Cerebellopontine Angle Tumors

Published on 13/03/2015 by admin

Filed under Neurosurgery

Last modified 13/03/2015

Print this page

rate 1 star rate 2 star rate 3 star rate 4 star rate 5 star
Your rating: none, Average: 0 (0 votes)

This article have been viewed 8541 times

Chapter 38 Cerebellopontine Angle Tumors

Clinical Pearls

Comprehensive knowledge of the complex anatomy of the cerebellopontine (CP) angle is a prerequisite for achieving good surgical results. The crucial neurovascular structures should be identified as early as possible during surgery, which enables their preservation and guides subsequent operative steps. Whatever the tumor size and extension, the anatomical relationships of the cranial nerves in the area of the fundus of the internal auditory canal and in the brainstem exit/entry zone are constant.

Most of the CP angle tumors are benign and their complete removal leads to excellent long-term outcomes. The only exception to complete tumor removal is the attempt to preserve function, such as in surgery for vestibular schwannoma in the only hearing ear.

The major principles of CP angle tumor removal include the following: important neural structures, such as the cochlear and facial nerves, should be identified early; the tumor should be initially debulked; the dissection from the surrounding structures should be performed only after sufficient internal decompression is achieved; the dissection should always be performed in the arachnoid plane; bipolar coagulation, especially in the vicinity of a cranial nerve, should be avoided.

Our preferred approach is the retrosigmoid approach. It is safe, relatively simple, and provides a panoramic view of the CP angle and petroclival area. Importantly, it is related to a very low procedure-related morbidity rate. The additional removal of the suprameatal tubercle provides access to tumors with extensions into Meckel’s cave, into the petroclival area, and even into the posterior cavernous sinus.

History of Cerebellopontine Angle Surgery

Tumors of the cerebellopontine (CP) angle are usually benign and their complete removal leads to the healing of the patient. However, because of the very complex anatomical structure of the area and the severity of the neurological dysfunction in case of iatrogenic damage, surgery in the CP angle has always been a challenge. The first successful complete removal of a CP angle tumor was performed in 1894 by Sir Charles Balance. The tumor was approached via a right posterior fossa craniectomy and removed with the finger inserted in an unsterile fashion between the pons and the tumor. Although the patient had facial anesthesia and complete facial palsy, he recovered from surgery and was alive for at least 18 years.1 The pathological nature of this tumor is a matter of controversy and the credit for the first removal of a vestibular schwannoma (VS) probably belongs to Thomas Annandale of Edinburgh. In 1895 he removed a tumor “the size of a pigeon’s egg” via a unilateral suboccipital craniotomy. Later, major contributions to the surgery of CP angle tumors were made by V. Horsley, von Eiselsberg, and F. Krause. Krause used for the first time faradic stimulation to differentiate the facial from the audiovestibular nerve.2 H. Cushing was the first to reduce the complication and mortality rates of VS surgery to an acceptable level by performing intracapsular tumor removal. Expectedly, the tumor recurrence rate in his series was very high. W. Dandy introduced the currently widely accepted concept of VS management. He argued that benign tumors should be removed completely in order to prevent recurrence at a later stage, even at the expense of a somewhat higher perioperative mortality rate. In the following decades, morbidity and mortality rates progressively improved with increased experience and knowledge of the normal and pathological anatomy of the CP angle, earlier detection of such tumors with the introduction of computed tomography (CT) and magnetic resonance imaging (MRI), routine use of intraoperative electrophysiological monitoring, and more reliable and safe anesthetic and operative techniques. Initial efforts at preservation of facial nerve function have expanded to preservation of hearing with increasing success. As a result of these efforts, modern CP angle surgery has been refined to a routine, safe, and low morbidity procedure.

Cerebellopontine Angle Anatomy

The CP angle is a triangular space located posterior to the pyramid, inferior to the tentorium, lateral to the pons, and ventral to the cerebellum.3 It is defined by the superior and inferior limbs of the CP fissure (Fig. 38.1).

image

FIGURE 38.1 Schematic diagram demonstrating the cerebellopontine angle formed by the superior and inferior limbs of the cerebellopontine fissure with the apex oriented laterally. Also demonstrated are the three neurovascular complexes, the upper, middle, and lower complex and the elements that form each of these complexes.

(Adapted from Rhoton AL Jr. Microsurgical anatomy of posterior fossa cranial nerves. In Barrow DL, editor. Surgery of the Cranial Nerves of the Posterior Fossa, Neurosurgical Topics. Rolling Meadows, IL: American Association of Neurological Surgeons; 1993:1-103.)

The CP angle cistern is located between the anterolateral surface of the pons and cerebellum and the posterior surface of the petrous bone and contains the trigeminal, abducent, facial, and vestibulocochlear nerves, the superior cerebellar and anterior inferior cerebellar arteries, a variable number of draining veins, the flocculus of the cerebellum, and the choroid plexus that protrudes through the foramen of Luschka. The facial nerve exits from the brainstem in the lateral part of the pontomedullary sulcus, 1 to 2 mm anterior to the entry zone of the vestibulocochlear nerve. The ninth, tenth, and eleventh cranial nerves are located in the lower part of the CP angle (Fig. 38.2A and B).

image

FIGURE 38.2 A, The upper neurovascular complex consisting of the trigeminal nerve (CN V, cranial nerve V) contained within the superior cerebellopontine cistern, the superior cerebellar artery (SCA), and the superior petrosal vein (Sup. Pet. V). The trigeminal nerve courses toward the petrous apex anterior to the superior petrosal vein, which is identified by its typical inverted Y configuration at its entrance into the superior petrosal sinus at the tentorial-petrosal dural junction. The superior cerebellar artery usually runs along the superior aspect of the fifth cranial nerve. The middle neurovascular complex consists of the facial nerve (CN VII) and vestibulocochlear nerve (CN VIII), which arise from the lateral part of the pontomedullary sulcus, and the anterior inferior cerebellar artery (AICA). The seventh and eighth cranial nerves course in the cerebellopontine cistern and enter the internal auditory canal. Also seen are the glossopharyngeal nerve (CN IX), vagal nerve (CN X), and the flocculus (Flocc.) of the cerebellum. B, Intraoperative view demonstrating the relationship of the anterior inferior cerebellar artery and its two major branches in the CPA, the internal auditory artery (Labyr. A), and the subarcuate artery (Subarc. A). The internal auditory artery travels into the internal auditory canal (IAC) and provides a blood supply to the inner ear and nerves in the IAC. Also demonstrated are the vestibulocochlear nerve (CN VIII), facial nerve (CN VII), the glossopharyngeal nerve (CN IX), vagal nerve (CN X), and the flocculus (Flocc.).

(A from Rhoton AL Jr. The cerebellopontine angle and posterior fossa cranial nerves by the retrosigmoid approach. Neurosurgery 2000;47:S93-S129. B from Rhoton AL Jr. Microsurgical anatomy of posterior fossa cranial nerves. Rhoton’s Anatomy–Part 3; October 2003:480, Figure B.)

Five nerves pass through the internal auditory canal (IAC): the facial, the vestibular (superior and inferior), the cochlear, and the nervus intermedius, accompanied by the labyrinthine artery and occasionally by branches of the anterior inferior cerebellar artery (AICA) or a loop of the AICA itself.4

In the area of the fundus of the IAC, the nerves have constant location: the facial nerve occupies the anterosuperior quadrant, the cochlear nerve occupies the anteroinferior quadrant, the superior vestibular nerve is in the posterosuperior quadrant, and the inferior vestibular nerve is in the posteroinferior quadrant (Fig. 38.3). Knowledge of these anatomical relationships is of utmost importance for the surgeon because early identification of the main neurovascular structures is a prerequisite for their preservation.

Tumors of the Cerebellopontine Angle

Tumors of the CP angle account for 5% to 10% of all intracranial neoplasms.5 VSs are the most common CP angle tumor and account for 80% to 94% of them, followed by meningiomas (3-10% of CP angle tumors) and the epidermoids (2-4%). Much rarer primary tumors are schwannomas of other cranial nerves: of the trigeminal nerve, of the facial nerve, or of the caudal cranial nerves; paragangliomas, chordomas, chordosarcomas, arachnoid or neurenteric cysts, dermoid tumors, and metastases.6,7 The CP angle could be secondarily involved by tumors extending from the brainstem or fourth ventricle: gliomas, ependymomas, choroid plexus papillomas, medulloblastomas, or lymphomas. Bilateral CP angle tumors are characteristic for neurofibromatosis 2 (NF2) and are typically VSs. Rare bilateral tumors not associated with NF2 are facial nerve schwannomas, plexus papillomas, endolymphatic sac tumors, metastases, or osteomas.8

Imaging

The precise radiological diagnosis of CP angle tumors requires a systematic approach and analysis of the lesions: site of origin, location, shape and margins, density, signal intensity, and contrast enhancement characteristics.6,9 The enhancing CP angle tumors are most frequently vestibular and nonvestibular schwannomas, meningiomas, metastases and paragangliomas, chondrosarcomas, and chordomas.7 Nonenhancing extra-axial CP angle lesions may be cystic, such as the epidermoid cysts, the arachnoid cysts, and the neurenteric cyst or may contain fat (dermoid cyst, lipoma). Intrinsic brain tumors with a significant exophytic extension into the CP angle (lymphoma, hemangioblastoma, choroid plexus papilloma, ependymoma, glioma, medulloblastoma, dysembryoplastic neuroepithelial tumor) may be difficult to differentiate from an extra-axial lesion based only on their radiological characteristics.6

Bone window thin slice CT shows the bony changes of the pyramid and of the IAC and is essential for surgical planning. Erosion or dilatation of the IAC is seen in 70% to 90% of the patients with VS. Nevertheless, the diagnostic tool of choice for all CP angle tumors is MRI.7,10,11 On T1-weighted sequences VSs are isointense to slightly hypointense and on T2-weighted sequences they are hyperintense. They enhance intensely and homogeneously after contrast application, with the exception of cystic portions of the tumors. Intrameatal VSs are best visualized with gadolinium enhancement. Meningiomas are most frequently isointense to slightly hypointense to brain parenchyma on T1-weighted MRI studies. On T2-weighted MRI studies they have higher intensity than that of VS and show a homogeneous contrast enhancement. The radiological differential diagnosis between VS and CP angle meningioma is based on several criteria. Meningiomas are centered usually away from the IAC and have broad contact with the petrous bone or the tentorium (Fig. 38.4A and B). The angle between the tumor and the pyramid is obtuse. The IAC is not widened and the tumor very rarely extends into the IAC. Although secondary invasion of the IAC might be observed in 10% to 20%,12 primarily IAC meningiomas are exceedingly rare.6,13 Calcification and cystic changes are frequent findings. A tail of enhancement along the dura (the dural “tail” sign), although not pathognomonic, is visible in 60% to 72% of meningiomas. VSs are centered at the widened IAC. They form an acute angle with the posterior surface of the petrous bone and almost always extend into the IAC (Fig. 38.5). In VS, calcifications are extremely rarely found.

Epidermoids are hypodense on CT and in up to 25% rim calcification is observed. On T1- and T2-weighted MRI they are usually isointense to cerebrospinal fluid (CSF).6,11 The signal intensity is related to the contents of the cyst: if cholesterol predominates, the cysts are hyperintense on T1-weighted images and hypointense on T2-weighted images. Fluid-attenuated inversion recovery imaging, diffusion-weighted imaging, and CISS spin-echo MRIs are more precise imaging modalities. Because of their similar characteristics it might be difficult to differentiate epidermoid cysts from arachnoid cysts. Two of the main differences between them are the smaller mass effect caused by the arachnoid cysts and their more homogeneous signal intensity on T2-weighted images.

Surgical Approaches to the Cerebellopontine Angle

The approaches to the CP angle are either posterior (through the posterior cranial fossa) or lateral (through the petrous bone). The most popular approach is retrosigmoid suboccipital, introduced by Fedor Krause (1903) and later modified and refined by many surgeons.14 The lateral approaches involve removal of a part of the petrous bone either via a subtemporal route (e.g., the middle fossa approach, the extended middle fossa approach, or the Kawase approach) or via the mastoid (the presigmoid approach, the retrolabyrinthine petrosectomy, the translabyrinthine approach (Rudolf Panse, 1904), and the transcochlear approach).4,14

Several factors have to be considered when selecting the surgical approach: tumor type, size, and origin; extension in the CP angle or in the IAC; patient’s age and general and neurological status, especially hearing level; and surgeon’s experience and institutional tradition. The approach should adhere to the following principles: it should provide sufficient exposure of the tumor; it should not be related to significant morbidity; the neural structures should not be at increased risk; any injury to the venous outflow system should be avoided. Most of the CP angle tumors are benign and their complete removal leads to healing of the patient. The preservation and even the recovery of neurological functions and of the patient’s quality of life should always be the first priority. The only exception to complete tumor removal is the attempt to preserve function, such as in surgery for VS in the only hearing ear. Excellent results have been achieved with different operative approaches, which depend more on the individual surgeon’s experience than on the advantages or disadvantages of each particular approach. Still, an ever growing amount of evidence suggests that the goals of CP angle surgery are best achieved with the retrosigmoid suboccipital approach (RSA).3,1416 It is safe and relatively simple and provides a panoramic view of the whole CP angle and petroclival area. Importantly, it is associated with a very low procedure-related morbidity rate. The additional removal of the suprameatal tubercle—the Samii technique, developed and introduced by the senior author in 1982—provides access to tumor extensions into Meckel’s cave, the petroclival area, and even the posterior cavernous sinus.

Retrosigmoid Approach

Each of the possible positions of the patient on the operating table—semisitting, lateral, or supine with the head rotated 90 degrees to the contralateral side—has its advantages and drawbacks in terms of providing sufficient access and avoiding complications. It remains, however, a matter of surgeon’s preference and institutional tradition. We prefer the semisitting position that allows the surgeon to work bimanually because there is no need for constant suction. Further, frequent coagulation during tumor removal is evaded thanks to the continuous irrigation of the operative field performed by the assistant. The head is held with a three-point head fixation frame; then it is flexed and rotated approximately 30 degrees to the involved side, avoiding occlusion of venous jugular outflow or hyperflexion of the cervical spine.

A drawback of this patient position is the risk of venous air embolism, paradoxical air embolism, tension pneumocephalus, or circulatory instability. However, in experienced hands these effects are not related to any lasting morbidity.17 Transesophageal echocardiography is the most specific and sensitive method in detection of air embolism but the combined monitoring of end-tidal carbon dioxide and precordial Doppler echocardiography yield similar results. If immediate measures are carried out at the first sign of venous air embolism, the related morbidity is insignificant.

Continuous neurophysiological monitoring should be performed throughout the surgery from the time of positioning of the patient to the skin closure. It includes monitoring of somatosensory evoked potentials (important during patient positioning in order to prevent spinal cord compression); electromyography of the facial nerve; and monitoring of the brainstem auditory evoked potentials in the case of preoperatively available hearing.18,19 Monitoring of the oculomotor, trochlear, abducens, and caudal cranial nerves is performed if needed according to the particular tumor extension and clinical presentation.

A slightly curved skin incision approximately 2 cm medial to the mastoid process is performed and the underlying muscles are incised in line with the skin. We make the bur hole approximately 2 to 2.5 cm below the superior nuchal line, two thirds behind and one third in front of the occipitomastoid suture. The asterion is not an absolutely reliable anatomical landmark and is variable both in the cranial-caudal plane and in the anterior-posterior plane.20 The sigmoid sinus descends along an axis defined by the mastoid tip and the squamosal-parietomastoid suture junction or over the mastoid groove. The course of the transverse sinus is more variable and the superior nuchal line gives a rough orientation of its location. The issue of whether to make a craniectomy or craniotomy is a matter of individual preference but we avoid making one-piece craniotomy due to the related high risk of injury to the underlying sinuses and the risk of tearing the dura by the craniotome. Excessive traction to the mastoid emissary vein/veins could lead to sinus laceration and increases the risk of venous air embolism. The vein should be skeletonized with a diamond drill until it is free of any bony encasement and can be safely coagulated. The lateral and superior limits of the approach are the borders of the sigmoid and of the transverse sinuses; their edges have to be exposed. Inferiorly, enough bone should be removed in order to provide access to the lateral cerebellomedullary cistern and allow a continuous egress of the CSF or irrigation fluid throughout the surgery.

The dura is incised in a curvilinear manner just 1.5 to 2 mm medial to the sigmoid and inferior to the transverse sinus. This allows for a primary watertight dural closure and avoids the need for using a dural substitute in almost all cases. The lateral cerebellomedullary cistern is then opened and CSF is drained. Thus, the cerebellum relaxes away from the petrous bone and the self-retaining retractor supports and protects the cerebellar hemisphere, instead of compressing it. The further operative steps depend on the pathological tumor type and its extension. However, several general principles should always be followed:

Following tumor removal, the opened mastoid air cells are sealed with multiple pieces of fat tissue that are fixed with fibrin glue. If necessary, fat pieces are glued on the sutured dura. Bone wax is avoided, except in case of significant bleeding from the bone edges.

Lateral Approaches

Buy Membership for Neurosurgery Category to continue reading. Learn more here