pressure measurement should be carried out. Similarly, an occluded straight sinus allows a much more radical resection than if it is not occluded. Cranial nerves VII and VIII are displaced inferiorly and nerve V will either be within the tumor or also displaced inferiorly.

Dissection within the tumor must be carried out in the direction of the nerves until they have all been identified. The attachment of the tumor should be resected where possible

Large meningiomas at the posterior part of the incisura can be approached through a combined occipital and suboccipital route by dividing the transverse sinus and the tentorium in front of it. Tumor resection must be carried out with due consideration given to the intricate arterial and venous anatomy in the pineal region.

Tumors at the anterior portion of the tentorial incisura are usually more difficult to approach. They are deep and involve the brain stem and the upper basilar artery and its branches. A tongue of tumor may also extend into Meckel's cave and the posterior cavernous sinus. Small and medium tumors extending supra- or infratentorially are approached by a combination of a trans-sylvian and anterior subtemporal approach. Addition of a zygomatic osteotomy provides the surgeon more room for a middle subtemporal approach if necessary. More extensive tumors in this region are dealt with by a combined posterior subtemporal and presigmoid route. To see a demonstration clickhere!

A frontotemporal craniotomy is performed and the zygomatic arch is removed after dividing it posteriorly at the root and anteriorly at the lateral rim of the orbit. The greater wing of the sphenoid is drilled down to make the exposure flat with the middle fossa floor. The sylvian fissure is opened widely, starting laterally and then extending all the way medially to the tentorial incisura. The veins at the temporal tip are divided, thus allowing the temporal tip to be mobilized out of the middle fossa. These manoeuvres along with the bony exposure allow the temporal lobe to be lifted out of the middle fossa without the arachnoid or the bone limiting it, thus resulting in a wide and shallow access to the anteromedial tentorial margin. The tumor is exposed and the relationship of the posterior communicating and anterior choroidal arteries and that of nerves III and IV is evaluated. The tentorial edge posterior to nerve III is everted to visualize nerve IV, and the tentorium is divided in a lateral direction posterior to this point. The trigeminal root and Meckel's cave are thus exposed. The tumor is debulked by working between the nerves, and gradually the capsule is dissected away from the surrounding structures. The consistency of the tumor and its vascularity will determine the ease or difficulty of dissection and the amount of trauma to the cranial nerves.

Because much of the blood supply to such tumors is derived from the intracavernous ICA, detaching the tumor from its anterior attachment early during the debulking provides for a relatively bloodless dissection of the tumor capsule. Small extensions into Meckel's cave can be removed by unroofing the dura over it, but significant extensions into the cavernous sinus require proximal control of the ICA and intracavernous dissection of the cranial nerves. The attachment of the tumor is resected where possible, and bleeding from the superior petrosal sinus and the cavernous sinus is controlled with Surgicel packing.

This is a versatile approach that is applicable to a wide variety of lesions in the area of the tentorial incisura, petrous ridge, and clivus. The approach relies on converting the supra- and infratentorial compartments into one by dividing the tentorium and thus allowing a wider access to the tentorial incisura. It may be useful to deal with a large tumor in two stages. In the first stage, through a frontotemporal approach, the upper pole of the tumor is removed; at the same time the tumor is detached from the posterior cavernous sinus, and its major blood supply is thus interrupted. At the second stage, performed within a week or two, the combined subtemporal and presigmoid approach is used to remove the rest of the tumor.

The patient is placed in a lateral decubitus position with the dependent shoulder being supported off the table and the head held in a three-point pin headrest. The posterior temporal, mastoid, and suboccipital region is exposed and either a combined temporal and suboccipital bone flap is raised, or this is done separately in two pieces after which the bone over the transverse sinus is removed. The mastoid is then drilled away in the retrolabyrinthine area to uncover the sigmoid sinus and the dura anterior to it and the area over the superior petrosal sinus. The temporal and presigmoid dura are opened, converging toward the superior petrosal sinus, which is clipped or coagulated and divided. The tentorium is incised from here medially. The vein of Labbe enters posterior to the point of division of the superior petrosal sinus and is retracted cephalad with the posterior part of the tentorium and the temporal lobe, thus avoiding traction on the vein. The tentorial incision is carried posterior to the tumor. The cerebellum and sigmoid sinus are retracted posteriorly, providing the required exposure.

The tumor is debulked by working between cranial nerves III. IV, V, VII, and VIII. Nerve VI may be imbedded in the tumor and must be traced carefully from the brain stem into the tumor. The trigeminal nerve involvement may extend into Meckel's cave and the nerve fascicles may be dispersed in the tumor. making its dissection and preservation difficult. Encasement of posterior fossa vessels must be noted on the MRI scans, and even under these circumstances they may be successfully dissected away from the tumor. Although the irrigating bipolar forceps and microscissors are the principal instruments, occasionally the ultrasonic aspirator may be a helpful debulking tool. The tumor may derive blood supply from the parenchymal vessels of the posterior circulation. and this must be determined carefully while these vessels are dissected from the tumor. Intimacy of the brain stem-tumor interface or tumor encasement of the important vessels may be the limiting factor in the excision of such tumors.

Although the mortality rates of such operations have reduced tremendously, tentorial meningiomas can be formidable tumors to manage (Table-1). Early diagnosis leading to the discovery and treatment of small tumors has a high likelihood of a favourable outcome. Anterior tumors are more difficult and the morbidity is related to brain retraction and cranial nerve palsy. Almost all the complications and poor outcome were among patients with medial tentorial tumors Venous sinus involvement should be dealt with according to the guidelines mentioned above. Radical resection must be weighed against the morbidity in the individual situation.

Complications can usually be anticipated on the basis of the preoperative studies. For tentorial meningiomas, complications are related to their location and size. Some of the complications can be avoided by meticulous attention to planning and technique. It is important to recognize the others early to avoid irreversible consequences.

These may occur from excessive and prolonged retraction or from venous outflow compromise and are potentially devastating complications. Like all complications, they are best avoided by careful planning and execution of the surgical approach. A purely posterior subtemporal approach for a lesion at the incisura is generally not preferred because of the possibility of venous injury as well as excessive retraction. A CT scan is routinely performed on the day after the operation to check for potential problems before they become symptomatic. A low threshold for early exploration and debridement is crucial to prevent severe consequences from temporal lobe herniation.

These injuries may occur during dissection of the tumor from the brain stem and the cerebellum, especially if the tumor has violated the pial plane. This situation can usually be detected in the preoperative T2-weighted MRI scans, which will show the increased signal intensity around the tumor. It is preferable to leave part of the tumor against the brain stem rather than risk a neurological injury.

During dissection of the tumor capsule, the surgeon must realize that parenchymal arteries may be supplying the tumor. No artery should be sacrificed until it is positively identified as a tumor vessel. It is difficult to repair a lacerated vessel at such a depth, but certainly the likelihood of a successful repair is increased if it is a clean cut produced by sharp dissection. The neurological deficit will depend on the site of injury (proximal or distal to the superior cerebellar artery or the posterior cerebral artery).

Temporary loss of function of cranial nerves III, IV, and V can occur, depending on the amount of dissection necessary. Difficulty with ocular motility should be managed in conjunction with a neuro-ophthalmologist. Recovery is the rule although its degree is variable. An anesthetic cornea is a potentially dangerous situation and proper lubrication and care must be taken. Concomitant nerve V and VII deficits are potentially devastating for the eye and should be treated with a tarsorrhaphy or a gold weight insertion into the upper eyelid

Authors	Total No.Patients	Outcome
		Excellent	Good	Fair	Poor	Dead
Sugita and Suzuki	49	76%	12%	6%	2%	4%
Sekhar et al.	27		72%	17%	4%	7%
Guidetti et al.	61		62%	26%	12%	3%