Cerebellopontine Angle Meningiomas

The cerebellopontine angle (CPA) is the most common location of infratentorial meningiomas. Their origin in relation to the inner auditory canal (IAC) is an important landmark in their classification into five groups: (1) ventral to the IAC; (2) inside the IAC; (3) superior to the IAC; (4) inferior to the IAC; (5) posterior to the IAC. Different symptoms occur according to the location. However, frequently these meningiomas show a slow but progressive growing pattern and reach large volumes before causing signs of brainstem compression. On magnetic resonance imaging (MRI), CPA meningiomas are hypo-to isointense on T1-weighted images and show strong enhancement after contrast medium. T2-weighted images, and particularly high-resolution Constructive Interference in Steady State (CISS) sequences, depict arachnoid cleavage planes between the tumor and brainstem and detect possible areas of pial invasion.

Careful evaluation of several aspects is recommended in patient counseling. Options include follow-up with MRI, microsurgery, radiotherapy, or multimodality treatment.

Close follow-up in asymptomatic patients can be the initial treatment of choice. However, it seems that small or medium sized tumors grow more rapidly and treatment should be offered as soon as growth is documented or upon symptoms progression. The process of surgical decision making, selection of different skull base approaches as well as complication avoidance are described in detail in this chapter. The microsurgical goal is maximum safe resection with preservation of the patient’s quality of life.

13.1 Introduction

Cerebellopontine angle (CPA) meningiomas comprise specific subset of tumors, which depending of their size, location, and growth pattern, can have various clinical presentations and prognosis, requiring individual approach to each patient in terms of treatment. Although infratentorial meningiomas are rare, comprising only about 10% of all intracranial meningiomas, ¹ the CPA is the most common location of origin in the posterior cranial fossa, followed by the petroclival region. ² Most of these lesions are benign, characterized by very slow growth, and only occasionally malignant histological types are found. ³^, ⁴ Meningiomas originate from arachnoid cap cells in this region and by growing, can compress and stretch cranial nerves, displace the brainstem, sometimes violating its pial plane, distort and encase vital brainstem vessels, even cause hydrocephalus. Their origin in relation to the internal acoustic canal is an important landmark in their classification in terms of surgical accessibility, choice of surgical approach, and possible risks for morbidity and mortality. According to the classification introduced by Nakamura et al., these lesions can be classified into five groups in relation to their location to the internal auditory canal ⁵—anterior to the inner auditory canal (IAC) (group 1), involvement of the IAC (group 2), superior to the IAC (group 3), inferior to the IAC (group 4), posterior to the IAC, originating between the IAC and sigmoid sinus (group 5). This classification proved to be very accurate in predicting the outcome of facial and cochlear nerve function in CPA meningiomas, depending on the topographic classification of these tumors. There are other classifications of CPA meningiomas, ⁵^, ⁶^, ⁷^, ⁸ but we find the above mentioned the most convenient and useful in terms of surgical planning.

Historically, anterior CPA meningiomas and especially, the petroclival type had been related to higher morbidity and mortality when total removal was attempted. ⁷^, ⁹^, ¹⁰^, ¹¹ In the recent years, with the advanced understanding of skull base anatomy, routine introduction of Intraoperative Neurophysiological Monitoring techniques (IOM) and advances of neuroimaging, a significant drop of mortality below 1% with notable reduced morbidity is reported. ¹²^, ¹³^, ¹⁴^, ¹⁵^, ¹⁶^, ¹⁷^, ¹⁸^, ¹⁹^, ²⁰ On the other hand with the progress in radiosurgery, ²¹^, ²²^, ²³^, ²⁴ and improvement in the understanding of the natural history of the tumors, ²⁵^, ²⁶^, ²⁷^, ²⁸^, ²⁹ a multimodality treatment option with less aggressive surgery and a more conservative approach towards these lesions is aimed. ¹⁴^, ¹⁵^, ¹⁷

Therefore, the decision for the treatment strategy for each particular case is very complex and numerous factors (from patient side, tumor characteristics, various treatment options including wait and see strategy, surgery, radiosurgery) have to be taken into consideration before a decision is made.

13.2 Clinical Presentation and Preoperative Evaluation

CPA meningiomas can have various presentations depending on their location, size, and aggressiveness (violation of brainstem arachnoid planes). Generally, the symptoms can be classified according to compression of cranial nerves, cerebellar signs (gait disturbance dyscoordination symptoms), brainstem symptoms (oculomotor symptoms and long tract signs), symptoms of increased intracranial pressure (ICP), and hydrocephalus. Headache is a common complain but is a nonspecific sign. Due to the indolent course of the disease symptoms and signs secondary to brain stem and cerebellar compression, are a late occurrence and often are not apparent even with very large tumors.

There is difference in the symptoms according to the location of the tumor in the CPA. If the meningioma originates posteriorly to the IAC, it manly presents with cerebellar compression, signs of cranial nerves VII and VIII deficit, and sometimes, symptoms of raised ICP. Tumors growing superior to the IAC can give symptoms of trigeminal neuralgia or numbness, due to compression of the trigeminal nerve. If the tumor is located in its main bulk inferior to the meatus, it may present with swallowing difficulties due to compression of the caudal group of cranial nerves. ¹² However, in the case of petroclival meningiomas, located anteriorly to the IAC, it may have different clinical presentations. The symptoms often arise from brainstem compression, cranial nerve deficits—more often trigeminal nerve symptoms (45%) oculomotor disturbances (more often abducens nerve palsy—64%) and ataxia (37%). ³⁰ According to Cho et al, hearing loss, facial weakness and trigeminal symptoms, gait disturbance, dysarthria, spasticity, and headache are among the predominant clinical findings in patients harboring petroclival meningiomas. ³¹

More often due to very slow growth of tumor, it takes a lot of time before the diagnosis is established—between 2.5 and 4.5 years according to the literature. ⁷ Therefore, it is very important that proper radiological evaluation is done when a CPA tumor is suspected.

13.3 Neuroradiological Evaluation

A complete and thorough neuroradiological evaluation is essential for the correct decision-making when consulting a patient with CPA meningioma. This evaluation includes thin slice computed tomography (CT), gadolinium enhanced magnetic resonance imaging (MRI) including MRI angiography (▶ Fig. 13.1). Additional information can be obtained when using three-dimensional (3D) computed tomography angiography (CTA) and in selected cases, digital subtraction angiography (DSA).

Computed tomography is very useful for the initial diagnosis as it presents the meningioma as isodense before, and hyperdense after intravenous contrast administration. Calcification can be present, but are rare, and according to some authors, can be associated with slower growth pattern. ²⁸^, ³²^, ³³ A bone erosion or hyperostosis may be present. It is generally accepted that bone hyperostosis represent tumor invasion and not reaction of the bone to the tumor. A bone window thin slice CT is useful for evaluation of the mastoid bone pneumatization and its extension over the sigmoid sinus. This is important for the planning of the extent of mastoid drilling in retrosigmoid craniotomy. The CT gives also information for the location and size of the mastoid emissary veins, size and anatomy of the sigmoid sinus, location and size of the jugular bulb. If transpetrous approaches are planned, the bone CT provide information for the pneumatization of the mastoid and petrous apex, proximity of the labyrinthine block to the IAC, location of the cochlea, course of the fallopian canal, size of the presigmoid space (▶ Fig. 13.1a).

In some cases, 3D CTA can give additional information for the displacement or encasement (such as anterior inferior cerebellar artery or superior cerebellar artery) of the major brainstem vessels from the meningioma. ³⁴^, ³⁵ Meningiomas can be very vascular tumors and will be very well-presented on a CTA. Some modern programs with 3D rendering capabilities, like Osirix (Pixmeo, Bern Switzerland) or Horos, could be used for preoperative planning in order to volumetrically present the tumor and bony structures as well the displaced vessels ³⁶^, ³⁷^, ³⁸^, ³⁹ (▶ Fig. 13.1b). Another important feature for the CTA is that it can very well present the associated venous anatomy including the dominance of the transverse and sigmoid sinuses, size of the jugular bulb, superior and inferior petrosal sinuses, and temporal lobe drainage pattern.

The diagnostic method of choice for the evaluation of CPA meningioma is the MRI with and without gadolinium contrast enhancement. The MRI examination most often reveals an isointense or hyperintense lesion on T1-weighted imaging, which is heterogeneous on T2-weighted imaging. The MRI T1-weighted modality (with and without contrast) is useful for evaluation of the size of the tumor, its location, and displacement of the neural structures. The MRI T2-weighted modality is very informative about arachnoid cleavage planes between the tumor/brainstem, possible areas of pial invasion (a high-intensity signal in the brainstem noted on T2-weighted imaging is typical of peritumoral edema due to brainstem pial transgression), relation to the cranial nerves, major vessels, and tentorium (▶ Fig. 13.1c). The flow voids on the MRI-T2 modality can indicate vessel displacement or encasement by the tumor, associated venous anatomy (venous sinuses size and variation). MRI venography can be very informative about the sigmoid sinus dominance and anatomy, vein of Labbé size and location, which is important information for the planning of a transpetrous approach.

Advanced techniques such 3 Tesla MRI Constructive Interference in Steady State (CISS) (high T2 sequences) sequence provide much more details regarding the arachnoid plane between the tumor/brain interface, as well as the cranial nerve displacement and involvement in the tumor. Diffusion tensor imaging (DTI) could be used to present some of the important brain circuitry (pyramidal tracts, pontocerebellar fibers within the brachium pontis), as well as cranial nerves (V, VII), displaced by the tumor.

DSA is less frequently used nowadays given the diagnostic efficacy of noninvasive magnetic resonance angiography and venography, as well as the 3D CTA. DSA is only planed if tumor embolization will be attempted.

The CPA meningiomas are supplied by the tentorial artery of Bernasconi-Cassinari clone of meningohypophyseal trunk), posterior branch of the middle meningeal artery, meningeal branch of the vertebral artery, petrosal branches of the meningeal arteries, and ascending pharyngeal branches of the external carotid artery. ¹²^, ⁴⁰ These arteries can be used in selected patients for preoperative embolization in order to decrease the intraoperative blood loss that often obscures the intraoperative view and makes the surgery even more challenging. Most authors agree that in order to have a clinical benefit complete or near complete embolization of the tumor feeders have to be achieved, ⁴¹^, ⁴²^, ⁴³ which is not always possible. Another important point is the timing of surgery after the embolization. One of the arguments for early surgery after embolization is the tumor swelling secondary to its necrosis, which is not always the case. More recent series advocate for delayed surgery after 24 hours. ⁴²^, ⁴³ Nevertheless, the embolization is an invasive procedure and is not without risks. Major complications of embolization include stroke, blindness, retroperitoneal hemorrhage, and cranial nerve palsies with estimated incidence between 0 and 16% of cases. ⁴³^, ⁴⁴^, ⁴⁵

In our experience, we rarely use preoperative embolization in case CPA of meningiomas. Intraoperative blood loss can usually be compensated, and the problem with blood filling the operative field is avoided with the use of semi-sitting position. ⁴⁶

Fig. 13.1 Preoperative radiological workup. (a) Thin slice computed tomography (CT) is required in order to carefully examine the petrous bone anatomy and pneumatization. This is important in retrosigmoid approaches (location and size of the sigmoid sinus as well as mastoid emissary veins) as well for transpetrosal approaches. Note the hyperostotic bone on the suprameatal tubercle caused by the meningioma. (b) CT angiography can give essential information about the displacement of the vessels by the tumor. Using software such as OsiriX software (Pixmeo, Bernex, Switzerland), a three-dimensional volumetric reconstruction can be created representing a large petroclival meningioma (this is an overlay of two images). Note the displacement of the superior cerebellar artery and basilar artery by the tumor; (c and d) T1 contrast enhanced and T2-weighted magnetic resonance imaging (MRI) are very useful in examination of the tumor/brainstem interface and to estimate the arachnoid plane allowing safe surgical dissection.

13.4 Patient Evaluation and Decision-Making

For selection of the appropriate treatment modality in each case, it is mandatory to understand the natural history, ²⁵^, ²⁶^, ²⁷ take into consideration patient age and comorbidities, preoperative predictive factors indicating the degree of safe resection and radiological finding pointing the risks for postoperative complications, ⁴⁷^, ⁴⁸ as well as the results of radiosurgery. ²¹^, ²²^, ²³^, ²⁴ Historically speaking, the natural history of these tumors was once associated with progressive neurological worsening and was ultimately fatal prognosis. ⁴⁹^, ⁵⁰ At present with the modern diagnostic tools, there is much better possibility for close follow-up of the patients and better estimate the degree of growth rate. A cooperative retrospective study of 21 untreated patients diagnosed with petroclival meningiomas, with clinical and radiological (MRI) follow-up, ranged from 4 to 10 years (mean, 82 months; median, 85 months) has shown radiological tumor growth in 76% of the cases. ²⁶ With 63% of the growing tumors, there was functional deterioration. According to the authors of the study the mean growth rates were 1.16 mm/year in diameter and 1.10 cm³/year in volume. Rapid growth spurts were documented in small and medium-sized tumors. Moreover, a change in growth pattern preceded functional deterioration.

Another study by Tatasaka et al. ²⁷ followed 15 patients (median follow-up period was 40 months) diagnosed with petroclival meningiomas. Sixty percent of the cases showed radiological tumor growth during the follow-up period. There was functional deterioration in 47% of the cases.

A recent study by Hunter et al. ²⁹ volumetrically studied 34 untreated patients with petroclival meningiomas using consecutive MRI. The mean follow-up was 44.5 months. According to the results, 88.2% showed progressive growth, with estimated mean annual volumetric growth rate of 2.38 cm/year (-0.63 to 25.9 cm/year). Tumor volume, T2 hyperintensity within the tumor, peritumoral edema, and ataxia and/or cerebellar symptoms at presentation were all significantly associated with greater rates of tumor growth.

Therefore, these results suggest that untreated tumors show progressive growth over time and close follow-up in asymptomatic patients have to be the initial treatment of choice. However, it seems that small or medium sized tumors grow more rapidly and treatment should be offered when there is documented growth or upon symptoms progression.

In case of symptomatic patient, treatment modality vary from surgery alone, radiosurgery or combination from surgery and radiosurgery for the tumor remnant. Taking in mind the data from natural history of the disease, a possible treatment strategy would be surgical resection for younger and healthy patients and focused beam radiation therapy (stereotactic radiosurgery or radiotherapy) for elderly patients or unhealthy patients who cannot undergo surgery. Radiotherapy could be used as single treatment option for small tumors (<3 cm). However, there are known complications from radiosurgery ⁵¹^, ⁵²^, ⁵³^, ⁵⁴ and with the advances of modern microsurgical techniques and intraoperative monitoring, some authors advocate excision even for small CPA and petroclival meniniomas. ¹⁸^, ¹⁹

If surgical treatment is the choice of approach, there are numerous factors that have to be taken into consideration in order to predict surgical resectability and possible complications including peritumoral edema, presence of nourishing pial vessels, age of the patient, location of the tumor, duration of symptoms, preoperative neurological status, vessel encasement, tumor size, presence and severity of the associated hydrocephalus, and comorbid conditions, such as hypertension and diabetes mellitus. ⁴⁷^, ⁴⁸^, ⁵⁵^, ⁵⁶ There are several proposed grading scales used in the evaluation of for the surgical risks in skull base meningiomas.

Sekhar et al ⁴⁸ studied multiple preoperative variables such as preoperative Karnofsky scale score, previous radiosurgery, preoperative radiological findings, intraoperative findings and correlated them to postoperative outcome at early and late follow-up. Statistical analysis revealed significant correlations between early functional deterioration and preoperative Karnofsky scale scores, absence of an arachnoid cleavage plane, between tumor and brainstem, edema of the brainstem, and direct blood supply from the basilar artery. Permanent functional deterioration was statistically associated with feeding from the basilar artery, tumor size, incomplete tumor resection, and early postoperative dysfunction. The authors proposed three stages of tumor relationship to the brainstem arachnoid and pial membranes: Stage 1 tumors show preservation of the arachnoid plane, presented as a high-intensity band between the meningioma and the brainstem on T2-weighted MRI. In stage 2 tumors, the arachnoid plane is lost, presented by absence of the high-intensity band between tumor and the brainstem on T2-weighted MRI. In stage 3 tumors, the pial membrane is violated and the arachnoid cleavage plane is absent, with presence of brainstem edema depicted on the MRI as hyperintensity signal on the T2-weighted images. In this stage, the dissection of the tumor from the brainstem can cause damage of the latter and injury of the basilar artery perforators which supply the pial brainstem surface. ⁴⁷^, ⁴⁸

Based on the results of their study, the authors recommend that patients with small or medium size tumors should be offered treatment, due to the higher chance for postoperative worsening in large and giant tumors. Another important recommendation is that in case of brainstem pial invasion of the tumor, a subtotal or near total resection is warranted, because of higher chance of postoperative deterioration

Other authors have also examined the risk factors associated with resection of skull base and particularly CPA meningiomas and devised a useful scale (the “ABC surgical risk scale”) to predict the postoperative outcome and the possible degree of resection. ⁴⁷ The authors identified five major variables in order to properly predicts the extent of tumor removal and postoperative neurological changes: (1) tumor attachment size; (2) arterial involvement; (3) brainstem contact; (4) central cavity location; and (5) cranial nerve group involvement.

These scoring scales as well the data from the natural history would help choosing the correct treatment approach for the patent.

In our experience, an important factor is tumor consistency. Firm and calcified tumors can be more difficult to dissect, with greater risk for postoperative cranial nerve deficit, compared to soft and aspirable tumors.

13.5 Preoperative Surgical Planning and Complication Avoidance

If the patient is selected for surgery, there are several factors that have to be considered in order to decrease the risk for morbidity and mortality. The detail preoperative clinical and radiological workup would reveal important factors for planning of the surgical strategy: location, extension, and size of the tumor, presence or absence of hydrocephalus, brainstem edema, tumor associated hyperostosis, venous anatomy, as well as patient age comorbidities, patent foramen ovale, preoperative hearing and facial nerve function, and surgeon’s preference. ¹¹^, ¹²^, ¹⁴^, ¹⁵^, ¹⁶^, ¹⁷^, ¹⁸^, ²⁰^, ⁴⁶^, ⁵⁷^, ⁵⁸^, ⁵⁹^, ⁶⁰^, ⁶¹^, ⁶²^, ⁶³

Preoperative brain edema and hydrocephalus are factors that could potentially increase the complications rate and have to be managed before surgery. In case of severe brain edema, the patient has to be loaded preoperatively with dexamethasone (8 mg intravenously initially followed by 4 mg every 8 hours). It takes approximately 13 to 18 hours before the dexamethasone takes effect; therefore, this regiment has to be started several days before the operation. ⁶⁴

Obstructive hydrocephalus may present in case of very large tumors. Management options include preoperative ventriculoperitoneal shunt placement, endoscopic third ventriculostomy (ETV), temporary external ventricular drainage. We do not recommend preoperative insertion of ventriculoperitoneal shunt, due to potential infection, as well as potential postoperative hemorrhagic complications after surgery of the tumor itself. According to our experience, successful removal of the tumor usually manages the hydrocephalus and placement of permanent ventriculoperitoneal shunt can be avoided in most cases. ETV is a minimally invasive option for the management of obstructive hydrocephalus caused by the tumor. ⁶⁵^, ⁶⁶^, ⁶⁷^, ⁶⁸ An alternative is the external ventricular drain with possibility of monitoring intracranial pressure. Lumbar puncture has to be avoided due to the risk of tonsillar herniation.

If sitting or semi-sitting position is planned, preoperative echocardiography excludes a patent foramen ovale, is the main part of the diagnostic workup. ⁴⁶

Careful review of the radiological data in order to examine petrous bone pneumatization, associated venous anatomy, 3D volumetric reconstructions of the from the CTA data, tumor vascularity origin, presence or absence of hyperostosis have to be done in order to properly plan the surgical approach. (See neuroradiological examination for more details.)

13.6 Surgical Approaches

There are various surgical approaches designed for the treatment of CPA meningiomas. ⁷^, ¹⁰^, ¹¹^, ¹²^, ¹³^, ¹⁴^, ¹⁶^, ¹⁷^, ¹⁸^, ²⁰^, ⁴⁸^, ⁵⁷^, ⁶⁰^, ⁶³^, ⁶⁹^, ⁷⁰^, ⁷¹^, ⁷²^, ⁷³^, ⁷⁴^, ⁷⁵ General principles are adequate bony exposure, early eradication of vascular supply, tumor debulking, maintenance of the arachnoid plane between the neurovascular structures and avoidance of dissection where the tumor has violated the pial brainstem surface. Depending on the location and size of the tumor, there are several approaches that can be used. The transpetrosal approaches can be subdivided into anterior (Kawase) approach, posterior—presigmoid retrolabyrinthine, translabyrinthine, transcochlear approaches, and combined petrosal approach. A variation of the retrosigmoid approach, the trans- and suprameatal type can also be included in the transpetrous approaches.

In order to better illustrate the area of the skull base that can be reached with the different petrosal approaches the clivus is divided into following zones (▶ Fig. 13.2) ¹²:

Zone I extend from the upper border of the dorsum sellae to the internal acoustic canal and is the area that can be reached using the anterior petrosectomy approach.
Zone II extend from the IAC to jugular tubercle and is the area reachable by posterior petrosal approaches. Tumors involving both zone I and zone II require a combined petrosal approach.
Zone III extends from the jugular tubercle to the lower edge of the clivus and is an area that can be reached by the lateral suboccipital-transcondylar approaches.

Fig. 13.2 Clivus zones demarcation, useful to plan the approach. ⁶ Zone I: Accessible through anterior petrosectomy; Zone II: Accessible through posterior petrosectomy; Zone III: Accessible through far lateral transcondylar approach. The combined petrosal approach allows exposure of zones I and II. The dashed area marks the area that is reachable with the retrosigmoid approach as well as its supratentorial extension (retrosigmoid intradural suprameatal approach).

There are advantages and disadvantages of one or the other approach. The advantages of the transpetrous approaches are that they offer a more lateral and oblique angle of view toward the clivus and decrease brain retraction.

On the other hand, one of the main disadvantages of the transpetrous approaches is that they have higher morbidity due to the approach itself [cerebrospinal fluid (CSF) leaks, cranial nerve injuries, vascular complications] and they are time-consuming.

According to our experience, most of the CPA meningiomas can be resected via the classic retrosigmoid approach or in combination with the suprameatal drilling [retrosigmoid intradural suprameatal approach (RISA)] gaining access to the petrous apex ⁵^, ¹¹^, ²⁰^, ⁶⁰^, ⁶¹^, ⁶²^, ⁶³^, ⁷⁴^, ⁷⁶ (▶ Fig. 13.2, dashed area). The retrosigmoid approach offers a straight view to the whole CPA (from III^rd to XII^th cranial nerve) ⁷⁷ and in most cases, the medial clivus area can be reached owing to the displacement of the brainstem by the tumor. Moreover the RISA, with drilling of the petrous apex and splitting of the tentorium, provides additional exposure to the posterior part of the middle fossa. ⁶⁰^, ⁶¹^, ⁷²^, ⁷³^, ⁷⁴^, ⁷⁸^, ⁷⁹

13.6.1 Anterior Petroesectomy Approach

The anterior petrosectomy approach is used for the treatment of petroclival meningomas arising at the petrous apex, but not extending below the VII/VIII nerve group (below the IAC) (▶ Fig. 13.2—zone I). This is basically a subtemporal extradural middle fossa approach, where one exposes the Meckel’s cave, lifts the trigeminal nerve and drills the petrous apex to the inferior petrosal sinus, up to the posterior fossa dura. This is followed by opening of the posterior and middle fossa dura, splitting of the tentorium and combining the two fossae. ⁷⁰^, ⁸⁰^, ⁸¹^, ⁸²^, ⁸³ One of the advantages of this approach is that it allows for early devascularization of the meningiomas because most of their blood supply originates from vessels traversing the petrous bone and the tentorium.

For each case, we use advanced electrophysiological monitoring including somatosensory evoked potentials (SEPs), motor evoked potentials (MEPs), facial nerve motor evoked potentials (FMEPs), direct intraoperative cranial nerve stimulation, auditory evoked potentials (AEPs), and electromyography (EMG)/MEP of the cranial nerve VII and the lower cranial nerves.

For antibiotic preoperative administration, we use second or third generation cephalosporin. Additionally, mannitol 1g/kg and dexamethasone 20 mg is given before the skin incision to achieve maximum brain relaxation. Preoperative lumbar drain is placed in order to decrease temporal lobe retraction.

The patient is positioned supine on the operative table with the head fixed on a Mayfield pin-holder and rotated 45° towards the opposite shoulder, so as the zygomatic arch is the highest point of the operative field and the sagittal suture is parallel to the floor. Alternatively, in elderly patients with stiff and spondylotic cervical spine, the lateral position can be chosen.

We use a straight skin incision with minimal hair shave (▶ Fig. 13.3A). The incision starts from the root of the zygomatic arch reaching 2 cm above the superior temporal line The dissection is made in anatomical layers. Care should be taken to preserve the superficial temporal artery (STA), which runs in the temporoparietal fascia, just below the skin. ⁸⁴ Therefore, only the frontal or parietal branch of the artery have to be divided and the trunk is usually preserved.

The temporalis muscle fascia is incised using scalpel or scissors, not electrocautery, in order to be able to adequately close the fascia at the end of the surgery. The temporalis muscle is incised along the same line as the skin incision and retrograde muscle dissection is used, preserving the deep temporalis muscle fascia along with muscle blood supply and innervation. ⁸⁵ The latter maneuver would potentially decrease the postoperative temporalis muscle atrophy.

A craniotomy centered over the root of the zygoma approximately 5/5 cm in size (below the superior temporal line) is done. After elevation of the bone flap, the remaining bone at the inferior end of the craniotomy is drilled until is flushed with the middle fossa floor. Some authors recommend dropping the zygomatic arch (left attached to the masseter muscle) in order to increase the exposure. ⁸³ In our opinion, this is seldom necessary and adds additional morbidity to the approach. One of the main factors in this stage is preservation of the integrity of the dura.

The next step is extradural dissection and exposure of the middle fossa floor (▶ Fig. 13.3b). Dural elevation from back to front is recommended, which is aimed to preserve the greater superficial petrosal nerve (GSPN). This nerve emerges from its respective foramen just in front of the arcuate eminence and is a direct branch of the facial nerve (geniculate ganglion) between its labyrinthine and tympanic segments. It provides innervation to the lacrimal gland and its lesion results in dry eye.

After elevation of the dura from back to front, first the arcuate eminence and then GSPN are identified. The GSPN is followed anteriorly towards foramen spinosum (middle meningeal artery) and foramen ovale (V3 branch of the trigeminal nerve). Neuromonitoring plays essential role in this step of the operation—using monopolar stimulation probe at low intensity (0.1–0.3 mA), the geniculate ganglion can be identified on the petrous bone surface. Sometimes, it is difficult to differentiate the V3 entrance to the foramen ovale. With the help of the monopolar probe, the nerve can be directly stimulated and the contractions of the temporalis muscle are visible. Alternatively, for orientation over the middle fossa floor, the neuronavigation can be used. At some cases, dehiscence over the petrous bone are present and the petrous (C2) segment of the carotid artery can be seen laying just below the GSPN.

The transition zone between the periosteal and endosteal dural layers is the place to start the elevation of temporal dura from the dura propria of the lateral wall of the cavernous sinus and exposure of the Meckel’s cave. ³¹^, ⁸⁶

The dissection and elevation of the dura is continued medially towards the superior petrosal sinus and the petrous ridge anteriorly. In order to expose the true petrous ridge, the Gasserian ganglion and V3 have to be gently lifted from the petrous bone. ⁸⁷^, ⁸⁸ At this stage, occasional bradycardia can be expected due to the occurrence of the trigeminocardiac reflex. ⁸⁹^, ⁹⁰ In such cases, temporary halt of the surgical activity usually resolves the symptoms.

The boundaries of the bone resection of the anterior petrosectomy (Kawase’s triangle or more precisely quadrilateral) are the GSPN lateral, V3 anterior, superior petrosal sinus medial, and the arcuate eminence (harboring the superior semicircular canal) posterior (▶ Fig. 13.3c). ⁷⁰^, ⁸⁰ The location of the IAC can be found using the neuronavigation or can be estimated as a line bisecting a 120° angle between the GSPN and the arcuate eminence. ⁷⁰^, ⁸² Drilling is started over the estimated position of the IAC. Care is taken not to damage the superior semicircular canal or the cochlea at the cochlear angle. ⁹¹^, ⁹²^, ⁹³^, ⁹⁴ Drilling of the IAC continues to the Bill’s bar, a small bone crest separating the facial from the superior vestibular nerve. After that the drilling is continued in anteromedial direction to the superior petrosal sinus until the posterior fossa dura is exposed. ¹² The inferior limit of the drilling is the inferior petrosal sinus. The lateral limit of the drilling is the GSPN and petrous carotid artery.

After the drilling is completed, the temporal lobe dura is incised parallel to the superior petrosal sinus. The temporal lobe is gently lifted and the brain spatula is advanced to the ambient cistern, which is opened in order to evacuate CSF and achieve further brain relaxation. The trochlear nerve is identified at its point where it pierces the tentorium.

The next step is the tentorium splitting, which have to be done behind the dural entrance of the trochlear nerve. The superior petrosal sinus has to be ligated. The bleeding encountered during this maneuver can be controlled with bipolar coagulation, hemostatic sponge, or by injecting a small amount of fibrin glue onto the sinus bleeding edges. ⁹⁵ The posterior fossa dura is also opened and the trigeminal nerve is visualized at its exit point from the brainstem. After completion of the dural and tentorial opening, one can have a wide view of the space between the III^rd cranial nerve to the VII^th/VIII^th nerve complex, with the trigeminal nerve in the center of the surgical exposure (from the brainstem to the Meckel’s cave) ▶ Fig. 13.3d, e)

As a general rule, safe removal of CPA meningioma requires extensive surgical debulking using microsurgical technique and ultrasonic aspirator first, which will decrease the volume of the tumor and allow for safer arachnoid dissection. Care has to be taken to identify and preserve the cranial nerves and respect the arachnoid layers which guide the dissection. Constant cooperation with the neurophysiological team is crucial for the safety of the surgery.

After completion of the surgical resection, measures have to be taken to assure adequate dural closure. For the purpose, we use temporalis fascia or pericranium as an onlay graft for closure of the posterior fossa. We place a fat graft in the petrous bone defect and a layer of fibrin glue. The dura is lifted to the bone with tack up sutures. A central tenting suture to the bone flap is placed

The bone flap is repositioned. The soft tissues are closed in layers. No subgaleal drainage is used as to avoid CSF leaks. A compressive head wrap is applied. The spinal drain is removed at the end of the surgery.

Fig. 13.3 Anterior petrosectomy approach. (a) 3D reconstruction using OsiriX software (Pixmeo, Bernex, Switzerland). This is an overlay of two images. A straight skin incision is used starting from the root of the zygoma to the linea temporalis superior. (b) Image representing middle fossa anatomy after the dura is elevated. (c) The V3 and Gasserian ganglion are displaced in order to expose the true petrous ridge. (d) The petrous apex is removed, the posterior fossa dura as well as the temporal dura are opened and the tentorium is divided. The basilar artery as well as the superior cerebellar artery are visible. (e) Endoscopic view of the approach demonstrating the third, fourth cranial nerves and the associated vessels. Note that the tentorium is sectioned before the dural entrance of the fourth cranial nerves (anatomical dissections made by Dr. Luigi Rigante)

BA, basilar artery; GSPN, greater superficial petrosal nerve; PCA, posterior cerebellar artery; SCA, superior cerebellar artery.

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