Keywords: meningioma, clivus, petrosal, petrosectomy, tumor
True “clival meningiomas” arise from a broad attachment to the central upper two-thirds of the clivus, distinct from the more lateral petroclival or more inferior foramen magnum meningiomas. Because of their midline origin, clival meningiomas grow to compress the brainstem and encase the basilar artery and multiple cranial nerves bilaterally. They are often large in size at the time of diagnosis and represent a considerable therapeutic challenge. Because of their proximity to the brainstem and often large size, surgical resection is the optimal treatment. Complete resection should be pursued to reduce the risk of recurrence. This chapter describes the preoperative assessment, techniques, potential complications and advise for their avoidance, and management in pursuing resection of clival meningiomas.
9.1 Preoperative Definition of Lesion Features
The classification of meningiomas involving the posterior fossa has evolved with time and is a nontrivial task, as proper classification of these tumors allows for a useful comparison of treatment outcomes and allows the surgeon to better predict the pathologic anatomy in planning surgery. 1 Cushing and Eisenhardt classified posterior fossa meningiomas into four groups based on the location of the tumor mass. 2 Castellano and Ruggiero revised the classification based on the site of tumor origin using postmortem analysis 3 and Yasargil et al refined the classification based on intraoperative microsurgical findings. They differentiated basal posterior fossa meningiomas into clival, petroclival, sphenopetroclival, lower clival, and cerebellopontine angle tumors. 4 Clival meningiomas are defined as those arising from a broad attachment to the central upper two-thirds of the clivus. These are distinct from the more lateral petroclival subtype and more inferior lower clival (or foramen magnum) meningiomas.
9.1.2 Anatomical Considerations
Because of their origin in the midline on the clivus, these tumors grow to involve and typically encase the basilar artery, its perforators, and a multitude of cranial nerves bilaterally. The tumor is obscured by the petrous temporal bone, and the brainstem is typically compressed and deflected in a posterior direction (▶ Fig. 9.1). This is in contrast to petroclival meningiomas which originate slightly more laterally and deflect the basilar artery rather than encasing it and compress the brainstem laterally. Clival meningiomas are typically large in size at the time of diagnosis and can extend to involve Meckel’s cave, the cavernous sinus, the internal auditory canal, and jugular foramen to either side.
Fig. 9.1 Contrast-enhanced (a) axial and (b) sagittal MRI showing the potentially extensive nature of clival meningiomas with spread throughout the skull base, encasement of the basilar artery, and compression and posterior displacement of the brainstem.
9.2 Surgical Indications
With rare exception, clival meningiomas’ typical large size, brainstem compression, and relentless natural progression necessitate surgical resection. 4, 5, 6 Small, asymptomatic tumors in elderly or medically ill patients may best be observed. Rarely radiation is the primary treatment and only in those patients who are poor surgical candidates due to advanced age or medical comorbidities with proven growing tumors.
9.3 Surgical Techniques
9.3.1 Preoperative Assessment
A critical component of the surgical management of clival meningiomas is the preoperative work-up and planning. All patients should be studied with contrast-enhanced magnetic resonance imaging (MRI), computed tomography (CT), angiography, venography, and audiogram. These studies play a critical role in planning the approach and anticipating the pathologic anatomy. MRI provides unparalleled detail of the tumor’s soft tissue extension and relationship to the brainstem as well as vascular encasement. CT scan delineates the patient’s skull base anatomy and the tumor’s involved bone. Typically, noninvasive angiography and venography is sufficient. We have found dynamic CT angiography (CTA) to be particularly useful. 7 Angiography details the relationship of the major arteries to the tumor as well as the tumor’s vascularity. Study of the venous anatomy is critical in approach selection and should include bilateral demonstration of the transverse and sigmoid sinuses, their connection at the torcula, the venous drainage of the temporal lobe and its relationship to the superior petrosal sinus, tentorium, and sigmoid sinus, and the location of the jugular bulb. 8 The audiogram aids in determining whether a hearing preserving approach is worthwhile.
9.3.2 Approach Selection
The ideal approach offers direct visualization and illumination of the tumor, brainstem, and critical neurovascular structures with a short operating distance, avoidance of brain retraction, multiple trajectories, freedom of movement, and early devascularization of the tumor. In approaching clival meningiomas, the temporal bone is obstructive and lateral skull base approaches are needed to optimize the approach. Since these tumors lie anterior to the brainstem and typically extend below the level of the internal auditory canal, they require either a combined petrosal approach 9 or a total petrosectomy with the patient’s hearing status being the determinant. However, midclival tumors represent a formidable challenge and even patients with intact hearing may warrant total petrosectomy. The venous anatomy, although important to understand prior to surgery and may necessitate adaption of the approach, rarely prohibits an approach. 1, 8, 10
Anesthesia and Intraoperative Neuromonitoring
The anesthetic care of the patient is a critical component in a successful operation and the surgeon must work closely with the anesthesiologist with an open line of communication. Total intravenous anesthesia is used to optimize brain relaxation and neuromonitoring. Mild hypothermia is induced, normotension is maintained throughout induction, and the duration of the case and hypervolemia is avoided. We avoid the use of mannitol to preserve the arachnoid dissection plane and avoid its anticoagulant effect. We also avoid the use of lumbar drains because of their potential catastrophic complications. 11 Skull base approaches allowing access to cerebrospinal fluid (CSF) egress without brain retraction obviate their need. In the event of a tumor that extends down to crowd the foramen magnum, awake fiberoptic intubation may be necessary.
Intraoperative neuromonitoring is a critical adjunct and is used in all cases. Somatosensory-evoked potentials, brainstem auditory-evoked responses, and cranial nerve monitoring, which may include any cranial nerve III to XII, are performed in all cases.
Combined Petrosal Approach
The patient is placed supine with an ipsilateral shoulder roll. The trunk is elevated to place the head above the heart and facilitate venous drainage. The head is turned slightly to the opposite side and the vertex dropped to allow the temporal lobe to fall away. Excessive turning or flexion of the head will compromise venous drainage and must be avoided. The abdomen is prepped for harvesting fat.
Soft Tissue Dissection
The scalp incision begins 1 cm in front of the tragus, curves anteriorly along the hairline, behind the frontalis branch of the facial nerve, gently curves posteriorly two fingerbreadths above the pinna of the ear, and finally turns down, two fingerbreadths behind the pinna to below the level of the mastoid tip. During the scalp incision, the superficial temporal artery is preserved and the scalp incision is reflected inferiorly separately from the underlying muscle and fascia. The temporalis muscle is incised along its inferior, anterior, and superior aspect and elevated off of the temporalis muscle in continuity with the sternocleidomastoid muscle which is incised posteriorly and reflected inferiorly. This creates a long, wide, thick vascularized combined myofascial flap for reconstruction. The zygoma is then sectioned allowing the temporalis muscle to reflect well inferiorly. The temporalis muscle can also be used as a vascularized flap.
Four burr holes are placed bridging the sinus anteriorly and posteriorly and additional burr holes are placed as needed to safely dissect the dura. A combined posterior fossa, occipital, temporal craniotomy is then performed. The burr holes bridging the sinus are connected with the drill rather than crossing the sinus with the footplate. Following the craniotomy, a piece of the mastoid cortex is harvested for later reconstruction. A mastoidectomy is then completed, skeletonizing the sigmoid sinus from its junction with the transverse sinus down to the jugular bulb and exposing the presigmoid dura. A middle fossa dissection is then performed to expose the petrous apex. The middle meningeal artery is identified in the foramen spinosum, coagulated, cut, and waxed. Posterior and medial to it, the greater superficial petrosal nerve is identified and preserved. The dissection is carried medial up to the posterior fossa dura and posteriorly to the internal auditory canal. Care must be taken during the dissection as there may be a dehiscence of the bone overlying the petrous carotid or the genu of the facial nerve. The area of bone to be drilled is bounded by the trigeminal ganglion anteriorly, the petrous carotid laterally, the internal auditory canal posteriorly, and the posterior fossa dura medially. The drilling is carried inferiorly to the level of the inferior petrosal sinus and can be extended medially into the clivus.
Dural Opening and Sectioning of the Superior Petrosal Sinus
The dura is incised both along the inferior temporal lobe and in the presigmoid dura. Separating the two incisions is the superior petrosal sinus and the tentorium. Prior to connecting the cuts by sectioning the superior petrosal sinus, it is critical that the venous drainage of the temporal lobe is identified. The sinus must be sectioned and the tentorium cut anterior to where the temporal lobe venous drainage enters. Once the sinus is ligated and cut, the incision is extended on the tentorium in an anterior and medial direction to the incisura. Prior to completing the cut, the fourth nerve must be identified and the final cut made posterior to where the fourth nerve enters the tentorium. Completing this cut releases the sigmoid sinus allowing it to mobilize posteriorly, opening up the presigmoid exposure, and connecting the posterior and middle fossa.
Intradural Dissection and Tumor Removal
Once the dura is opened, the first objective is to open the cisternal arachnoid to allow for CSF egress. Once CSF has been released and the brain relaxed, a wide and shallow exposure to the tumor without the need for retraction is obtained. The tumor is then devascularized from its attachment along the clivus. Once the tumor has been devascularized, the arachnoid membrane is dissected off of a safe portion of the tumor, allowing it to be entered and intracapsular debulking to be performed. Debulking must be done with caution as the basilar artery, its perforators, and branches can be encased by the tumor. Once the tumor is debulked, using high magnification and meticulous technique the critical neurovascular structures are dissected free utilizing the arachnoidal plan. Maintaining the dissection within the proper arachnoid plane is the key to the safe and total removal. In cases where an arachnoidal plane is not present, total removal is not possible and it is better to leave a small residual than risk injury to the critical neurovascular structures.
Primary closure of the dura is not typically possible and it must be grafted. Following a good dural grafting, the cavities created in the petrous apex and mastoid are obliterated with fat. The vascularized myofascial flap is reflected to cover the defect. The bone is replaced along with the mastoid cortex using standard cranial fixation techniques and the wound is closed in layers. 1
The soft tissue dissection for the total petrosectomy is similar to that for the combined petrosal approach; however, it includes the sectioning and blind sac closure of the external canal. The bone work also begins similarly but following the mastoidectomy, the drilling is continued to include translabyrinthine and transcochlear exposures. Although the facial nerve can be completely liberalized from its bony canal and reflected posteriorly, we prefer to leave it protected in a thin bony canal which has allowed for good facial nerve outcomes when using this approach. 8 The middle ear is emptied and the eustachian tube obliterated prior to completion of the drilling including the cochlea and internal auditory canal 1, 12(▶ Fig. 9.2).
Fig. 9.2 (a) Axial contrast-enhanced MRI showing a midclival meningioma extending bilaterally, encasing the basilar artery, and compressing the brainstem posteriorly. (b) Postoperative CT scan demonstrating the bony removal and fat packing following a total petrosectomy and the complete resection of the tumor.