Retrosigmoid Approach to Tumors of the Cerebellopontine Angle

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Chapter 50 Retrosigmoid Approach to Tumors of the Cerebellopontine Angle

Robert K. Jackler, David W. Sim

image Videos corresponding to this chapter are available online at www.expertconsult.com.

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. 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). The principal disadvantages are a substantially higher incidence of persistent postoperative headache and a higher incidence 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 acoustic neuroma resection.

SURGICAL ANATOMY

Historically, the earliest approach to the posterior fossa was undertaken through the suboccipital convexity. Krause1 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, as a result 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, 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. 50-1). Although 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.

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FIGURE 50-1 Incision used in retrosigmoid approach is located approximately 6 cm behind postauricular sulcus. After craniectomy and retraction of cerebellum, tumor becomes visible within cerebellopontine angle. Anterior edge of craniotomy is placed immediately behind sigmoid sinus and just inferior to lower margin of transverse sinus.

FIGURE 50-2. Operative view of cerebellopontine angle as seen through retrosigmoid approach. Superiorly is trigeminal nerve (CN V) and petrosal vein. Inferiorly are lower cranial nerves (IX, X, and XI). In midsection of the exposure, a medium-sized acoustic neuroma is seen in relation to facial and audiovestibular nerves.

The anatomic 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. 50-2).2–5 Access to the central nervous system includes the lateral cerebellar hemisphere and the lateral surface 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, which 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.

Anatomic 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 a 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.6 A highly placed jugular bulb restricts access to the internal auditory canal (IAC), and can make the dissection of the inferior bony trough between the canal and the bulb difficult. Occasionally, the bulb may extend superiorly to overlap the IAC, partially obscuring access to the medial aspect of the canal.7

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]). In nonacoustic 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 an acoustic neuroma.8

Numerous factors affect the selection of posterior fossa craniotomy for tumors of the CPA.7,9,10 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. In most cases, 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. Less common complications that are discussed include CSF leak, meningitis, cerebrovascular accident, and death.11 Although blood transfusion is seldom required, we encourage the patient to donate 1 U of autologous blood.

PATIENT SELECTION

Common Indications in Neurotology

Hearing Preservation

The primary aim of acoustic neuroma management is removing the threat of progressive tumor growth, while avoiding injury to the central nervous system. Preservation of cranial nerve function (facial movement, facial sensation, and hearing), which has become the primary focus of acoustic neuroma surgery in recent years, 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. Criteria that place a patient into this group include poor hearing (<30% speech discrimination, >70 dB speech reception threshold), large CPA component (>3 cm), and deep penetration of the IAC. Conversely, patients with good hearing (>70% speech discrimination, <30 dB speech reception threshold), small CPA component (<1 cm), and shallow IAC involvement are considered excellent candidates for a hearing conservation approach.7 It is difficult to codify a set of rules concerning selection of a hearing conservation approach for the numerous patients who lie between these parameters. Each surgical team must rely on its own criteria, based on experience, 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 ear is often of little practical benefit. When hearing in the contralateral ear is impaired or threatened, such as in cases of bilateral acoustic neuromas associated with neurofibromatosis type 2, a conservative approach to hearing conservation is prudent, occasionally even at the expense of complete tumor excision.12

Hearing preservation is seldom achieved when tumors with a CPA component exceeding 2 cm in diameter are removed.13,14 This rule should not be applied in nonacoustic CPA tumors (e.g., meningiomas), however, because hearing preservation is frequently achieved even with large tumors.15

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.16

Use of Retrosigmoid Approach in Combined Therapy of Acoustic Neuroma

Numerous studies have shown that functional outcome after conventional microsurgery is 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.17 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.18 If the remnant grows on serial imaging, it may be treated with stereotactic radiation with a greater than 90% probability of halting its growth.

Acoustic Neuroma in a Patient with Chronic Otitis Media

Although patients with acoustic neuromas rarely have concomitant chronic middle ear infection, in patients who do the translabyrinthine approach for acoustic neuroma 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 tympanoplasty, antibiotics, or both, before tumor surgery whenever possible.7

Tumors Extending into 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. Acoustic neuromas seldom extend into the inferior reaches of the CPA. Even when they do, the capsular peel is readily mobilized superiorly after tumor debulking. Meningiomas and other extra-axial tumors usually do not mobilize easily, however, and 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 cases, 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 acoustic neuroma 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 Meckel’s 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 Meckel’s cave for tumor removal.19–21

Revision Surgery

The retrosigmoid approach is favored in cases of recurrence after a previous translabyrinthine removal of an acoustic neuroma 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, tumors extending into the lateral one third of the IAC are not resectable by the retrosigmoid approach without destroying hearing. In such cases, the translabyrinthine approach ensures complete resection and reduces operative morbidity.7

Extension into 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.

Large Tumors

Although large CPA tumors (>3 cm) may be approached through either the retrosigmoid or the translabyrinthine technique, when we intend radical removal we prefer to use the latter because it provides ample exposure and minimizes the need for cerebellar retraction. In addition, when the pons and the cerebellar peduncle are substantially displaced medially and posteriorly, the translabyrinthine approach, by virtue of its more anterior placement, provides a more favorable angle of view posteriorly toward the brainstem interface. In cases of facial nerve disruption, which is more common in large tumors, the translabyrinthine approach affords more reconstructive options through mastoid meatal rerouting.7

PATIENT PREPARATION AND POSITIONING

At most centers, the operation is performed by a multidisciplinary team consisting of a neurotologist, neurosurgeon, neuroanesthesiologist, neurophysiologist, and specialized operating room nurses. The operation is done with the patient under general anesthesia. A short-duration muscle relaxant is used to facilitate endotracheal intubation. Thereafter, anesthesia is maintained with inhalational agents alone, avoiding the use of muscle relaxants, which would prevent effective intraoperative cranial nerve electrophysiologic monitoring. In addition to the routine neuroanesthesia monitoring equipment, antithrombotic stockings and a urinary catheter are used. The retrosigmoid approach may be carried out in one of three surgical positions: supine, lateral supine (park bench position), and sitting.10 Supine is the favored position because it affords excellent exposure and carries the lowest risk of complication (discussed later).

The patient is secured in the optimal operating position by means of a head holder attached to the bed frame (e.g., Mayfield). This apparatus facilitates exposure of the suboccipital region while the patient is in the supine position. Optimal surgical field exposure is obtained by a combination of head rotation, neck flexion, and ipsilateral shoulder elevation. Excessive neck torsion should be avoided to prevent cervical injury and to reduce the risk of cerebellar swelling secondary to compromised flow through the vertebral venous system. The cranial nerve–monitoring electromyographic electrodes are placed into the muscles supplied by CN V, VII, and XI. When intraoperative auditory brainstem monitoring is indicated, scalp electrodes are placed, and an earphone is inserted into the ipsilateral external auditory canal.22

We favor using an operating room table with enhanced lateral rotation capability (up to 30 degrees), which permits optimal visualization of the lateral end of the IAC at a comfortable working angle. When the surgeon works at relatively extreme rotations, the patient must be securely supported on the operating table by a lumbar support and placed on the contralateral side to the operative exposure, with chest and thigh safety straps. The bed is reversed, with the patient’s head on the foot section to allow the surgeon to sit during the microsurgical portion of the procedure.

A perioperative prophylactic antibiotic with good CSF penetration (e.g., ceftizoxime, 2 g) is administered intravenously. Mannitol (1 g/kg) is administered intravenously when the scalp incision is made so that its effectiveness in reducing brain swelling coincides with dural entry. We do not routinely give corticosteroids except in patients with larger tumors (>3 cm) or in patients with peritumoral brain edema, when dexamethasone (10 mg) is administered intravenously. To reduce the risk of CSF fistulization, an indwelling lumbar CSF drain is used when extensive peri-IAC pneumatization is encountered.

SURGICAL SITE PREPARATION

We ask the patient to wash his or her hair thoroughly either the morning of surgery or on the evening before with an antiseptic shampoo. In the operating room, after induction of anesthesia, the hair over the suboccipital area is removed with electric clippers. It is also acceptable to use a racing stripe technique. The upper neck is included in the operative site, allowing potential access to the great auricular nerve in case a graft is required for facial nerve reconstruction. The scalp is washed with povidone-iodine soap, and the clipped area is shaved. To improve attachment of adhesive drapes, the surgical site is defatted with alcohol and dried. Sterile drapes are placed 1 cm from the hair edge around the prepared scalp and held in place with surgical adhesive (e.g., Mastisol). The field is prepared with povidone-iodine solution and dried. Sterile towels are placed around the operative field and held into position by an adhesive plastic sheet.

SPECIAL INSTRUMENTS

Various instruments are used for the retrosigmoid approach to the CPA, including craniectomy instruments, retractors, high-speed surgical drill with a selection of cutting and diamond burrs, suction and suction-irrigation tips of the fenestrated and nonfenestrated types, bipolar cautery, microdissection instruments, and a binocular operating microscope. We perform the craniectomy with an Acra-Cut disposable cranial perforator burr-hole maker in a Hudson brace, a system that allows rapid bone removal, while minimizing the chance of dural or venous sinus injury. The craniectomy is completed with rongeurs. To retract the thick suboccipital musculature, a deep-bladed Weitlaner-type retractor is used. For brain retraction, several sizes of malleable blades are used that may be held in position in several ways. We prefer to use the Apfelbaum base, which combines a Weitlaner-type retractor with a movable arm to affix the retractor. Other options for basing the brain retractors during retrosigmoid craniotomy include a C-clamp placed on the head holder frame or a table-based system (e.g., Greenberg).

Either an electric or an air-powered drill is suitable to use for this approach. When the exposure is narrow, an angled handpiece is advantageous because it is less obstructing to the surgeon’s point of view. An operating microscope with an inclinable optical pathway is desirable to accommodate the various exposure angles required during the procedure while maintaining a comfortable operating position. Insulated bipolar cautery forceps are essential for obtaining hemostasis during CPA tumor surgery. Large tips for handling substantial vessels and slender, fine tips for use when the coagulation must be confined to a narrow region are needed. We have found that a self-irrigating system (e.g., Malis bipolar irrigating system) is valuable because it discourages tissue adhesion to the forceps tips.

We use a microsurgical instrument set that includes sharp and blunt dissectors in various shapes and sizes, needles, and small bone curettes (e.g., Rhoton microneurosurgical instruments). A set of sharp scissors of different sizes and angles is also important. Many special tools are available to facilitate rapid intracapsular debulking of the tumor. We prefer to use a Cavitron ultrasonic surgical aspirator (CUSA), which allows debulking without traction or torsion, minimizes hemorrhage, and respects tumor capsular planes, avoiding inadvertent neural or vascular injury. Other options include the surgical laser and a rotatory surgical aspirator (House-Urban).

Operating room electric circuitry and neuroanesthesia electric monitoring equipment should be grounded and electronically quiet to minimize 60 Hz noise production, which interferes with the cranial nerve electrophysiologic monitoring setup. The specialized equipment for intraoperative cranial nerve monitoring used in our institution has been described elsewhere in detail.22

SURGICAL TECHNIQUE

Acoustic neuroma excision by the retrosigmoid approach to the CPA can be subdivided into seven stages: (1) craniectomy, (2) exposure of the CPA, (3) exposure of the IAC, (4) tumor resection, (5) hemostasis, (6) IAC closure, and (7) craniotomy closure.

Craniectomy

A curvilinear paramedian incision 3 cm behind the postauricular sulcus is made down to bone. The cervical muscles are detached anteriorly and posteriorly, exposing the mastoid and suboccipital areas. Emissary venous bleeding is controlled with bone wax. The mastoid tip is exposed, and the posterior belly of digastric muscle is elevated from its groove. Dissection directly on the bone preserves the occipital nerves and vessels. A posterior fossa craniotomy window of approximately 3 × 3 cm is made in the retrosigmoid approach. It is bounded anteriorly by the sigmoid sinus and superiorly by the transverse sinus. The craniectomy begins with two or three closely approximated burr holes. The burr holes are joined up with rongeurs, creating a craniotomy window. The bone fragments are collected and stored in sterile antibiotic-saline solution for replacement in the cranial defect at the end of the procedure.

Development of the craniectomy anteriorly usually opens the mastoid air cell system to a variable degree. When the bony craniectomy is complete, the opened mastoid air cells are sealed with bone wax. Wax is also used to control bleeding from diploic bone at the craniotomy margins. Many styles of dural opening are described in the literature. We use a posteriorly based dural flap to enter the posterior fossa. The posterior fossa dura is opened 2 to 3 mm from its junction with the sigmoid and transverse sinus dura and at a similar distance from the inferior bony margin. The dural flap is reflected posteriorly. Small, relaxing incisions are made superiorly and inferiorly in the marginal dura to create small anterior and superior dural flaps, which are retracted with stay sutures, completing the dural opening.

Exposure of Cerebellopontine Angle

When the dural flap has been reflected posteriorly, it and the craniotomy margins are covered with moist Telfa strips. To drain CSF from the cisterna magna, the cerebellum is gently retracted superiorly with a polytef (Teflon)-coated malleable retractor. The arachnoid of the cistern is lanced with a bayoneted suction tip, which decompresses the posterior fossa, relaxes the cerebellum, and allows it to fall away medially. Premature medially directed cerebellar retraction, before draining the cisterna magna, risks inducing massive cerebellar swelling. After this maneuver, the retractor is withdrawn and repositioned anteriorly to develop posteromedial cerebellar retraction. Retraction in this manner, accompanied by division of arachnoid bands and bridging veins, opens the CPA. The degree of CPA exposure required varies with the size and location of the tumor being addressed. Superiorly, the petrosal veins (or Dandy’s veins), which lie just below the tentorium cerebelli and run parallel to the course of the trigeminal nerve, may hinder exposure or appear to be in jeopardy of tearing with retraction. When necessary, these may be coagulated and divided. After their division, the superior pole of the cerebellar hemisphere falls posteromedially away from the tentorium, providing access to the superior aspect of the CPA.

The cerebellar flocculus often overlies the brainstem root entry zones of CN VII and VIII, and must be gently mobilized from the cerebellar peduncle and lateral pontine surfaces. A tuft of choroid plexus, emanating from the lateral recess of the fourth ventricle, is also frequently encountered in this area. Mobilizing these structures from the root entry zone need not be performed during hearing conservation procedures when the proximal portion of the nerves are not involved with tumor because this maneuver places the internal auditory artery at risk. The cranial nerve electrophysiologic monitoring circuitry is tested by stimulating CN XI, which is usually readily accessible at the inferior pole of the exposure. Particular attention is paid to the location of the anterior inferior cerebellar artery (AICA) and its branches. Inferiorly, the posterior inferior cerebellar artery may be seen in relation to the lower cranial nerves, and superiorly the superior cerebellar artery may be identified coursing through the region of the tentorial notch. In acoustic neuroma surgery, we prefer to begin the drill excavation of the IAC at an early stage, before extensive opening of the arachnoid planes above and below the CPA component. This method helps reduce bone debris contamination of the subarachnoid space.

Exposure of Internal Auditory Canal

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