Surgical taming of petroclival meningiomas: King Cobra of all skull base tumors
“Neurosurgeons must be careful not to turn the cure into something worse than the disease. In particular, the benefits of complete resection must be weighed against the possibility of associated morbidity and even mortality. The serious complications of resection, while less frequent today, are no less devastating for patients who suffer them.“
Petroclival meningiomas (PCMs) are technically challenging lesions. True petroclival meningiomas are those originating along the upper two-thirds of the petroclival junction medial to the fifth cranial nerve, cephalad to the jugular tubercle with dural attachment centered on the petroclival junction. These tumors, as they grow, displace the brainstem and basilar artery posteriorly, and to the contralateral side. Meningiomas which arise lateral to the trigeminal nerve are called ‘petrosal meningiomas' and are further split into ‘anterior' and ‘posterior' petrosal meningiomas, depending on their origin anterior or posterior to the internal auditory canal. Anterior petrosal meningiomas are often grouped along with true petroclival meningiomas as their surgical decision making is similar to that of true petroclival tumors. These tumors may extend into the cavernous and petrosal sinuses, middle cranial fossa, parasellar region, tentorium, foramen magnum, Meckel's cave, and/or various other cranial nerve foramina before they become clinically apparent. In addition, they may also displace or surrounding cerebrovascular structures of the region and may invade through the dura and infiltrate the underlying bone. Resection of these tumors has been associated with a high level of morbidity and mortality in the past. The management of petroclival meningiomas, even with advanced surgical technology and instrumentation, remains a formidable technical challenge to the skull base surgeon.
Gosal et al., in this issue, should be congratulated for publishing an honest, long-term outcome of 33 cases of large and giant (>5 cm) petroclival meningiomas operated in their institute. In 75% of the cases, there was tumor extension to both posterior and middle fossae, and an equal percentage had involvement of the Meckel's cave. A gross total excision could be achieved in 36% of cases; it was near-total in forty six percent of the cases. Tumor extension below the internal auditory meatus (IAM) was seen in 60% of tumors, and in the rest, the tumor was located predominantly above and medial to the IAM. Majority (48%) of the patients were operated by the Kawase approach, while 21% and 18% of patients underwent retrosigmoid and pretemporal temporopolar half-and-half approach. At a mean follow up of 36 months, 13 patients (39%) had a poor outcome (dead 6 and severely disabled 7). However, the relationship of mortality and severe morbidity to the two groups (large and giant), to various surgical approaches, and to the extent of resection is not mentioned. Some of the patients operated by the Kawase approach obviously had tumor extension below the IAM and it would have been nice if author had mentioned the morbidity of this group. The concept of a two-stage operation using two simple approaches for these large and giant tumors, which span both the compartments, will definitely reduce morbidity.
Nowhere is the adage truer that there are many ways to ‘skin a cat' than in the variations in the operative management of petroclival meningioma. In this critical review, we discuss the technical pearls in the operative management of petroclival meningiomas, focusing mainly on the middle fossa approaches to these tumors. The choice of the approach is dependent on the location and epicenter of the tumor, the direction of tumor extension, the tumor size, the patient's age, the medical comorbidities, and the proposed extent of resection. The four basic approach to petroclival meningiomas include:
Each of these strategies has its benefits and short-comings that should be carefully analyzed. Multiple simultaneous approaches or staged procedures may also be considered.
The pterional trans-Sylvian approach is accepted as the standard technique to access the interpeduncular area from an antero-lateral perspective. However, this approach must traverse though the anterior aspect of the temporal lobe, and obtaining adequate space in the pretemporal area without injuring the temporal lobe is difficult. The pretemporal temporopolar half-and-half approach, which combines the benefits of both the trans-Sylvian and the subtemporal routes is based on retraction of the temporal lobe posteriorly with sacrifice of the Sylvian veins., It offers a more anterior line of vision compared with the subtemporal approach and visualization of both P1 segments of posterior cerebral arteries (PCA) is improved. Primary concern with this approach is the ligation of the temporal tip bridging veins to the sphenoparietal sinus. The sacrifice of these veins may result in venous congestion if collateral flow is insufficient and can have negative consequences; and, there is no fool-proof method of predicting whether or not sufficient collateral flow exists to mitigate the risk. Due to this fact, various modifications of this approach have been introduced. These modified temporo-polar approaches can be classified into two major types according to the method used to retract the temporal lobe posteriorly, either intradurally (intradural anterior temporal approach) or extradurally (extradural temporopolar transcavernous approach).
The intradural technique requires extensive dissection of the superficial middle cerebral veins (SMCVs) without producing cortical or vascular injury. This obviously requires a high level of microsurgical skill to skeletonize not only the SMCVs, but also the anterior temporal artery (ATA) before performing the safe retraction. The skeletonized SMCVs may pass across the operative field and extreme care may have to be undertaken to avoid avulsion of these vessels during tumor manipulation. Adequate mobility of the temporal lobe is mandatory for the successful conduction of this approach. After a wide Sylvian fissure splitting, the frontal and/or temporal cortical branches of the SMCVs are carefully detaching from the frontal and temporal lobes. If several SMCVs are present, dissection between them facilitates separation of the frontal and the temporal lobes. The key point is the careful dissection of the thick arachnoid at the point where SMCVs penetrate the dura at the medial sphenoid ridge. The ATA, which usually branches from the M1 portion of the MCA and attaches to the anteromedial part of the temporal lobe tip should be dissected from the temporal lobe. Only after complete mobilization of the SMCVs and the ATA should the arachnoid membrane between the uncus and the oculomotor nerve be incised. By performing this essential maneuver, the temporal lobe can be retracted posteriorly and an adequate retrocarotid space may be obtained to access the petroclival tumor. An orbitozygomatic (OZ) approach can be added for large tumors with extension to the suprasellar, parasellar, retrosellar and cavernous sinus areas. This approach allows exposure of the anterior and middle cranial fossae, the area of the upper third of the clivus, and the posterior fossa. Although the OZ approach improves exposure of the surface of the craniotomy and allows an inferior-to-superior view, it does not appreciably improve exposure of the anterior incisural space, as anterior and posterior clinoid processes limit the working spaces between the optic nerve and the internal carotid artery (ICA) or between the ICA and third nerve, and often the middle cerebral artery (MCA) or the posterior communicating artery (PComA) runs horizontally across the operative field, restricting the upward access. To mitigate this, the OZ craniotomy can be combined with an intradural anterior clinoidectomy, optic canal opening and incision of the dural ring, which allows for a safe mobilization of the ICA and the optic nerve.
The extradural temporopolar transcavernous approach is designed to preserve the temporal bridging veins by mobilizing the temporal pole under the protection of dura propria., Though more complex in terms of surgical steps, the risk of venous insufficiency is largely mitigated and it provides an extremely clear operative view of the central skull base. In this approach, an anterior and/or posterior clinoidectomy is performed, thus expanding the carotid–oculomotor window. Removal of the anterior clinoid process disconnects its attachments to three main surrounding structures, namely the roof of the optic canal, the optic strut and the roof of the orbit. The dura is opened along the Sylvian fissure as far as the optic nerve, the falciform fold is cut, and the dural opening continued to the inferior frontal dura in a T-shaped fashion over the distal dural ring. Deroofing of the optic canal, cutting of the falciform fold and the distal dural ring, facilitate medial mobilization of the ICA, and an improved exposure of posterior clinoid, dorsum sella and upper part of clivus. Opening of the roof of the cavernous sinus medially and parallel to the oculomotor nerve helps in freeing this nerve and the trochlear nerve. The posterior clinoid process can be removed radically, if required, thereby extending vision and reaching up to the midclival region of the posterior fossa. These technically challenging maneuvers of the transcavernous approach should not be a part of the routine operative steps to the petroclival meningiomas, but need to be performed based upon the complexity of these tumors, and are directly related to a progressive need for more surgical space or access. The important limitation of the temporopolar transcavernous approach is venous bleeding, which may occur from the cavernous sinus and the basilar venous plexus.
Lateral transpetrosal approaches
Complete visualization of the meningioma and all adjacent vascular structures was once a must for making surgery more feasible and safer. As the main anatomic obstacle to a good exposure of the petroclival area is the temporal bone, approaches with partial or complete resection of the petrous pyramid have been described. The extradural subtemporal approach through the middle fossa has become one of the frequently performed operative procedures in skull base surgery. Approaches through the petrous bone have among their many advantages, a decreased operative distance to the tumor and neurovascular structures; moreover, the access for dissection to the lateral and anterior brainstem is improved. Specific approaches through the petrous bone include removal of the petrous apex in the middle fossa approach, resection of the presigmoid retrolabyrinthine petrous bone in the posterior petrosal approach, and the performace of a complete petrosectomy. The posterior petrosal approach is ideal for larger tumors extending below the internal auditory meatus in patients with serviceable hearing. Should a larger tumor extend across the clival midline or into the anterior cavernous sinus, a combined anterior and posterior petrosal approach may be used. If hearing is lost, additional exposure may be afforded by a complete petrosectomy, with removal of the labyrinth and cochlea. Careful study of the venous anatomy is critical in these cases. A greater risk may be incurred in patients with a dominant or isolated sigmoid or transverse sinus on the side of the tumor or with venous drainage through the tentorium. In the latter scenario, should the vein of Labbé drain into the tentorium or the superior petrosal sinus before the sigmoid-transverse junction, it is crucial that the tentorial incision be made anterior to the insertion of the vein of Labbé, with sparing of the petrosal sinus.
Anterior transpetrosal approaches and its variants
The goal of the basic lateral middle fossa approach is the identification of the IAC in the floor of the middle fossa for accessing intracanalicular vestibular schwannomas. This extradural approach is a basic building block for the extended middle fossa and Kawase's approaches, which are utilized for surgical access of the petroclival meningioma. The temporal lobe dura is elevated in a posterior-to-anterior fashion. The foramen spinosum and foramen ovale are skeletonized, and the middle meningeal artery is coagulated and divided. The dura propria is elevated from the trigeminal third division by sharp dissection using a no. 15 knife blade. The greater superficial petrosal nerve and the location of the geniculate ganglion are ascertained. The petrous ridge is identified medially and after placing tapered retractors, the IAC is exposed medially first by drilling the porus acusticus, and then, by proceeding laterally. Many techniques are described for ascertaining the approximate location of the IAC. The bisection angle between the greater superficial petrosal nerve (GSPN) and the arcuate eminence approximates the lie of the IAC. The absence of neurovascular structures at the porus acusticus makes this area relatively safe for bone removal. Once the dura of the IAC is seen, the exposure continues laterally toward the fundus. At the fundus, bone resection is limited anteriorly by the basal turn of the cochlea and posteriorly by the ampulla of the semi-circular canal (SSC) and the vestibule. Injury to either structure results in hearing loss. Therefore, the dural exposure is narrowed as the dissection proceeds laterally. Drilling in the angle between the GSPN and the IAC can easily perforate the underlying cochlea. The dura mater of the IAC is exposed just laterally to the vertical crest and opened longitudinally along the IAC axis. The dural opening is made posteriorly first over the superior vestibular nerve to avoid injury to the facial nerve. Although identification of this structure via the middle fossa may seem like a straightforward exercise, small errors can lead to injury of the seventh nerve and hearing. The extended middle fossa approach to the internal auditory meatus and the cerebellopontine angle builds on the basic middle fossa approach with removal of additional medial petrous bone lying within the Kawase rhomboid. The anterior petrous bone is also resected upto the horizontal segment of the petrous ICA. The Kawase rhomboid is bound by the GSPN laterally, the petrous ridge medially, the IAC axis posteriorly, and the V3 anteriorly. The limits of the extended middle fossa exposure are the inferior petrosal sinus inferiorly, the middle ear ossicles laterally, the Gasserian ganglion More Details anteriorly, and the SSC posteriorly. The anterior petrosectomy exposes the dura mater inferiorly to the level of the inferior petrosal sinus. Opening the dura mater across the superior petrosal sinus exposes the posterior fossa. The Kawase approach differs little from the extended middle fossa dissection., The extent of the anterior petrosectomy and dural opening is unchanged. The main addition is the transverse division of the tentorium to the incisura, which then allows for a full communication between the intradural middle and posterior fossa compartments. The only two structures crossing the field of view are the fourth cranial nerve medially and the fifth nerve laterally. Elevating the divided edges of the tentorium with retention sutures provides a wide exposure of the petroclival region, comfortably to the mid-clivus, that is, at a level with the porus acousticus. Although, all of the middle fossa approaches are designed to preserve hearing, sacrifice of the labyrinth and thus hearing can sometimes be used to allow for an additional exposure of the posterior fossa. However, the middle fossa approaches have some disadvantages, especially in patients with a large epidural venous route, such as that of the sphenobasal and sphenopetrosal veins. In the latter situation, a modified technique for the anterior petrosectomy via an intradural approach, that overcomes these disadvantages, is mandated. The intradural approach has enabled the tailoring of the extent of anterior petrosectomy to the individually required exposure; however, the risk of injury to the temporal bridging veins is higher in this approach than in the epidural approach, and also, the lack of familiar epidural landmarks impair the identification of the petrous apex. Following a subtemporal craniotomy and basal opening of the dura mater, the vein of Labbé is first identified and protected. Cerebrospinal fluid is drained via a spinal catheter. The tent is incised behind the entrance of the trochlear nerve toward the superior petrosal sinus (SPS), which is coagulated and divided. The dura is stripped from the petrous pyramid. Drilling starts at the petrous ridge and proceeds laterally and ventrally. The trigeminal nerve is unroofed. The internal acoustic meatus is identified and drilling is continued laterally, as needed. The bone of the Kawase triangle toward the clivus can then be removed down to the inferior petrosal sinus, if necessary. The anterior exposure can be extended to the carotid artery, if required.,,
Posterior transpetrosal approaches
The posterior transpetrosal approaches provide a lateral operative corridor to lesions located on and around the upper and middle clivus to access the PCM. Although many variations of this approach have been described in the literature, all are extensions of the basic mastoidectomy. In general, the posterior transpetrosal approaches are divided into 3 variations: retrolabyrinthine, translabyrinthine, and transcochlear. Each differs in the extent of petrous bone resection and have been well described. Progressively aggressive bone removal provides better anatomical exposure and increases the degree of surgical freedom. Nevertheless, the increase in exposure and surgical freedom comes at the expense of potentially increasing complication rates. The retrolabyrinthine approach involves the smallest amount of petrous bone resection. It avoids entering the labyrinth and skeletonizing the facial nerve, thus preserving hearing and facial nerve function. Although, the retrolabyrinthine approach is the safest method in terms of cranial nerve morbidity, it does not provide an easy access to the central clival depression. If seventh and eighth cranial nerve functions are to be preserved, and if an anterior exposure to the brainstem is not needed, the retrolabyrinthine approach can be selected. In the translabyrinthine approach, all 3 semicircular canals are completely removed and the lateral segment of the IAC is completely skeletonized. Additional bone resection in the translabyrinthine approach increases access to the anterolateral brainstem and inferior clivus. However, the translabyrinthine approach sacrifices the labyrinth at the expense of hearing function. The transcochlear approach requires resection of the cochlea and division of the greater superficial petrosal nerve. It, therefore, provides the maximal exposure to both the petroclival surface and the anterior surface of the brainstem. Thus, this approach should be considered for patients who are already without functional hearing and who have severe preoperative facial weakness. Its current application is limited and it is rarely used.
Combined transpetrosal approach
The combination of anterior and posterior transpetrosal approaches further increases the exposure, and in some cases, it may be associated with hearing preservation. To preserve hearing, one can use a partial labyrinthectomy and partial apicetomy, the transcrural variant, which is thought to provide an increased exposure and operative freedom to the the central clival depression while minimizing cranial nerve morbidity. The combined petrosal approach, incorporating the retrolabyrinthine approach and the anterior transpetrosal approach, that facilitates hearing and maximizes the petroclival exposure by removing the petrous apex, is recommended for patients with a large PCM who have serviceable hearing and an intact facial nerve function.
Although the combined transpetrosal approach provides a wider surgical field, it also has several disadvantages, including an increased risk of postoperative cerebrospinal fluid leakage, damage to the facial nerve and functional hearing, temporal lobe retraction, an increased risk of injury to the vein of Labbé, and an increased operative time. The initial enthusiasm for performing extended skull base approaches with wide bone resection was thus countered by an unacceptably high mortality and morbidity. Growing experience especially with vestibular schwannoma surgery directed many skull base surgeons to the simpler and safer subocciptal retrosigmoid approach in treating most of the PCM.,, It has a very low procedure related morbidity and is one of the most frequently used operative routes in neurosurgery, which offers an excellent panoramic view of the whole cerebellopontine angle. It is a hearing preserving technique that permits a wide exposure of the tumor and offers more safety than the middle fossa approaches while working in the vicinity of the brainstem. The dissection of the meningioma at all stages is done under direct visual control utilizing various corridors between the cranial nerves, which are identified quite early during the surgery. A suprameatal extension of the approach by intradural drilling of the bone located above and anterior to the IAC increases the degree of surgical freedom at the trigeminal porus acusticus and the Meckel's cave. However, it is difficult to remove lesions with a large extension in the middle fossa. It is the most ideal approach for PCMs which extend below the level of the IAC.
Anterior endoscopic approaches
As indications for the endoscopic endonasal approaches continue to expand, ever since the technique was first used for pituitary surgery, the transclival application of the technique to remove midline tumors of the region became a logical extension of the approach. There are only a few publications related to the resection of petroclival meningiomas, which have compared the lateral approach with the endoscopic transclival resection. The reason behind this is probably because, compared to chordomas, meningiomas are more vascular, and are frequently situated more laterally in the petroclival region. These issues in dealing with meningiomas increase the degree of difficulty during the performance of the midline endonasal endoscopic approach. Although endoscopes do not allow a three-dimensional perspective, they do provide a close and wide view of the operative field from different angles The main disadvantage of the lateral approaches is that the basilar artery, frequently encompassed by the tumor, is seen late during the course of the operation at the distal extreme of the operative field. In the endoscopic endonasal approach (EEA), the basilar artery is found early in the resection process and it also serves as an anatomical guidepost to direct the resection from an inferior-to-superior direction. Theoretically, it provides the most direct trajectory to these medially-situated tumors, eliminating brain retraction and minimizing the risk of inadvertent injury to the brainstem. Compared to the open techniques, it is more difficult to control the bleeding from the meningioma during the utilization of the skull base endoscopic approaches since there is less working space to cauterize the tumor capsule. For petroclival meningiomas that extend superiorly up to the dorsum sellae, visualization of the superior pole of the tumor might require the upward mobilization of the pituitary gland and a posterior clinoidectomy. Pure clival meningiomas tend to displace the cranial nerve (CN) V laterally and superiorly, CN VI laterally and posteriorly, CNs VII, VIII, IX, X, XI posteriorly and the brainstem posteriorly. This type of pathological anatomy is favorable for approaching the tumor ventrally through the EEAs. The main advantages of approaching the ventral posterior fossa through an endoscopic transclival approach include the ability to avoid any cerebral retraction and the tumor resection can be carried out without crossing the cranial nerve. The early access to the meningioma's vascular supply (cranial base dura) can greatly reduce intraoperative blood loss and facilitate tumor removal. Further advantages of this approach include removal of the involved bone and dura as a part of the approach, which allows a Simpson Grade I resection. The surgeons must be very careful when indicating an EEA for a petroclival meningioma as the cranial nerve VI may be pushed medially blocking the safe access. The cases where CN VI is pushed inferiorly (dorsum sellae meningiomas) or superiorly (tuberculum jugulare meningioma) are potentially good candidates for EEA. Cerebrospinal fluid leak is always a concern following any endoscopic endonasal approach. If the majority of the meningioma is situated on the petrous bone rather than the clivus, lateral approaches are still preferable.
Surgical techniques: The dissection techniques are the same for all the approaches. After coagulation of the dural attachments, intracapsular piece-meal resection between the cranial nerve windows is carried out. Preservation of the cranial nerves requires patience and persistence. Most surgeons agree that the first step in performing an epi-arachnoid dissection is by converting a large tumor to a smaller one by an ultrasonic aspirator. The tumor capsule can then be folded on itself and used to free the tumor from the brainstem, cranial nerves and blood vessels. The encirclement of the basilar artery and its branches as well as the vertebral artery by the tumor is common. The non-involved segment of these vessels is first identified and then followed up to the involved portions. Dissection along the epi- arachnoid plane around the vessels and cranial nerves is necessary to preserve these structures and the integrity of brainstem. The surgeon should realize that ‘discretion is the better part of valor' in those cases where tumor resection may not be possible because of its dense attachment to the brainstem and other vital structures.
Familiarity with all the approaches permits the surgeon to have the greatest range of options. Creatures of habit that they are, many surgeons remain married to one technique and become master of that approach. If one is not familiar with all the available approaches, one should use the approach with which one can personally achieve the best results. The extent of tumor removal is the most important predictor of outcome, and every effort should be made to remove the tumor completely at the initial surgery itself, knowing very well that even total tumor removal does not preclude late recurrences. Leaving a small part of the tumor or the tumor capsule is consequently reasonable, if the attempt to remove it completely would cause injury to essential neural and vascular structures. Trying to obtain total removal, but at a high risk of adding further neurological dysfunction to the preexisting ones is not acceptable in the present era. Less aggressive cranial base approaches, including the retrosigmoid exposures combined with an OZ craniotomy, are effective alternatives to the transpetrosal approaches in the treatment of these challenging lesions. The recently introduced EEA, is yet another alternative for debulking these tumors in selected patients. Approaches should be tailored depending on the expertise and proficiency of the surgical team and preservation of neurologic functions and quality of life after surgery should have the highest priority.