Keywords: petroclival meningioma, retrosigmoid approach, stereotaxic radiosurgery, transpetrous approach
Petroclival meningiomas are one of the most challenging pathology because of their deep-seated locations and intimate relationship with important neurovascular structures. Patients could be asymptomatic or affected by symptoms due to raised intracranial pressure or due to cranial nerves, cerebellum, or brainstem compression. The choice of the approach should be tailored on the patient and tumor characteristics, while the goal of the surgical treatment should be the radical resection, without neurological damage.
Stereotaxic radiosurgery is an effective and safe tool that gives good control of small residual tumor. We considered the retrosigmoid approach for tumors localized in posterior fossa, and the pterional, subtemporal, and orbitozygomatic for tumors with middle cranial fossa extension. Follow-up should be carried out according to neuro-oncological principles.
Petroclival meningiomas are defined as those originating at the upper two-thirds of the clivus, lateral to the midline, at the level of the petroclival junction and medial to the trigeminal nerve. 1, 2, 3, 4 These lesions represent a permanent challenge for neurosurgeons because of their deep-seated locations and intimate relationship with important neurovascular structures. Petroclival meningiomas are a rare entity accounting for 0 to 15% of all intracranial tumors and comprising only 3 to 10% of the posterior fossa meningiomas. Nevertheless, these lesions are still one of the “hottest” topics for debate in neurosurgical community, mostly with regards to the choice of the surgical approach as well as to the growing application of stereotactic radiosurgery.
Because of slow growth rate, and relatively scarce symptoms, sometimes they can reach extensive dimensions at the time of diagnosis, extending into posterior portion of the cavernous sinus and middle cranial fossa, with involvement of the petrous apex and cave of Meckel. Spheno-petroclival meningiomas involve the entire cavernous sinus (including its anterior portion), sella turcica, with wide middle cranial fossa extension. Sometimes cavernous sinus is infiltrated bilaterally, with clivus and sphenoid sinus involvement. 2, 3 Treatment of these tumors will also be considered in this chapter. Other meningiomas of posterior fossa, including clival meningiomas (midline clivus), foramen magnum meningiomas (lower third of clivus) and anterior and posterior petrosal meningiomas (lateral to trigeminal nerve) do not share anatomic relations with true petroclival meningiomas and will not be addressed further. 1
10.2 Anatomical Considerations
Petroclival meningiomas grow at the posterior cranial fossa, where they occupy the space between brainstem and cerebellar hemispheres posteriorly, pyramid laterally, and clivus anteriorly. Usually, neurovascular structures are displaced in the typical pattern, III and IV nerves above the tumor, V and VII nerves laterally and posteriorly, VI nerve medially, 5 and lower group (IX–XI) on the caudal pole of the tumor. These structures might also be encased by the tumor as it gradually enlarges, although seldom there is any apparent cranial nerve palsy. As this is the most frequent cause of morbidity related to petroclival meningioma surgery, it is of great importance to accurately predict their localization preoperatively and high definition magnetic resonance tractography seems a promising tool to identify displaced and/or encased cranial nerves. 6, 7
One of the most important features is the relationship with the brainstem; usually it is displaced by the tumor posteriorly and contralaterally, along with vertebral and basilar arteries and their branches. 1 MRI T2 sequences are useful to identify the arachnoidal layer of the brainstem, which appears as a space between the tumor and the brainstem, predicting a more favorable tumor resection; absence of this radiological sign indicates that this border is disturbed, thus probability of brainstem and perforating blood vessels damage during the resection is higher. 8, 9
Spheno-petroclival lesions are the most extensive of these lesions involving one or both cavernous sinuses or establishing a close relationship with optical nerves, chiasm, intradural portion of carotid artery, and their branches.
10.3 Clinical Presentation
Patients with petroclival meningiomas can develop nonspecific symptoms due to raised intracranial pressure (ICP), either from the tumor mass or obstructive hydrocephalus, or specific symptoms due to cranial nerves compression, mass effect on the cerebellum and brainstem. 10, 11 Headache is the most frequent complaint, and cerebellar signs are the most common clinical signs. 12, 13, 14, 15 Cranial nerves V (approximately 65%) 3, 16 and VIII (51.5%) 3 are most frequently involved, presenting with face numbness, trigeminal neuralgia, or hearing loss. 13, 17 Facial nerve palsy occurs in 24.4 to 50% of patients. 2, 11, 14, 16 The lower cranial nerves are involved less frequently (28.6%), as well as III, IV, and VI (18.3%), in less than half of the cases. 2, 11, 18 Spastic weakness can appear due to brainstem motor pathway compression in 15 to 57% of patients.
10.4 Preoperative Evaluation
CT is usually the first examination in diagnosing intracranial lesions. It shows hyperdense lesions, usually with strong contrast enhancement. The CT sequence known as a “bone window” is useful in evaluating the anatomy of the skull base, identifying hyperostosis, bone erosion as well as tumor calcifications (▶ Fig. 10.1). Contrast-enhanced brain-MRI is standard for assessing tumor characteristics and evaluating relationship with neurovascular structures and brain tissue. Petroclival meningiomas usually appear T1-isointense, T2 and fluid attenuated inversion recovery (FLAIR) hyperintense, showing strong, homogenous gadolinium uptake. T2-weighted MRI is important to determine whether there is a good arachnoid plane between the tumor and brainstem, also brainstem edema, suggesting vascular engulfment that might be considered as an adverse prognostic sign. 3 Venography is useful to evaluate the size of the transverse and sigmoid sinuses as well as their collateral flow, anatomy of major venous blood vessels, mostly vein of Labbe. MR angiography, CT angiography, and digital angiography can be used to assess displacement, stenosis, or occlusion of the basilar or internal carotid arteries and their branches as well as blood supply of the tumor. 14 It can also give excellent assessment of venous structures (▶ Fig. 10.2).
Fig. 10.1 (a, b) The CT scan shows a right spheno-petroclival hyperdense lesion with strong post-contrastographic enhancement, in absence of evident bone erosion or calcifications. It occupies the space between brainstem and right cerebellar hemisphere posteriorly, the pyramid of the temporal bone laterally and clivus anteriorly in the posterior cranial fossa. The tumor extends to the middle cranial fossa involving both cavernous sinus.
Fig. 10.2 Post-contrastographic brain-MRI study in a 62-year-old man: (a) axial spin-echo T1 weighted and (b) coronal spin-echo T1 weighted sequences showing a left petroclival lesion, with strong contrast enhancement. This lesion extends from the left lateral part of the clivus to the petroclival junction involving the posterior portion of the left cavernous sinus.
Meningiomas are known for their extensive vascularization that sometimes can lead to massive blood loss during surgery; though when dealing with skull base tumors such as petroclival meningiomas, devascularization cannot be achieved before tumor debulking. In these lesions, blood supply is usually provided by clival branches of the meningohypophyseal trunk and by external carotid branches. In case of high-vascularized tumors, preoperative embolization should be considered preferably 7 to 10 days before the operation. Feeders from meningohypophyseal trunk are difficult to access, and care should be taken in cases of partial embolization of only external carotid branches, because it might cause increased blood flow in residual blood vessels and extensive bleeding during the surgery. 19, 20 Embolization can cause sudden intratumoral necrosis and hemorrhage that can lead to tumor volume enlargement and worsening of mass effect, so that whether brainstem edema is already present on preoperative MRI, embolization should be avoided 21 Cranial nerve palsy can also occur as complication of accidental embolization of their feeders.
The authors attempted to reduce tumor vascularization of skull base meningiomas by mean of stereotaxic radiosurgery with targeted on tumor dural attachment. Operation was performed 2 to 3 months after irradiation, with significant reduction of blood supply, but with increased firmness of tumor.
Finally, it is mandatory for patients to undergo a baseline audiogram to evaluate preoperative hearing function as well as careful evaluation of other cranial nerve function since their assessment is crucial for the choice of operative approach.
10.5 Surgical Indications
Until 1970s petroclival meningiomas have been considered as unresectable lesions 1, 22, 23 and universally described as progressive, eventually lethal diseases. Nowadays, more data are available about natural history of these tumors. Volumetric analysis of untreated petroclival meningiomas showed annual growth rate from 0.81 to 2.38 cm/year, with 76% of patients diagnosed with progression, among which 63% presented with neurological worsening. 9, 12
This suggests that carefully designed treatment is necessary for this group of patients. Policy of “wait and see” might be justified only in elderly patients with very small tumors or with serious comorbidities that are suitable neither for operative treatment nor stereotaxic or conformal radiotherapy. In this group of patients, ventriculoperitoneal (VP) shunt might be considered if hydrocephalus is present.
A second important issue is the choice of therapeutic approach. In the era of expansion of stereotaxic radiosurgery, gamma knife treatment of small, deep-seated meningiomas might be reasonable. In several studies, good local control was achieved after stereotaxic radiosurgery, with tumor control rate of 80 to 91.2% after 5 years and 77.2 to 81% after 10 years. 24, 25
There is some concern about eventual anaplastic transformation but series with follow-up period of 10 to 15 years show a 2.2% rate of malignant transformation. As low rate of malignant transformation is also noticed in nonirradiated meningiomas only further investigation, that should consider detailed pathohistological analysis, can give accurate evaluation of late adverse effects of stereotaxic radiotherapy.
It can be concluded that stereotaxic radiosurgery, as sole treatment, can be ideal solution for small tumors in elderly, due to the good tumor control and low probability of malignant transformation in period of their life expectancy.
For young patients, radical surgery should be considered as the treatment of choice, because it gives a chance for cure without any further treatment. On the other hand, sacrifice of neurological function in order to maximize extent of resection (EOR) is not acceptable, and as mentioned before, stereotaxic radiosurgery is a comfortable single session treatment that provides good tumor control, especially in patients with very small tumor remnants. There have been a lot of suggestions for classification and therapeutic protocols for petroclival meningiomas, but none of them is univocally accepted. 15
Presence of hydrocephalus at the time of diagnosis with signs of elevated intracranial pressure, in authors’ experience, should be resolved with VP shunt placement. A period of 7 to 14 days is recommended between shunt procedure and tumor removal, which enables good brain relaxation and easier approach to these deep-seated tumors.
The choice of approach should be left to experienced surgical team. Optimal surgical approach for petroclival meningiomas is still controversial and depends on the location, size, and extension of the tumor as well as patient age, neurological status, particularly hearing and facial nerve function, and the neurosurgeon’s preferences. 26, 27, 28, 29
Two main groups of approaches can be considered, the first including “standard” transcranial approaches as the retrosigmoid, for tumors localized in posterior fossa, and the pterional, subtemporal, and orbitozygomatic for tumors with middle cranial fossa extension. The second large group stands for skull base approaches that demand more extensive bone removal as the transpetrosal approach (anterior, posterior, and combined).
The best surgical route should give the widest exposure enabling maximal EOR with minimal approach-associated morbidity.
10.6 Surgical Techniques
10.6.1 Retrosigmoid Approach
In University of Naples “Federico II” and Clinic of Neurosurgery, Clinical Center of Serbia at Belgrade (authors’ institutions), tumors without significant extension into middle cranial fossa are preferentially treated via a retrosigmoid approach, (▶ Fig. 10.3) while when a large middle fossa extension is noticed, a two-step approach with combination of the retrosigmoid and the frontotemporal is performed. 29, 30 As brainstem decompression is a critical step in petroclival meningioma treatment, infratentorial part of tumor should be resected first (▶ Fig. 10.4). The major advantage of the retrosigmoid approach resides in its simplicity with avoidance of extensive petrous bone resection, reducing the risk of VII and VIII cranial nerves injury and eventually a postoperative cerebrospinal fluid (CSF) leak. Samii advocates the so called “retrosigmoid intradural suprameatal approach” that consists of intradural drilling of petrous bone above and anterior to internal acoustic meatus to extend the view of Meckel’s cave and trigeminal nerve, with tentorial incision for tumors with supratentorial extension. 30
Fig. 10.3 Post-contrastographic brain-MRI in a 62-year-old woman: (a) axial spin-echo T1 weighted and (b) coronal spin-echo T1 weighted. The study shows a large left spheno-petroclival lesion with strong contrast enhancement. It originates from the left lateral part of the clivus and the petroclival junction extending to the middle cranial fossa. The tumor displaces posteriorly and contralaterally the brainstem and left cerebellar hemisphere encasing the origin of the left V cranial nerve as well as involving both cavernous sinus. (c) Postoperative axial spin-echo T1 weighted and (d) coronal spin-echo T1 weighted. These sequences show the subtotal removal of the lesion by retrosigmoid approach. The residual tumor remains in both cavernous sinus.