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.

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

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.

Exposure of Internal Auditory Canal

Exposure of the IAC and its contents involves removal of the bone surrounding the posterior, superior, and inferior aspects (Figs. 50-3 to 50-6). Optimal canal visualization may be obtained through a combination of rotation of the operating table away from the side of the surgeon and microscope positioning. These maneuvers bring the posterior petrous face into view centered over the region of the IAC. To locate the canal, the opening of the meatus is gently probed with a blunt, right angle hook. Before IAC opening begins, the operative field is set up to contain as much bone debris as possible and to prevent its dissemination into the subarachnoid space. Absorbable gelatin sponge (Gelfoam) pledgets are placed into the superior and inferior portions of the CPA. A rectangular-shaped rubber dam is fashioned from a surgical glove, placed over the occluding pledgets, and held in place with the cerebellar retractor.

FIGURE 50-3 Step 1 of exposure of internal auditory canal (IAC) during retrosigmoid approach. After localization of porus acusticus through palpation with a ball hook, an H-shaped dural incision is created over the long axis of IAC. Dural flaps are reflected anteriorly and posteriorly to expose posterior aspect of petrous pyramid. Dura usually can be dissected from posterior surface of endolymphatic sac. To maximize the possibility of hearing preservation, care must be exercised to avoid avulsion of sac from its aqueduct. Before drilling, Gelfoam pledgets are positioned above and below 7 to 8 neurovascular bundle in posterior fossa in an effort to minimize the spread of bone dust onto arachnoidal surfaces.

FIGURE 50-4. Step 2 of exposure of internal auditory canal (IAC) during retrosigmoid approach. A cutting burr is used to excavate rapidly bone overlying IAC. When canal dura is encountered, a diamond burr is used. Drilling is carried out from medial to lateral (from porus to fundus), in part to minimize the possibility that drill could accidentally run into posterior fossa.

FIGURE 50-5. Step 3 of exposure of internal auditory canal (IAC) during retrosigmoid approach. A diamond burr is used to create deep troughs around IAC that should extend well deep to canal plane to provide adequate room for microdissection with the angled instruments needed to remove facial nerve from tumor safely. Particular care should be exercised while superior trough is developed because facial nerve may lie immediately beneath dura in this location.

FIGURE 50-6. Step 4 of exposure of internal auditory canal (IAC) during retrosigmoid approach. In preparation for tumor removal, dura of IAC is incised. After an incision along the length of the canal is created with upbiting scissors, two small relaxing incisions are created at porus and fundus to develop dural flaps. These are reflected anteriorly and posteriorly to expose canal contents.

An H-shaped dural incision, centered on the long axis of the IAC, is outlined on the posterior petrous face by use of a bipolar cautery. When the dura has been incised with the tip of a No. 11 blade, superior and inferior dural flaps are elevated with a small Lempert mastoid elevator. The surgeon should exercise caution when incising inferiorly because the jugular bulb is occasionally dehiscent on the posterior petrous face. Similarly, the incision should not be carried too far laterally because laceration of the sigmoid sinus can occur. Care is taken to identify and preserve the endolymphatic sac and duct, which are located posterolaterally. The dura usually can be elevated off the endolymphatic sac. The entry point of the vestibular aqueduct into bone is a useful anatomic landmark. When the bony dissection of the IAC does not extend lateral to the operculum of the aqueduct, the labyrinth is unlikely to be breached.

The posterior IAC wall is rapidly removed by the drilling of a trough over the posterior petrous face. Drilling from medial to lateral in the line of the IAC reduces the risk of the burr slipping into the CPA. The canal should be opened only as much as required to expose the lateralmost aspect of the tumor. Excessive bony opening does not enhance exposure further, but may increase the risk of CSF leak through the opening of additional petrous air cells. Initially, the bone is removed with a cutting burr until the IAC dura is identified through a thin bony plate. To expose the dura of the posterior aspect of the IAC, the dural cuff of the meatus is first elevated from the thin residual plate. Then the remaining bony shell over the posterior aspect of the IAC is drilled away. To reduce the risk of traumatizing the IAC dural lining or its neural structures, removal of the last eggshell of bone is accomplished with diamond burrs. Diamond burrs are more controllable by virtue of their reduced tendency to run, and are less likely to cause injury if they come into contact with soft tissue structures.

Bony troughs, 3 to 4 mm in diameter, are developed above and below the canal. These troughs are important for three reasons: (1) to provide working space for the insertion of angled instruments needed to establish a plane of dissection between the tumor and the facial and cochlear nerves, (2) to permit visualization of the facial nerve when it is acutely angled superiorly or inferiorly as a result of tumor displacement, and (3) to enhance exposure of the anterior aspect of the CPA.

In preparation for the drilling of bony troughs around the canal, the IAC dura is elevated from the upper and lower canal walls with a blunt dissector. The troughs, which should be widest at the level of the porus, are excavated with a cutting burr. As the troughs are developed, a thin shell of bone is left over the dura of the superior and inferior walls of the IAC. When the troughs are fully developed, the remaining bony shells are progressively thinned with the side of the diamond burr until the dura is exposed. Copious irrigation is used to prevent thermal injury to neural structures. Often, the IAC dura can be gently retracted with a fenestrated suction, permitting completely atraumatic removal of the remaining bony eggshell fragments, which can be elevated from the exposed IAC dura. This technique exposes 180 to 270 degrees of the IAC circumference.

Caution must be exercised in development of the superior and inferior troughs because of the proximity of the facial nerve and the jugular bulb. The width of the inferior trough varies with the location of the jugular bulb. When the jugular bulb is unusually high, creating an inferior bony trough at the level of the meatus may be impossible, although exposure of the fundus is typically unhindered. Compensating for the limited inferior access associated with a high jugular bulb is usually possible through creation of an unusually wide and deep superior trough. Additional exposure of the IAC from above may also be gained through retraction of the tentorium.

In hearing conservation attempts, the lateral extent of the IAC opening should be restricted to approximately the medial two thirds of the IAC because opening of the lateral one third to expose the fundus may result in a breach of the vestibule or crus commune, militating against hearing conservation. The decision as to how far laterally the IAC is opened depends on the lateral intracanalicular extent of the tumor, which may be accurately predicted from the preoperative gadolinium-enhanced MRI.^16,23 Alternatively, the lateral opening can be limited on the premise that an indirect inspection and clearance of the tumor from the lateral IAC can be satisfactorily achieved. This method has the attendant risk of leaving residual tumor in the lateral IAC, however. To avoid this problem, some surgeons have advocated blind curettage using special right angle curettes followed by inspection of the fundus with a small mirror or endoscope to validate the extent of tumor resection.^24,25

With these methods, distinguishing residual tumor from the transected vestibular nerves and traumatized dura is sometimes difficult. Dissection of tumor from the fundus without direct visualization risks leaving well-vascularized residual tumor with the potential for clinically significant recurrence.^26,27 We advocate exposure of the IAC laterally to a point beyond the tumor interface, where the naked CN VII and residual CN VIII may be visualized. This process sometimes may require opening the canal to the fundus, with resultant entry into labyrinthine structures and sacrifice of residual hearing. It has been proposed that enhanced visualization of the fundus can be achieved by skeletonization of the posterior and superior semicircular canals.²⁸ Similarly, it has been shown that partial resection of the posterior semicircular canal may be helpful in augmenting fundal exposure.²⁹

After completion of the IAC exposure, the rubber dam and Gelfoam pledgets are removed. The dura of the IAC is opened along the long axis of the canal with sharp, upturned, right angle microscissors working from a medial-to-lateral direction. This incision is placed slightly eccentrically and is biased to the superior side to avoid the creation of a long flap over the facial nerve course. The dural flaps are reflected superiorly and inferiorly, exposing the IAC contents.

Acoustic Neuroma Resection

Attention is now turned to planning the actual resection of the tumor; the size of the tumor largely dictates the actual sequence and pattern of removal (Figs. 50-7 to 50-9). We prefer to dissect the IAC initially because this step helps ascertain the probable course of the facial nerve outside of the porus into the CPA, and allows early identification of the facial nerve. A test run of the neural monitoring system can be performed in which positive identification of the nerve by its anatomic relationships is possible. In many cases, ascertaining whether the tumor has arisen from the superior or inferior vestibular nerve is possible. When only one of these nerves is visible on the posterior surface of the tumor, it may be assumed that the other was the nerve of origin.

FIGURE 50-7 Step 1 of removal of acoustic neuroma via retrosigmoid approach. After intracanalicular portion of tumor is debulked, lateralmost extension of tumor is reflected medially, and plane between tumor capsule and facial nerve is developed. This maneuver ensures complete removal of tumor from fundus. It also affords an early opportunity to confirm function of cranial nerve monitoring system by stimulation of distal facial nerve under direct vision in a region where it is characteristically not especially adherent to tumor surface.

FIGURE 50-8. Step 2 of removal of acoustic neuroma via retrosigmoid approach. Main portion of tumor in cerebellopontine angle is rapidly debulked. To facilitate rapid and safe tumor removal, we use a Cavitron ultrasonic aspirator. Tumor capsule is first liberated from cerebellum and middle cerebellar peduncle. Pontine surface, including root entry zones of CN VII and VIII, can be exposed. In larger tumors, lesion also must be microdissected from trigeminal and lower cranial nerves (IX and X).

FIGURE 50-9. Step 3 of removal of acoustic neuroma via retrosigmoid approach. Characteristically, facial nerve is most adherent to tumor capsule between brainstem surface and anterior lip of porus acusticus. Liberation of nerve from tumor surface in this location often requires particularly delicate microdissection techniques.

FIGURE 50-10. Closure of internal auditory canal defect at completion of retrosigmoid craniotomy. After waxing of cut bony walls to seal any transected air cells, muscle graft harvested from nuchal area is mortised into bony defect. Graft is retained in position by sutures, which are anchored in dural flaps previously developed from posterior petrous surface.

Dissection is begun laterally by identification of the plane between the facial nerve and the tumor. A fine-tipped dissector is insinuated between the superior dural leaf of the canal and the tumor while gentle downward pressure and a rotating motion are applied. Gradually, this process brings into view the interface between the facial nerve and the lateral end of the tumor. When this plane has become established, a sharp, right angle instrument is used to dissect the tumor from the posterior surface of the facial and cochlear nerves. All tissue superficial to this plane, including the tumor and both vestibular nerves, is transected either with curved microscissors or through an upward motion with the sharp edge of the dissector. When the intracanalicular tumor component is bulky, it may require debulking to a variable degree to permit microdissection of the capsular peel from the facial and cochlear nerves. This initial dissection of the intracanalicular portion should proceed only to the lip of the porus acusticus or just beyond it, to avoid dissection of the typically most adherent section at this stage. It is important to avoid inducing neurapraxic injury, which might impair later electric identification of the facial nerve medially at its brainstem exit.

After removal of the intracanalicular portion of the tumor, the CPA component is addressed. The posterior capsule is swept with the neural monitoring probe to ensure that the facial nerve is not on this surface (a rarity in acoustic neuroma). A rectangular incision is made in the posterior capsule with the point of a No. 11 blade, and the peel is resected with scissors. Intracapsular debulking may be done with cupped forceps, sharp dissection with scissors, an ultrasonic aspirator (e.g., Cavitron), a rotatory aspiration device (e.g., House-Urban), or the surgical laser. We favor using the CUSA because it efficiently removes the tumor core while respecting its capsule, avoiding potential injury of adherent nerves and vessels. Tumor resection proceeds with alternate intracapsular debulking, followed by microdissection of the thin capsule from the brain surface and cranial nerves, and, ultimately, resection of the liberated capsular segment.

The most crucial aspect of CPA tumor removal is identification and preservation of the cranial nerves and blood vessels that lie draped on the capsular surface. In larger tumors, the medial tumor brain dissection plane begins posteriorly along the middle cerebellar peduncle. When this arachnoid plane has become established, it is gradually developed onto the lateral surface of the pons. Attention is turned inferiorly to the probable root entry zone of the seventh and eighth cranial nerve complexes. As an aid to facial nerve identification, electric stimulation is periodically performed along the meniscus of dissection. The course and appearance of the nerve vary depending on its displacement by the tumor. It may be thinned and fanned to a variable degree, making it difficult to delineate from surrounding thickened arachnoid tissue without the use of the microneural stimulator.

The brainstem entry of CN VIII is usually encountered lateral to and immediately above the seventh cranial nerve entry zone. A small branch of AICA typically passes between the two nerves and may be a useful guide in orienting the surgeon. In hearing conservation approaches, the vestibular fibers must be separated from the cochlear fibers and divided proximally to establish a tumor dissection plane. When no effort is being made at hearing preservation, CN VIII may simply be transected, a maneuver that simplifies identification of the proximal seventh cranial nerve. When the proximal plane over these two nerves is established, an arachnoid plane can be developed between them and the tumor capsule. While the tumor capsule is dissected, large and small arteries, potential AICA branches, are meticulously preserved. Vessels directly entering the tumor capsule can generally be safely coagulated and divided at the capsular surface without adverse consequences.

While the tumor neural plane is dissected, use of the microneural stimulator (e.g., Xomed Treace-Yingling) with a curved, pliable wire allows blind stimulation of the yet undissected anterior capsule. By localizing the facial nerve course before dissecting the tumor nerve interface, the surgeon may rapidly resect uninvolved capsule and direct meticulous efforts along the actual course of the nerve. Although the course of the facial nerve varies, it characteristically lies anterior to the tumor, occasionally with an anterosuperior or anteroinferior bias. In small tumors, the entire dissection may be accomplished from a medial-to-lateral direction. In larger tumors, medial-to-lateral dissection becomes difficult when the nerve is anteriorly angulated toward the porus acusticus. When this occurs, we return to the lateral tumor nerve interface at the end of the IAC and work medially.

Alternatively, the anterior tumor capsule with attached nerve may be lifted and rotated to bring the facial nerve course into the surgeon’s view. This action is quite traumatic to the facial nerve, however, and risks disruption of its attenuated fibers. We prefer to dissect the tumor from the facial nerve in situ without mobilizing it from its bed, where it lies supported by an arachnoidal mesh. When the facial nerve is splayed and tightly adherent to the tumor capsule, removing the last remnant of capsule may be impossible without disruption of the nerve.³⁰ In such cases, we prefer to perform a near-total removal, leaving a thin velum of capsule, only 1 to 2 mm thick, attached to the nerve. We believe that this minuscule residual capsule, hanging free in the CPA, is unlikely to generate a recurrent tumor.²⁶ By contrast, tumor left in the distal IAC or in contact with brainstem possesses a vascular supply, and the possibility of regrowth is greater.

Several modifications in the strategy of tumor removal are used during hearing conservation approaches. The direction of dissection should be from medial to lateral, whenever possible, to reduce the risk of traumatic avulsion of the delicate cochlear nerve fibers from their entry into modiolus. Throughout the cochlear nerve dissection, changes in auditory brainstem responses relative to the previously recorded baseline waveforms obtained at the start of the procedure are reported. Continuity of the cochlear nerve is maintained if possible; however, tumor adherence to it may necessitate its resection. Even when the cochlear nerve is well preserved during dissection, hearing is often lost because of interruption of the cochlear blood supply. This may occur either in the CPA, where the labyrinthine artery branches from a loop of AICA, or in the IAC, where it courses between the inferior vestibular and cochlear nerves.

After tumor resection, anatomic and electrical continuity of the cochlear and facial nerves are checked. The facial nerve stimulation threshold voltage at the root entry zone and the intraoperative auditory brainstem response waveform pattern and latencies are recorded. We believe that electrophysiologic monitoring of the auditory nerve is not clearly beneficial, other than in the prognostic sense, in the maintenance of hearing. Monitoring of the facial nerve is indispensable, however, if an optimal outcome is to be obtained.

DRESSING

After closure of the wound, it is cleaned, dried, and covered with a Telfa strip, to which a sterile adhesive, Op-Site, or similar dressing is applied. To discourage subcutaneous accumulation of CSF, a mastoid-type padded pressure bandage is applied. The dressing is removed 48 hours after surgery, and the wound is inspected and left open to the air. The skin sutures are removed 7 to 10 days after surgery.

POSTOPERATIVE CARE

The anesthesiologist awakens the patient, ideally with a smooth extubation that avoids straining and coughing. Antiemetics are given prophylactically to prevent vomiting, which could cause aspiration and associated pneumonitis during recovery from anesthesia. Postoperative monitoring is carried out initially in the postanesthesia care unit and then in the neurosurgical intensive care unit for 24 hours after surgery. After the initial 24-hour period, patients spend an average of 5 to 6 days on a hospital unit staffed by nurses experienced in postcraniotomy care. In addition to the monitoring of temperature, cardiorespiratory status, consciousness level, and fluid balance, the nursing staff and patients are instructed to identify and report any CSF wound leakage or rhinorrhea.

If a postoperative facial palsy is present, its grade is recorded according to the House-Brackmann scale, and preventive eye care is instituted. When eye closure is incomplete, artificial tears are applied hourly, or more often as needed, while the patient is awake. During sleep, a plastic eye shield is placed to prevent drying and development of corneal abrasion. Special attention needs to be directed toward patients with dysfunction of the facial and trigeminal nerves. When the cornea is dry, exposed, and insensitive, early gold weight placement is performed even when facial nerve recovery is expected.

Moderate to severe headache for several days is typical and may require narcotic analgesia for a variable period. Global headache that is delayed in onset by several days may signify the evolution of aseptic or bacterial meningitis and is discussed later. We try to wean patients from narcotics quickly and try to get their headaches under control with nonsteroidal anti-inflammatory preparations. In the few cases in which corticosteroids have been used, they are tapered over a 7 to 10 days. Vertigo can be controlled with parenterally administered antivertiginous agents, if the condition is severe, and with oral agents, if it is mild. We generally avoid vestibular suppressants in the postoperative period because they may retard vestibular compensation.

Diet and increasingly independent mobilization are encouraged under the guidance of a dietitian and physical therapist. We usually restrict the fluid intake for 3 days to a total of 1.5 L/24 hour period. Most patients are able to start a light diet 24 to 48 hours after surgery. Constipation and straining are avoided by the administration of stool softeners to prevent aggravation of headache and possible development of CSF leakage. Most patients begin mobility around 48 hours after surgery, although they have been encouraged to exercise their legs actively while they are recumbent in bed to reduce the risk of deep venous thrombosis. Antiembolism stockings are used until the patient is mobile. Mobilization usually takes the form of initially sitting at the bedside chair, followed by accompanied walks to the bathroom and then farther afield to the hospital corridors, and then onto a trial of practice on the stairs. Walking aids are provided by the physical therapist as required by the patient, depending on his or her progress. We usually discourage hair washing until 1 week after surgery to prevent the wound from getting wet and macerated. Patients may use a dry shampoo if desired.

Most patients are usually ready for discharge 5 to 7 days after surgery. Even when all other functions have recovered fully, easy fatigability often persists for 1 to 3 months postoperatively. The convalescent period required before returning to full-time employment and all the previous activities of daily living varies, but is usually 2 to 3 months.

RESULTS

Historically, the primary issue in acoustic neuroma surgery was the survival of the patient. With the evolution of microsurgical techniques, mortality from acoustic neuroma surgery has become very low—less than 2% in most recent series. Contemporary emphasis includes tumor control and, particularly, functional preservation. Before the data from our own experience and the data published in the literature are addressed, however, it is important to appreciate that limited international standardization exists in the criteria used for reporting results on degree of resection,³¹ facial nerve function,³² and hearing preservation.³³

In our opinion, the goal of acoustic neuroma resection should be tumor control and not complete resection in every case. Nevertheless, we perform a complete removal in most cases. Incomplete removal can be considered in two categories: subtotal and near-total excision. Subtotal removal, in which a substantial bulk of tumor remains, used to be reserved for elderly or infirm patients with a short anticipated life span in whom shortening of the operative procedure is thought to be in the patient’s interest. In recent years, it is increasingly used in large tumors, backed up by stereotactic radiation if the remnant grows. As previously discussed, near-total excision, in which a thin peel of capsule is left on the most adherent portion of the facial nerve, is occasionally used. Although few data are published on the recurrence risk for this group of patients, we have observed many individuals with serial gadolinium-enhanced MRI and have found a 3% risk of recurrence. The decision to undertake a near-total resection depends on the patient’s age (i.e., less desirable in a younger individual) and preference as to whether the slightly higher risk of recurrence is justified by the improved facial nerve outcome.

There are numerous articles in the literature on the subject of facial nerve preservation in acoustic neuroma surgery citing varying degrees of success. Because these results are difficult to compare and draw conclusions from, we confine our commentary to our own series. In our experience, facial nerve outcome from the retrosigmoid approach is similar to that from the other methods of removing acoustic neuromas for tumors of similar size.³⁴ In our acoustic neuroma patients, anatomic continuity of the facial nerve was maintained in 99.2% of cases. Anatomic continuity does not imply functional integrity. The probability of a grade I or II facial function at 1 year after surgery in the context of tumor size was 100% for tumors less than 1 cm; 90% for tumors 1 to 3 cm; and 82% for tumors greater than 3 cm.

With regard to hearing preservation, most published series address residual “measurable” hearing in contrast to the much more relevant concept of “useful” hearing.³⁵ For a patient with a unilateral acoustic neuroma, it could be argued that unless the conserved hearing maintains an interaural difference of less than 30 dB hearing loss with good speech discrimination (>50%), it would be likely to be beneficial. Preservation of useful hearing has been reported to be achieved in 25% to 58% of hearing conservation candidates.¹³ Very little information is available on the long-term follow-up of patients with preserved hearing. In two published series, significant late decline occurred in 22% to 56% of ears with successful hearing conservation.^36,37

Factors relevant to success in hearing conservation approaches to acoustic neuroma include tumor size in the CPA, the depth to which the tumor penetrates the IAC, pure tone hearing level, and auditory brainstem response results. A full discussion of these criteria is beyond the scope of this chapter. It is not yet well established whether intraoperative auditory monitoring materially improves hearing conservation results. In one study using auditory brainstem response monitoring, it was found to be of marginal benefit overall, with the possible exception of tumors less than 1 cm in diameter.³⁸

Several retrospective studies have compared hearing preservation rates after the retrosigmoid and middle fossa approaches.^39–41 In each study, the middle fossa approach yielded significantly better hearing results. Although that choice has not yet been universally accepted, the trend among centers undertaking a large volume of acoustic neuroma surgical procedures is to use the middle fossa approach as the preferred means of attempting hearing conservation. In our institution, the upper limit on the use of the extended middle fossa approach is a tumor in the 15 mm range of extracurricular diameter. The retrosigmoid approach is reserved for acoustic neuromas when three conditions are met: (1) excellent hearing, (2) a cisternal component 15 to 20 mm, and (3) no tumor involvement of the distal one third of the IAC. The retrosigmoid approach is still used for selected nonacoustic tumors of the CPA (e.g., meningiomas and epidermoids). Incomplete resection also has a potential role in hearing preservation, particularly in patients with neurofibromatosis type 2 or in patients with a tumor in an only hearing ear.^12,42

COMPLICATIONS

The common complications of the retrosigmoid approach to the CPA are persistent headache and CSF leakage.^11,43,44 Less common complications include (aseptic or bacterial) meningitis, hydrocephalus, cerebellar dysfunction, vascular compromise (thrombosis and hemorrhage), and problems associated with patient malpositioning during surgery. Medical complications, such as pulmonary thromboembolism and pneumonia, may also occur, but are not specific to surgery of this region. Although the potential complications of acoustic neuroma surgery are similar among the various operative approaches, their relative incidence varies considerably. In the retrosigmoid approach, persistent headache and CSF leakage occur more frequently than with the other technique used in approaching CPA tumors.

Acknowledgment

Figures 51-1 to 1-10 were adapted from artwork produced by the authors from Jackler RK: Atlas of Skull Base Surgery and Neurotology, 2nd ed. New York, Thieme. 2009. The original drawings in this chapter were produced by Christine Gralapp, MA.