Introduction

The first successful complete removal of a cerebellopontine (CP) angle tumor was performed on November 19, 1894, by Sir Charles Balance in London. At the first stage, a right posterior fossa craniectomy was performed, and 1 week later the tumor was removed with the finger inserted in an unsterile fashion between the pons and the tumor. The tumor was found to be firm and well encapsulated, and it had a wide attachment to the dura of the petrous bone. According to Cushing, this tumor was most probably a meningioma.1 In 1928, Cushing and Eisenhardt2 described their experience with the surgical management of seven patients with meningiomas “simulating acoustic neuromas.” During the following decades, surgery of CP angle tumors fascinated and attracted many of the most outstanding neurosurgeons. New operative approaches and management concepts were introduced that allowed dramatic reductions in mortality and morbidity rates. Gradually, the focus changed from performing “life-saving” to “function-pre-serving” surgeries. Nowadays, surgery of tumor in the CP angle is safe, major morbidity is exceptional, and preservation of the function of the facial and cochlear nerves is the rule.

Classification

Meningiomas arising from the dura of the posterior surface of the pyramid lateral to the trigeminal nerve are defined as “CP angle meningiomas.”3 We consider those meningiomas that arise from adjacent areas but have their main bulk in the CP angle also as CP angle meningiomas. Meningiomas are the second most frequent tumor of the CP angle after vestibular schwannomas and comprise 6 to 15% of all CP angle tumors.4–6

The variable attachment sites and direction of growth determine the high heterogeneity of CP angle meningiomas in terms of clinical presentation and operative challenge. The two main groups are the premeatal and retromeatal meningiomas, determined according to their relation to the internal acoustic meatus (IAM).3 This classification has operative and prognostic significance—the more medially located tumors are related to greater surgical complexity and to poorer outcome. We subdivide CP meningiomas further into premeatal, postmeatal, suprameatal, inframeatal, and centered at the IAM.7 As presented in a previously published evaluation of the senior author’s experience with CP angle meningiomas,7 33% of them originated at the petrous ridge anterior to the IAM (mean tumor diameter 3.1 cm), 20% originated superior to the IAM (mean tumor diameter 3.4 cm), 12% inferior to the IAM (mean tumor diameter 4 cm), 13% posterior to the IAM (mean tumor diameter 4.1 cm), and 22% with involvement of the IAM (mean tumor diameter 3.4 cm). A specific pattern of dislocation of the seventh and eighth cranial nerves is observed in each of the subgroups.

According to their extension pattern, CP meningiomas can be classified as follows:

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  Postmeatal meningiomas

  
  
  
  
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    Without extension into the IAM

    
    
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    With extension into the IAM

  
  
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  Premeatal meningiomas

  
  
  
  
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    Medial and superior extension (with or without extension into the Meckel cave, supratentorially, or into the IAM

    
    
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    Medial and inferior extension (with or without extension into the jugular foramen or into the IAM; extending to the level of the foramen magnum or not)

    
    
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    Combination thereof

  
  
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  Large meningiomas with pre- and postmeatal extension

Neuroimaging

Tumor features and extension are best appreciated on contrast-enhanced magnetic resonance imaging (MRI). On T1-weighted MRI studies, meningiomas are mostly isointense to slightly hypointense relative to brain parenchyma.4 On T2-weighted MRI studies, they show higher intensity than that of vestibular schwannomas. Contrast enhancement is usually homogeneous in both meningiomas and vestibular schwannomas. The radiological differential diagnosis between CP angle meningioma and vestibular schwannomas is based on several criteria.8,9 Usually, meningiomas are centered away from the IAM and have broad contact with the petrous bone or the tentorium. The angle between the tumor and the pyramid is obtuse and the IAM is not widened. Although secondary invasion of the IAM might be observed in 10 to 22%, primary IAM meningiomas are very rare.10,11 A tail of enhancement along the dura (the dural tail sign), although not pathognomonic, is visible in 60 to 72% of meningiomas. In contrast, vestibular schwannomas are centered at the widened IAM, form an acute angle with the posterior surface of the petrous bone, and almost always extend into the IAM. Thin-slice computed tomography (CT) with bone window settings is essential for the approach planning and provides information on the degree of bone involvement, whether there is erosion or hyperostosis, and the presence or absence of tumor calcification. Magnetic resonance angiography (MRA) and venography (MRV) demonstrate the degree of tumor vascularity. Digital subtraction angiography as a diagnostic and/or planning tool is required in exceptional cases, such as giant tumors with wide extension and involvement of main vessels. Preoperative embolization of tumor feeding arteries may be very helpful in highly vascular meningiomas.

Clinical Presentation

Meningiomas cannot be distinguished from other CP angle tumor types based on their clinical presentation alone. Common findings are hearing loss, vertigo, headache, and trigeminal and cerebellar symptoms.6,10,12–14 Symptoms and signs secondary to brain stem and cerebellar compression with obstructive hydrocephalus occur late.

Premeatal and retromeatal meningiomas have distinctive clinical development and symptomatology.3,5,7,15 Premeatal tumors are diagnosed earlier and consequently have smaller size. Their clinical presentation is with trigeminal symptoms (numbness and/or trigeminal neural-gia) and facial and cochlear nerve signs. Typical for the larger retromeatal meningiomas are cerebellar signs and symptoms. Patients with large meningiomas that extend anterior and posterior to the IAM can present with a combination of these symptoms.

Management

Treatment options for CP angle meningiomas are observation, surgery, radiotherapy/radiosurgery, or a combination thereof. In each case the decision should be taken individually, considering the patient’s biological age, expectations, activities, and general and neurological status, as well as the tumor’s size and extension. The wide availability of MRI allows earlier meningioma detection during the “preclinical” stage and is a reliable method for control of tumor growth. Initial follow-up and surgery are frequently recommended only when the tumor shows growth or new symptoms appear. On the other hand, earlier active management is justified because the outcome of surgery is related to tumor size and is optimal in neurologically intact patients. We recommend observation as the initial treatment option in elderly or medically unstable patients with small tumors and mild stable symptoms or in those unwilling to undergo surgery.

Complete operative tumor removal is the optimal treatment of CP meningiomas but should not be achieved at the expense of new neurological dysfunction or deterioration of the patient’s quality of life. Different surgical approaches are used for resection of CP angle meningiomas: retrosigmoid, extended retrosigmoid, translabyrinthine, transcochlear, transpetrosal, and modified far lateral, as well as approaches through the middle cranial fossa.4,7,12,14,16 However, the general concepts that are important for the successful removal of most meningiomas are similar: adequate exposure, interruption of the blood supply along the dural attachment, internal decompression, and cautious dissection of the tumor capsule from the brain stem and cranial nerves in the arachnoid plane.

In the early ages of the skull base surgery, an extensive bony exposure allowing complete visualization of the meningioma and all adjacent neural and vascular structures was considered a major prerequisite for complete and safe tumor removal. Approaches providing such exposure, however, have higher risks of complications, including a higher rate of facial nerve palsy and venous-related complications.

Another important issue consists of surgeons’ attempts to preserve hearing. Some authors suggest that hearing preservation should be attempted only in retromeatal meningiomas.15 Hearing-destructive approaches through the pyramid have been proposed for patients with severe hearing deficits. In CP angle meningiomas, contrary to vestibular schwannomas, hearing loss is usually due to compression of the cochlear nerve. Hearing improvement is often observed after tumor removal and has been reported even in cases of large meningiomas.7,17,18

Having extensive experience with all skull base techniques, the senior author believes that simpler and safer approaches are the prerequisite to improving outcome and reducing morbidity. The retrosigmoid suboccipital approach (RSA) offers an excellent panoramic visualization of the whole CP angle, wide exposure of the tumor (whatever its size), and increased safety when working in the vicinity of the brain stem. It has a very low procedure-related morbidity rate and allows hearing preservation. For CP meningiomas, the RSA is our favorite technique. In CP meningiomas with medial extension toward the petroclival area or the Meckel cave, we utilize the retrosigmoid suprameatal approach (Samiis technique), introduced by the senior author in 1982.