Large Acoustic Neuroma and other Posterior Fossa Tumors Expanded Transtemporal Approaches

Publication: Fishman AJ, The Expanded Transtemporal Approach for Large CPA Lesions in Master Techniques in Otology Neurotology Roland JT [Ed.] Wolters Kluwer 2019.

INTRODUCTION

The expanded transtemporal approach is intended to address large cerebellopontine angle (CPA) lesions. It is a non–hearing preservation approach, indicated for surgical treatment of tumors in which total or near-total surgical extirpation is the primary objective.

This operation can be conceptualized as a wide translabyrinthine dissection with the addition of the obliteration of the middle ear and mastoid and a facial nerve bridge. The ear canal becomes a blind sac and is oversewn at the initial stage. This technique does not necessarily include resection of the cochlea or routine skeletonization of the carotid canal. Although these modifications can be added to address extension of the disease into the cochlea and petrous apex, the more classic description of the transcochlear approach would be more appropriate for these conditions, which may also require transposition of the facial nerve. I have found, however, that the majority of large CPA tumors are amenable to the expanded transtemporal approach and do not require a complete transcochlear dissection, thus sparing time and the blood supply of the facial nerve.

The advantages over a more traditional translabyrinthine approach are in the expanded angle of view. The view can be particularly expanded posteriorly with the loss of the obstructing canal wall, as well as anterosuperiorly toward Meckel cave, and over the superior aspect of the internal auditory canal (IAC). This results in a generalized increased illumination to the surgical field because the canal wall structures block a considerable quantity of delivered light. When appropriate, the petrous apex can also be accessed by additional removal of the cochlea and delineation of the carotid artery.

Optimum outcomes with regard to the extent of resection and preservation of neurovascular function often require prolonged operative times. Therefore, the expanded transtemporal approach is often staged in two sessions. There are many important considerations when staging a procedure. The distribution of surgical segments and timing of the stages must be individualized for the specific patient’s status and tumor characteristic in order to maximize safety and outcomes.

HISTORY

Magnetic resonance imaging (MRI) reveals many CPA lesions incidentally. A complex array of modern therapeutic options is available for the removal of small and medium-sized lesions. In such cases, the presence or absence of vertigo, the level of hearing, and the exact size and location are among the clinical parameters that influence the decision-making process. The expanded transtemporal approach is indicated for the largest of tumors.

These larger tumors may present with evidence of central nervous system (CNS) compromise, hydrocephalus, and regional cranial nerve deficits in addition to hearing loss. It is important to recognize their signs and symptoms. Additional measures, including external ventricular drainage, prolonged postoperative cerebrospinal fluid (CSF) diversion, and/or shunting, may be indicated for some patients. Careful collaboration with a multidisciplinary team is required for safe and effective care.

PHYSICAL EXAMINATION

All patients should be subjected to a comprehensive physical examination, including the cranial nerves and formal tests of hearing acuity. The examination should include flexible fiberoptic laryngoscopy when appropriate. A comprehensive neurologic examination is imperative to look for signs of brainstem compression and other forms of CNS compromise.

Careful examination of the facial nerve is necessary because there may be subtle or overt signs of impairment. In these cases, imaging must be carefully studied in order to assess the possibility of tumors of facial nerve origin and to determine whether or not facial nerve preservation is possible. Computerized tomography (CT) should be obtained to supplement MRI. In some cases of facial nerve involvement, the procedure may include either concomitant or staged reanimation.

Corneal reflexes and testing of trigeminal nerve sensation is also important in these larger tumors, especially when there is preexisting or predictable facial nerve compromise. Regardless of whether the facial function is expected to recover, the presence of corneal anesthesia puts the affected eye at increased risk of abrasion and permanent visual loss if not addressed aggressively in the perioperative period.

INDICATIONS

The expanded transtemporal approach is intended to address large CPA tumors. It is a non–hearing preservation approach, indicated for surgical treatment of tumors in which total or near total surgical extirpation is the primary objective. It is typically used for removing large acoustic neuromas, meningiomas, epidermoid tumors, and any other larger lesion of the CP angle that either does not require extensive dissection into the petrous apex warranting a formal transcochlear approach or would otherwise be adequately served by a single-staged traditional translabyrinthine dissection. Ultimately, the selection of the approach is based largely on experience and preference of the cranial base surgery team, as many of the tumors encountered can be addressed by a variety of approaches discussed in this textbook.

CONTRAINDICATIONS

Although there are few absolute contraindications to surgery when large tumors are identified in the CPA, there are certainly relative risk factors that must be weighed and assessed prior to operating. Signs of acute CNS compromise may require urgent external ventricular drainage. There may be instances in which the patient is deemed not stable or healthy enough to undergo a gross or near-total resection. In these cases, an expeditious and limited translabyrinthine exposure with central debulking and decompression may be a wiser course of action to address immediate concerns.

PREOPERATIVE PLANNING

Imaging

Imaging is primarily by MRI. I include stealth protocol images compatible with neuronavigation systems. These require thin-cut axial images with no gantry tilt such that three-dimensional reconstructions can be formulated. It has recently become my practice to obtain CT angiography when possible. I have encountered a number of previously undetected vascular aneurysms and feel that these studies are important in the preoperative planning process when performing surgery around posterior, as well as the anterior circulation. The CT angiogram is also acquired with no gantry tilt, so they may be uploaded into navigation systems.

Strategy for Staging

Staging can be performed on consecutive days or with a delay of 7 to 10 days or longer between approach and definitive resection. There are advantages and disadvantages of both. My early experience with

consecutive day staging revealed that a fair number of patients can exhibit a partial facial palsy. This was thought to occur after the nerve was dissected circumferentially, leaving only a thin bone bridge and accompanied by cochlectomy. This was seen to occur despite lack of electromyogram (EMG) irritability and without evidence of direct trauma. I concluded that it was likely a devascularization phenomenon: an event that would not have been otherwise recognized in the absence of staged surgery and likely would have been attributed to neurapraxia from tumor dissection. Because the severity of postoperative palsy can progress, it was considered imperative to limit the time between stages so that good neuromuscular EMG monitoring could be performed during the definitive resection.

Counteracting this objective, however, is the matter of watertight closure of the oversewn external auditory canal. Despite best attempts, if the blind sac closure technique is exposed to an underlying CSF collection, it is vulnerable to breakdown and difficult to repair once this occurs. For this reason, there is a benefit to allowing the blind sac to heal, as well as allowing time for the eustachian tube closure to scar over.

Ultimately, I found that limiting the dissection around the stylomastoid foramen at the initial stage and avoiding a cochlectomy have essentially eliminated these early facial nerve effects. This allows the second stage to be postponed such that good healing can occur in the vulnerable areas.

In patients with sufficient CNS compromise, it may be preferable to proceed as a single stage or perform the surgery on subsequent days to balance benefits with risks. Regardless, the basic principles of the approach are maintained.

SURGICAL TECHNIQUE

Given the consistency of anatomical relations, temporal bone surgery should be conceptualized as a series of repeatable maneuvers performed in series. With experience, these maneuvers become routine, expeditious, and safe.

The initial step for almost all transtemporal procedures is a wide complete mastoidectomy (Fig. 26.1A). Though the intent is ultimately to enter the subarachnoid space, it is preferable, though not imperative, to avoid significant CSF exposure during the first stage. Once the mastoidectomy is complete, the dissection proceeds to remove the bone overlying the exposed middle cranial fossa, sigmoid sinus, as well as the pre- and postsigmoid posterior fossa. This dissection (Fig. 26.1B) can be termed a retrolabyrinthine drillout.

Clear identification of the facial nerve is important. It should be preserved within a thin intact layer of bone. The white and pink nerve sheath with its blood supply can easily be seen through translucent bone under adequate irrigation.

The endolymphatic sac and operculum will be encountered and transected to access the medial edge of the porus acusticus. The sigmoid sinus should be traced to its turn into the jugular bulb. Sufficient posterior fossa dura should be exposed in the postsigmoid region to adequately compress the structure posteriorly. Although the bone can easily be removed from the surface of the sigmoid sinus, I avoid excessive removal of bone over the jugular bulb, especially in the region where the sinus bends superiorly to join the bulb. This area is very adherent, and inadvertent entry occurs easily. This can, however, be promptly controlled with topical application of a small segment of muscle and directed gentle pressure until venous bleeding has stopped. As this may occur during the first surgical stage, it will be healed by the time the wound is reopened and will pose little risk of reopening. It is best to carefully drill this area with a diamond burr.

FIGURE 26.1 A. Complete mastoidectomy. B. Retrolabyrinthine dissection. dm, digastric muscle; es, endolymphatic sac; fn, facial nerve; in, incus; jb, jugular bulb; lc, lateral semicircular canal; mf, middle cranial fossa plate; pc, posterior semicircular canal; pf, posterior fossa dura; sp, superior petrosal sinus; ss, sigmoid sinus.

FIGURE 26.2 A. Labyrinthectomy. Note the five lumens: the lateral canal (lc) has two openings, one for the ampulated end anteriorly and a second posteriorly for the nonampulated end. The superior canal (sc) ampulated end is distinct and the nonampulated end connects to the common crus (cc). The posterior canal ampulated end (pc) is situated lower and deep to the facial nerve. Its nonampulated end also connects to the common crus but has been removed. This yields five identified openings at this stage in the dissection. B. The complete removal of the bone overlying the vestibule gives the appearance of a lower case letter “a.” It is helpful to sculpt the labyrinth in this fashion so that proper orientation can be maintained relative to the deeper structures.

Upon completion of this phase, a labyrinthectomy is performed (Fig. 26.2A and B). Subsequently, the remainder of the sinodural angle and IAC is delineated from the floor of the vestibular to the porus acusticus (Fig. 26.3). It is preferable to leave a thin layer of bone covering the IAC and to avoid extensive dissection anteriorly along the petrous ridge if a delayed second stage is planned. This will minimize the risk of gross dural violation. A CSF fistula at this time may increase the risk of breaking down the blind sac closure. A widened dissection along the petrous ridge and apex is executed at the outset of the second stage. The labyrinthine facial nerve is not delineated until the second stage, as I have found that if exposed, it tends to heal over with vascularized granulation tissue, which can impede visualization. A thin layer of bone is left covering the IAC for the same reason. This stage of the surgery will result in a nearly complete translabyrinthine drillout.

Next, the external auditory canal skin is transected and the canal wall is taken down so that the tympanic structures can be removed and the eustachian tube accessed (Fig. 26.4A and B). There is no need to carefully preserve the most medial skin. This skin is friable and contributes little to the integrity of the blind sac closure. I simply introduce a #15 blade across the immediately accessible canal skin and

cut firmly against the anterior canal wall. The lateral segment is elevated with a micro elevator and trimmed until a good tube is formed. The medial segment is removed and discarded.

The canal wall is taken down to the level of the facial nerve. Overly aggressive dissection in the region of the stylomastoid foramen should be avoided to minimize devascularization-induced palsy. In cases where additional dissection in this area is needed, it should be performed at the outset of the second stage.

The eustachian tube is scarified and obstructed with a segment of muscle tissue (Fig. 26.4C). It is helpful to gently force it with a small piece of surgical cellulose to gain some traction and deliver it in securely. The eustachian tube should be inspected for any overt carotid canal dehiscence and appropriate caution heeded.

The ear canal is then oversewn forming a blind sac closure (Fig. 26.5A and B).

The cavity can be filled with either gelatin foam or a free adipose tissue graft. A layer of gelatin foam is placed atop the exposed sigmoid sinus to prevent adhesion to the adipose tissue. The wound is closed in layers, and a mastoid pressure dressing is applied.

FIGURE 26.3 The medial dissection in the form of a nearly completed translabyrinthine approach. This is the form that should be taken prior to the transection of the external auditory canal. Note that the petrous ridge is preserved during stage one of the dissection (*). ca, cochlear aqueduct; fn, facial nerve; jb, jugular bulb; mf, middle cranial fossa dura; pf, posterior fossa dura; sp, superior petrosal sinus; ss, sigmoid sinus.

At the second stage of surgery, the wound is reopened and the adipose tissue material is gently removed and set aside in a sterile container. Care is taken not to remove excessive material that has scarred down over the eustachian tube, as this has already healed over to block the egress of CSF.

At this point, the IAC dissection is completed. The labyrinthine facial nerve is identified. The bony covering of the IAC is carefully removed. Dissection is extended along the petrous ridge above and into the medial petrous apex below the IAC (Fig. 26.6A).

FIGURE 26.4 A. The external auditory canal skin is transected and the canal wall is removed first with a bone rongeur. B. The canal wall is taken down to the level of the facial nerve, and the middle ear structures are then removed. C. The eustachian tube is obstructed with a segment of muscle (*) taken from the wound. eac, external auditory canal skin; fn, facial nerve; in, incus.

The posterior fossa dura is opened, creating a superior and inferior flap or “H” incision (Fig. 26.6B). These flaps are retracted apart with temporary sutures (Fig. 26.7A).

The tumor can be entered and centrally decompressed with a combination of ultrasonic aspiration, sharp dissection, and bipolar cautery (Fig. 26.7A). Once this has been accomplished, the tumor can be separated from the cerebellum and inferior pole. The lower cranial nerves are identified and preserved (Fig. 26.7B). The interface of the tumor with the brainstem should be explored while preserving its delicate vasculature until the root entry zone of the facial nerve is identified and stimulated (Fig. 26.7C). Dissection proceeds until all that remains is a distinct segment of tumor occupying the porus acusticus and IAC.

It is preferable to maintain an arachnoid layer between the dissection plane and the brainstem. If performed correctly, this will help to preserve the delicate surface vasculature. The facial nerve can still be stimulated at low amplitudes of monopolar current through the arachnoid layer for localization and confirmation of integrity. Great care should be taken to minimize cautery to the brainstem surface. In geographical regions where neurophysiologic monitoring systems are unavailable, the arachnoid layer technique can produce excellent results.

Ultimately, the lesion is delivered from the IAC with lateral to medial dissection. The facial nerve is connected with the segment identified and traced from the CPA (Fig. 26.8A and B). The integrity of the facial nerve is confirmed by stimulation at the brainstem root entry zone at the conclusion of dissection and hemostasis (Fig. 26.8C).

FIGURE 26.5 A. Absorbable polyfilament sutures are passed through the cuff of the external auditory canal skin and then pulled through with a surgical clamp. They are tied externally everting the skin tube. B. Additional sutures are passed on the medial side to secure the closure. C. A free adipose tissue graft is placed and gelatin foam is used to

protect the sigmoid sinus to prevent adhesion prior to wound closure. eac, external auditory canal skin; ss, sigmoid sinus.

A primary dural closure is not possible with these techniques. A sling suture is fashioned to support free adipose tissue grafts (Fig. 26.9A–C). The adipose tissue is covered with an anteriorly based pericranial flap fashioned at the initial stage of surgery. The wound is closed with an interlocking 3-0 nonabsorbable monofilament suture to secure a watertight repair.

FIGURE 26.6 A. Second-stage dissection. Completion of the IAC bone removal and identification of the labyrinthine facial nerve are shown. The petrous ridge is followed anteriorly as far as necessary (*). B. The “H” incision creating dural opening and flaps. fn, facial nerve; iac, internal auditory canal; lab fn, labyrinthine facial nerve; pg fn, postgeniculate facial nerve.

FIGURE 26.7 A. Intracapsular debulking with ultrasonic aspirator. B. Identification and preservation of lower cranial nerves. C. Stimulation of the facial nerve at the brainstem root entry zone. bs, brainstem; iac dura, internal auditory canal dural edge kept intact; jb, jugular bulb; lcn, lower cranial nerves; ss, sigmoid sinus; T, tumor; va, vertebral artery.

FIGURE 26.8 A. Intact arachnoid layer protecting brainstem and facial nerve. B. The last segment of tumor has been delivered from the opened internal auditory canal and dissected from the intracanalicular facial nerve. C. Integrity of the facial nerve is confirmed by stimulation at the brainstem by EMG monitoring. fn, facial nerve; iac, internal auditory canal tumor mass; jb, jugular bulb; pg fn, postgeniculate facial nerve; T, tumor.

FIGURE 26.9 A. Sling suture in place. B. Placement of a free abdominal adipose tissue graft. C. Pericranial flap in position.

POSTOPERATIVE MANAGEMENT

The postoperative course after the first stage is often similar to a routine mastoid surgery. Because most of these patients have already lost considerable vestibular function in the operated ear preoperatively, they rarely suffer from significant acute vertigo. A pressure dressing is maintained for approximately 48 to 72 hours, and so long as there are no other neurologic concerns, the patient can often be discharged pending the second stage.

Following definitive tumor resection, patients are maintained in the neuro intensive care unit until stable for transfer. When postoperative lumbar CSF diversion is employed, it is kept in place for 3 to 7 days to reduce CSF pressure. In our facility, patients with CSF diversion are preferably maintained in the ICU; however, there is no absolute contraindication to following these patients in an otherwise well- monitored setting. A mastoid pressure dressing is maintained for 72 hours. Patients are gradually advanced to chair and ambulation with assistance. Given the often-significant mass effect from these large tumors, perioperative steroids are usually administered. Efforts are made, especially in the absence of new neurologic deficits, to wean the patient off the steroids expeditiously, thereby helping to stave off the potential adverse effects on wound healing.

Postoperative MRI is obtained to confirm the extent of tumor removal. The images are repeated at 3 to 4 months to establish a baseline for future comparison. By this time, early postoperative signal changes have stabilized (Fig. 26.10A–D). These patients are typically monitored annually for a period of 5 years or more, after which time the scanning interval can be extended, especially if there is no evidence of residual or recurrent disease. The acquisition of fat suppression images will differentiate between adipose tissue graft and tumor, as they are both hyperintense on T1 contrast MRI.

COMPLICATIONS

Major complications are rare and include all the potential risks of intracranial surgery. Specific attention is paid to minimizing the risk of CSF leak after lateral skull base surgery, particularly with the very large tumors, and our surgical techniques have been modified over the years with this focus in mind. The expanded transtemporal approach was developed so that staging could be undertaken while still maximizing wound healing. Despite these preventative measures, approximately 5% of patients develop a postoperative CSF fistula either from the ear, the wound, and CSF rhinorrhea or by the development of pseudomeningocele.

In cases of CSF rhinorrhea or otorrhea, I usually employ a course of CSF lumbar drain diversions for a duration of 5 to 7 days, adjusted by response. This results in resolution for 87% of these patients. For cases of wound leak, I initially oversew to reestablish a watertight seal and reapply a pressure dressing. In some instances, lumbar CSF diversion is added, especially when presenting with a significant underlying pseudomeningocele or hydrocephalus.

RESULTS

Outcomes are typically judged by the extent of tumor removal and preservation of the facial nerve. A gross total resection can be expected in the vast majority of cases. Facial nerve anatomic preservation can be expected in greater than 90% of patients with very large tumors. Good to excellent functional outcome is seen in at least 90% of cases (Fig. 26.11A and B). Lower cranial palsies and strokes are rare. Hearing preservation is not attempted in these patients.

PEARLS

• Lateral skull base surgery can be conceptualized as a series of distinct dissections that are modified, combined, and expanded to address a specific tumor. The surgery is most safe and expeditious when performed as “steps,” keeping this principle in mind.

FIGURE 26.10 A. T2 axial MRI of a patient presented in the online video section. B. T1 contrast coronal MRI. C. T1 contrast sagittal MRI. D. Postoperative axial T1 contrast fat- suppression image.

Staging can be helpful to avoid surgeon fatigue and to achieve the best outcomes.
Allowing for adequate healing time between stages will minimize the risk of CSF fistula from the

oversewn ear canal.

Gentle preservation of the arachnoid plane will protect the delicate vascular supply to the brainstem and cranial nerves.

Keep the IAC dura intact and moistened until the final stages of the procedure. It can become desiccated and adherent to the underlying nerves during prolonged exposure to high-intensity microscope lamps and air.

PITFALLS

Very large tumors can attenuate the facial nerve fibers as they compress against the anterior porus acusticus. Do not hesitate to leave a very small remnant of tumor capsule on the attenuated nerve to avoid transection.
of the IAC too early will render all the traction forces onto the facial nerve. Even when the internal canal is opened, do not immediately transect the cochlear nerve, as it shares some of the force load when dissecting tumor directly off the facial nerve in a lateral to medial direction.
In the rare cases of facial nerve transection, it is best to acknowledge and treat promptly with a combination of regional nerve and muscle transpositions, as the earliest timing of these procedures is associated with best functional outcomes.

FIGURE 26.11 Patient presented in the online video section. Patient presented initially with ataxia and partial facial palsy. A. Eyes open examination recorded at 7-month follow-up. B. Eyes closed examination recorded at 7-month follow-up.

INSTRUMENTS TO HAVE AVAILABLE

A set of otology microinstruments is recommended. Rhoton or equivalent neurosurgical picks are often too long when using transtemporal approaches.
Ultrasonicaspirator.
Multichannel continuous EMG nerve monitoring system. Monitoring can be performed on the facial as

well as other cranial nerves.

A monitoring endotracheal tube can be used to assess integrity of the vagus nerve. This specific device however is not MRI compatible and is not used when intraoperative MRI is being considered.

Acknowledgement

I would like to acknowledge cooperation with numerous exceptional colleagues over the years. Skull base surgery is a team endeavor and requires good-spirited collaboration to be successful.

www.andrewfishmanmd.com

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Large Acoustic Neuroma and other Posterior Fossa Tumors Expanded Transtemporal Approaches