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Retrosigmoid Approach to Tumors of the Cerebellopontine Angle
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The retrosigmoid approach is a versatile type of craniotomy that creates a panoramic view of the posterior fossa from the tentorium cerebelli to the foramen magnum. The indications for the retrosigmoid approach include (1) resection of extra-axial lesions, such as schwannoma, meningioma, and epidermoid; (2) cranial nerve neurectomy (e.g., CN V, VIII, and IX); (3) vascular decompression of cranial nerves (e.g., CN V, VII, and IX); (4) vascular disorders of the vertebrobasilar system; and (5) parenchymal lesions of the brainstem and cerebellum.
The primary advantages of the retrosigmoid approach are the potential for hearing preservation and an unhindered exposure of the inferior portion of the cerebellopontine angle (CPA), especially for lesions extending inferior to the level of the jugular bulb. The principal disadvantages are a substantially higher incidence of persistent postoperative headache and of cerebrospinal fluid (CSF) leaks compared with transtemporal approaches. Although the retrosigmoid approach is technically capable of addressing most lesions involving the CPA, it is best used selectively to gain optimal benefit from its advantages while avoiding its occasional disadvantages. This chapter concentrates on the use of the retrosigmoid approach for tumors of the CPA, with an emphasis on vestibular schwannoma resection.
Surgical Anatomy
Historically, the earliest approach to the posterior fossa was undertaken through the suboccipital convexity. Krause first employed this technique during the latter portion of the 19th century. Until the 1970s, the technique in widespread use was the so-called suboccipital approach. In this procedure, a large bone window is removed and the anterior limit of the craniectomy is the first mastoid air cell encountered. Curtailment of the anterior opening at the first contact with pneumatization was predicated on the assumption that the mastoid was bacterially contaminated, and that opening its air cell tracts created an increased risk of meningitis. Because of its more posterior angle of view, the suboccipital approach required a greater degree of cerebellar retraction, and sometimes necessitated a partial cerebellar resection.
In recent years, because of increased experience with CPA surgery, the classic suboccipital approach has been modified to become the retrosigmoid approach, which is now the preferred method for exposing the CPA behind the sigmoid sinus. In this technique, the bone is removed anteriorly up to the level of the posterior border of the sigmoid sinus and superiorly to the inferior margin of the transverse sinus ( Fig. 45.1 ). Although the mastoid air cells are frequently transected during this maneuver, experience has not shown an increased incidence of postoperative infection. The slightly higher risk of CSF leak associated with this more anterior exposure is more than offset by its more favorable angle of view into the CPA and the markedly reduced need for cerebellar retraction with this approach.
The anatomical exposure of the posterior fossa provided by the retrosigmoid approach is bounded superiorly by the tentorium cerebelli and inferiorly by the jugular foramen and foramen magnum ( Fig. 45.2 ). Access to the central nervous system includes the lateral cerebellar hemisphere and the lateral surfaces of the pons and upper medulla. CN V through XI are visible at their root entry zones and over their cisternal courses. Although the theoretical anterior limit of exposure is the clivus and the apical portion of the petrous pyramid, in practice, access to these ventral structures is usually limited by CN VII and VIII superiorly and CN IX through XI inferiorly; these latter nerves bridge across the CPA, restricting ventral access to narrow intervals. Exposure of the prepontine cistern is largely obstructed by the lateral aspect of the pons, which does not tolerate medial retraction well.
Anatomical variations may affect the CPA exposure provided by the retrosigmoid approach. A posteriorly placed sigmoid sinus course results in the anterior edge of the craniectomy being placed more posteriorly. This placement creates a deeper field of action and a less favorable angle of view with the consequent need for more cerebellar retraction. This disadvantageous exposure may be compromised further by a low transverse sinus course, particularly if the patient also has a short neck and prominent shoulder. This problem of restricted exposure may be overcome by combining the retrosigmoid approach with an anterosigmoid, retrolabyrinthine decompression to allow anterior retraction of the sigmoid sinus. A high-lying jugular bulb restricts access to the inferior portion of the internal auditory canal (IAC), and makes it difficult to remove the bone between the IAC and the jugular bulb, thereby reducing access to the tumor located within the IAC. Occasionally, the bulb may extend superiorly to overlap the IAC, partially obscuring access to the medial aspect of the canal.
Preoperative Evaluation And Patient Counseling
The minimal preoperative evaluation for a patient with a CPA tumor comprises a clinical history, a physical examination, pure-tone and speech audiometry, and an imaging study (preferably gadolinium-enhanced magnetic resonance imaging [MRI]). For nonvestibular schwannoma tumors, computed tomography (CT) scanning for evaluation of the osseous characteristics of the cranial base and angiography to address vascular anatomy and possibly to perform embolization are occasionally indicated. Neither vestibular diagnostic testing nor auditory evoked responses are routinely obtained in patients already diagnosed with a vestibular schwannoma.
Numerous factors affect the selection of posterior fossa craniotomy for tumors of the CPA. , , As advocates of selective management of these lesions according to the unique attributes of each tumor and the potential surgical options, we involve the patient in the discussion of the relative advantages and disadvantages of each technique. With most patients, an obvious choice can be made, whereas in others, patient preference is important. Our customary preoperative counseling includes the anticipated and potential risks to hearing, balance, and facial motor function. The less common complications that are discussed include CSF leak, meningitis, cerebrovascular accident, and death.
Patient Selection
Common Indications in Neurotology
Hearing preservation
The primary aim of vestibular schwannoma management is removing the threat of progressive tumor growth while avoiding injury to the central nervous system. Preservation of the cranial nerve function (facial movement, facial sensation, and hearing), which has become the primary focus of vestibular schwannoma surgery, is a secondary goal. Patients with acoustic tumors can be classified into three groups in terms of potential for hearing preservation. Patients for whom hearing preservation is highly improbable generally undergo translabyrinthine removal, with the benefits of a more direct view and wider opening for tumor dissection and less cerebellar retraction. The criteria that place a patient into this group include poor hearing, large CPA component (>3 cm), and deep penetration (lateral extension) of the IAC. Conversely, patients with good hearing, a small CPA component (<1 cm), and shallow IAC involvement are considered excellent candidates for a hearing conservation approach. With the advancement of medical devices aimed at audiological rehabilitation and the evolving role of cochlear implantation in vestibular schwannoma, there is no strict hearing criteria for what might have been previously defined as poor or good hearing. , Each surgical team must rely on its own criteria for hearing preservation, based on experience and the available treatment options, together with the patient’s wishes, in coming to a selection of surgical approach. Neurotologists would always favor undertaking a hearing conservation approach, even when the chances of success were remote, were there not potential adverse consequences from the endeavor. The lower morbidity of the translabyrinthine approach, especially in terms of persistent headache and CSF leak, leads the clinician away from the retrosigmoid hearing conservation approach when the chances of success are limited.
The concept of useful hearing is context-dependent. In a patient with a normal contralateral ear, imperfect residual hearing in the tumor-affected ear is often of little practical benefit. When hearing in the contralateral ear is impaired or threatened, such as in cases of bilateral vestibular schwannomas associated with neurofibromatosis type 2, a conservative approach to hearing conservation is prudent, occasionally even at the expense of complete tumor excision.
Hearing preservation is seldom achieved when tumors with a CPA component exceeding 2 cm in diameter are removed. , This rule should not be applied in nonvestibular schwannoma CPA tumors (e.g., meningiomas), however, because hearing preservation is frequently achieved even with large tumors.
The retrosigmoid approach exposes a variable amount of the IAC without violating the inner ear while the canal is being drilled open. Two factors should be considered in the decision of whether hearing conservation via the retrosigmoid approach is feasible: the depth to which the tumor penetrates the IAC and the degree of IAC exposable in that patient. The relationship between the inner ear and the lateralmost extension of the tumor into the IAC may be predicated by preoperative gadolinium-enhanced MRI. It is useful to assess the amount of fluid in the lateral IAC on T2-weighted imaging.
Use of retrosigmoid approach in combined therapy of vestibular schwannoma
Numerous studies have shown that functional outcomes after conventional microsurgery are substantially poorer in patients with acoustic neuroma larger than approximately 3 cm. In these patients, the incidence of persistent facial dysfunction is high. There is also an increased risk of persistent balance dysfunction because of infarction of the middle cerebellar peduncle. In an effort to improve functional outcome, some centers have begun approaching larger tumors with subtotal resection, leaving a rind of tumor on the pons and along the course of the facial nerve. When the patient has serviceable hearing, the retrosigmoid approach is typically used. To reduce the risk of recurrence, it is essential to remove the IAC component. Such surgical remnants resume growth in approximately one-third of cases. If the remnant grows on serial imaging, it may be treated with stereotactic radiation with a greater than 70% probability of halting its growth in sporadic cases.
Vestibular schwannoma in a patient with chronic otitis media
Although patients with vestibular schwannomas rarely have concomitant chronic middle ear infection, in patients who do, the translabyrinthine approach for vestibular schwannoma resection is contraindicated. The retrosigmoid approach may also open into potentially contaminated mastoid air cells lying behind the sigmoid sinus, and into air cells that may surround the IAC. To avoid potential intracranial infection, chronic middle ear infection should be controlled with tympanomastoidectomy, antibiotics, or both before tumor surgery whenever possible.
Tumors extending into the inferior portion of cerebellopontine angle
The retrosigmoid approach provides the best access to the lower portion of the CPA and can be extended to expose the foramen magnum when required. Transtemporal approaches to the CPA are limited in their inferior exposure by the sigmoid sinus and the jugular bulb. Vestibular schwannomas seldom extend into the inferior reaches of the CPA. Even when they do, the capsular peel is readily mobilized superiorly after tumor debulking. Occasionally patient anatomical factors such as a very high-riding jugular bulb or a large dominant and anterior sigmoid sinus may make the approach to a vestibular schwannoma more favorable than a translabyrinthine approach. Meningiomas and other extra-axial tumors usually do not mobilize easily, however, and they are often entwined with the lower cranial nerves (CN IX through XII) and vital vascular structures (e.g., posterior inferior cerebellar and vertebral arteries). In such patients, the retrosigmoid approach is chosen for its superior ability to expose this region.
In patients with neurofibromatosis type 2, concurrent schwannomas on the lower cranial nerves are a common finding at the time of vestibular schwannoma surgery. The retrosigmoid approach permits a thorough inspection of the jugular foramen contents and the dural lining of the posterior cranial base for possible early meningioma formation. Small, asymptomatic schwannomas on the lower cranial nerves are typically left alone, whereas early meningiomas are excised.
Tumors with limited extension into Meckel ’ s cave
The retrosigmoid approach is also useful in approaching extra-axial posterior fossa tumors that possess minor extensions into the Meckel cave (cavum trigeminale). Most such tumors are trigeminal schwannomas and petroclival meningiomas. Added exposure is obtained by removing the apical petrous bone between the IAC and the tentorium. This maneuver provides access to approximately 1 to 2 cm of the posterior aspect of the Meckel cave for tumor removal.
Revision surgery
The retrosigmoid approach is favored in cases of recurrence after a previous translabyrinthine removal of a vestibular schwannoma to avoid the dural scar from the prior procedure and to allow identification of the facial nerve as it emerges from the fat graft located in the surgical defect.
Relative Contraindications
Deep extension into internal auditory canal
Generally, achieving gross total resection by the retrosigmoid approach in tumors extending into the lateral one-third of the IAC is difficult. The access to the lateral third of the IAC is limited by the extent of the bony removal at the endolymphatic duct to preserve the otic capsule. Indirect dissection or endoscope-assisted dissection is possible, but direct microscopic visualization of this area is not possible while keeping hearing intact. In such cases, the translabyrinthine approach ensures complete resection and reduces operative morbidity.
Extension into the cranial base
Tumor penetration into the posterolateral cranial base is a relative contraindication to the retrosigmoid approach. CPA tumors that invade the temporal bone (other than the medial two-thirds of the IAC), the jugular foramen, or the hypoglossal canal are generally best addressed via a lateral, transbasal craniotomy.
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