Surgery for Acoustic Neuroma: Retrosigmoid Approach
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Vestibular schwannomas (VSs), also called acoustic neuromas, account for 8% to 10% of intracranial neoplasms. More specifically, they account for more than 80% of the cerebellopontine (CP) angle tumors, making them the most common tumor in this location. Most VSs arise from one of the vestibular nerves, most commonly from the superior one.
VSs are slow-growing benign neoplasms arising from the transition zone between the central and peripheral myelin, a point of origin typically found in the medial aspect of the internal auditory canal (IAC), approximately 8 to 12 mm from the pial surface of the pons. As they grow, VSs typically cause dysfunction of cranial nerve (CN) VIII, followed by dysfunction of CN VII. As the tumor reaches a giant size, it causes brainstem and cerebellar dysfunction.
Two genetic forms of VSs have been described: sporadic VSs and those associated with neurofibromatosis type 2 (NF2). Sporadic VSs are unilateral and account for about 95% of all VSs, whereas bilateral VSs are pathognomonic of NF2.
NF2 is a result of mutation in the NF2 gene that codes for Merlin, a tumor suppressor found on chromosome 22. Mutations in the NF2 gene have also been found in sporadic forms of VSs, as well as other tumors such as meningiomas. Attempts to distinguish sporadic from NF2-associated VSs based on specific NF2 gene mutations have not been successful, nor have attempts to correlate specific mutations with NF2 disease severity or aggressiveness of sporadic VSs.
Histologically, these different forms of VSs are indistinguishable, with the exception of a cystic subtype of sporadic VSs that harbors a higher degree of cellular atypia. This subtype tends to have a more aggressive clinical course and poorer surgical outcomes because it tends to be more adherent to the surrounding neurovascular structures.
VSs are classified based on their size because tumor size is the most important prognostic factor; 2 cm is an important threshold size above which outcomes have been shown to worsen.
The most common presenting symptom of a patient with VS is gradual hearing loss, but occasionally hearing loss may be acute. Other common presenting symptoms include tinnitus and vertigo. Less common symptoms often indicative of large tumors include headache, facial numbness, ataxia, and nausea/vomiting.
Physical exam findings include unilateral sensorineural hearing loss. Large and giant tumors cause imbalance, abnormal corneal reflex, facial hypesthesia, nystagmus, and facial palsy. Abnormal extraocular muscle function and papilledema are less common and indicate the presence of hydrocephalus.
Otolaryngologic auditory testing assesses residual useful hearing and influences the choice of surgical approach. Pure-tone audiograms and speech discrimination testing are standard, commonly classified according to the Gardner-Robertson scale.
Auditory brainstem response testing can diagnose VSs, but it is less sensitive than magnetic resonance (MR) imaging, and therefore plays little role in the management of these tumors. The functions of the lower cranial nerves are formally established for large and giant tumors with inferior extension.
Diagnostic imaging includes a computed tomography (CT) scan to evaluate the bony anatomy that will be removed during creation of the surgical corridor. Recognition of the extent of pneumatization of the temporal bones and appropriate preparation for intraoperative repair can minimize the risk of a postoperative cerebrospinal fluid (CSF) leak. The position of the venous anatomy, specifically a high-riding jugular bulb or posteriorly displaced sigmoid sinus, may affect the choice of the operative corridor and the extent of bone removal. Tumor anatomy and its approximate relation to the nearby neurovascular structures are determined with MR imaging.
The relationship of the tumor to the basilar and posterior inferior cerebellar arteries may be appreciated; such vessels should be carefully protected if ultrasonic aspirator devices are used during surgery for tumor decompression. Despite extension of the large and giant tumors through the jugular foramen, they are microsurgically dissected off of the lower cranial nerves without significant risk.
The presence of edema in the brainstem indicates a high risk of brainstem pial violation during microdissection of the tumor capsule. If significant brainstem edema is evident, staging the surgery may be considered because the interval between the two stages allows the tumor to deliver itself into the resection cavity created during the first operative session. Subtotal removal is an alternative option to avoid brainstem injury.
VSs may be occasionally difficult to distinguish from CP angle meningiomas on MR imaging. Both are often homogeneously enhancing, but meningiomas are typically centered away from the IAC and have a broad dural attachment (tail) to the posterior aspect of the petrous ridge or tentorium. Meningiomas rarely originate in the IAC, and although they can spread secondarily into the IAC, the IAC is usually not widened. The angle between the tumor and the petrous pyramid is wide.
Centered around a widened IAC, VSs form an acute angle with the petrous pyramid and are likely to have a cystic component and be partially surrounded by a CSF cleft. The T2 sequences determine pial invasion (brainstem edema) or encasement of the vasculature.
The majority of patients do not require treatment for their hydrocephalus beyond tumor resection; however, a minority require postoperative ventriculoperitoneal shunting.
As expected, increased tumor size is a risk factor for the development of postoperative hydrocephalus. If symptomatic obstructive hydrocephalus is present, I prefer to place an external ventricular drain (EVD) at the time of the craniotomy with the goal of weaning the patient off the drain postoperatively or shunting if necessary.
Indications for Surgery and Treatment Options
Management options for VSs include observation with serial imaging, microsurgical resection, stereotactic radiosurgery (SRS), and a combination of resection and SRS. Small asymptomatic tumors or minimally symptomatic tumors in older patients and patients with high surgical risk may be observed with repeat MR imaging.
The majority of VSs will continue to grow, but their natural history is unpredictable, with some tumors remaining unchanged or rarely even regressing over time. Treatment is indicated for progressively enlarging tumors. If the tumor is small (<3 cm), SRS is a reasonable option that has demonstrated excellent tumor control with low treatment-related morbidity.
Patients harboring recurrent or residual tumors are referred for radiosurgery rather than reoperation. Outcomes for facial nerve function and hearing are more favorable with radiosurgery than with traditional open surgery. Radiosurgery has recently been advocated as the primary therapy for larger tumors, but reliable data currently do not exist about the outcomes of radiosurgery for this specific population
Tumors causing symptoms or neurologic dysfunction secondary to mass effect are surgically resected with the goal of radical subtotal removal to preserve facial function. This form of resection supplemented by SRS for a growing residual tumor is an emerging paradigm for the treatment of skull base tumors.
In summary, I have a low threshold for the use of radiosurgery in the setting of small and medium sized tumors (<3cm) if the mass effect is not considered prohibitive for radiosurgery. Very young patients with medium size tumors fair better from microsurgery long-term. Large and giant tumors deserve surgical removal. Patients with NF2 present a daunting challenge in their management as the goal is to protect some hearing long-term; their care has to be individualized.
Routine use of intraoperative facial nerve electromyography (EMG) and brainstem auditory evoked response potentials (BAERs) monitoring to identify and monitor facial nerve and cochlear nerve/brainstem integrity, respectively, is important. The significant attenuation and atrophy of the facial nerve in giant tumors, as apparent from preoperative facial weakness, often complicates mapping the exact location of the nerve splayed over the tumor capsule.
Staging the operative session should be strongly considered and preplanned for giant tumors. Fatigue of the surgeon and the operative team during the later (and more critical) parts of the operation is an important factor in poor outcomes.
A high-riding jugular bulb close to the IAC should be recognized on preoperative imaging and the surgeon should be cognizant of this anatomic finding when drilling the IAC.
The neural anatomy of the posterior fossa, especially near the porus acousticus, is relevant to microsurgery of VSs.
What is the Best Surgical Approach in VS Surgery?
VSs have been resected via the translabyrinthine (TL), middle cranial fossa (MCF), and retrosigmoid (RS) approaches, with the choice of approach based on the size/location of the bulk of the tumor, status of the patient’s hearing, and the preference of the operating surgeon. Other factors that play a role in selection of the approach include the patient’s age and overall health status, the anatomy of the vestibule and cerebellopontine (CP) angle, and involvement of the brainstem and the internal auditory canal (IAC). The RS approach is favored by neurosurgeons and offers an opportunity for hearing preservation while tackling large tumors via a panoramic view of the CP angle.
VSs have been stratified into the following tumor size/location categories: intracanalicular (IC), <1.5 cm (small), 1.5 to 3.0 cm (medium), and >3.0 cm (large). The MCF option may be superior to RS surgery for hearing preservation in patients with tumors <1.5 cm. Some neurosurgeons argue that the RS corridor results in less neurologic dysfunction during resection of the IC tumors than the MCF route. The RS pathway is potentially associated with less dysfunction than the MCF or TL approaches for tumors that are 1.5 to 3.0 cm.
Postoperative headaches tend to be more prevalent after the RS than TL route. The risk of CSF leak is greater following the RS craniotomy than with either the MCF and TL approaches. Some investigators have reported that the incidences of mortality, major noncranial nerve neurologic complications, residual tumor, tumor recurrence, and dysfunction of other CNs are not significantly different with the various approaches. There are no class I data that strongly support any one approach over another.
In summary, the MCF route appears to be safest for hearing preservation among smaller tumors. The RS trajectory is potentially the most versatile corridor for most tumor sizes, but it is associated with a higher risk of postoperative pain and CSF fistula. Finally, the TL osteotomy is associated with complete hearing loss, but may be beneficial for patients with large tumors and poor preoperative hearing.
Venous anatomy also plays a role in selection of the surgical approach. A high-riding or anteriorly located sigmoid sinus favors the RS over the TL route. Younger patients with “full” cerebellum and large tumors can benefit from the TL osteotomy because there is less need for cerebellar mobilization and retraction during this approach.
Opponents of RS surgery quote cerebellar retraction as a risk for postoperative ataxia and maintain that this approach carries a higher incidence of postoperative headache. This approach may also provide only a limited visualization of the fundus of the IAC, which may necessitate blind dissection to remove the entire tumor. However, the RS approach is a versatile route for tumors of any size regardless of the patient’s preoperative hearing status.
In the modern era, when the goal of zero mortality and minimal morbidity is achievable, hearing conservation should be pursued; however, with large and giant tumors, hearing preservation is the exception rather than the rule.
RESECTION OF VESTIBULAR SCHWANNOMA VIA A RETROSIGMOID CRANIOTOMY
For moderate size tumors, I place a lumbar drain to facilitate relaxation of the cerebellum during the dural opening. For very large tumors causing significant obstructive hydrocephalus, an external ventricular drain is installed. This early release of CSF avoids cerebellar herniation that commonly occurs because of the mass effect of VSs.
The patient may be positioned in the sitting/semisitting or horizontal (supine, oblique, park bench) positions. The sitting position is associated with a risk of venous air embolism; precordial Doppler and end-tidal CO2 monitoring are used.
Some operators report less blood loss, shorter operative time, and lower risk of cranial nerve dysfunction with the sitting position. This position facilitates a clear surgical field via gravity drainage of blood and CSF. The use of suction device and bipolar coagulation are therefore minimized around the cranial nerves. In addition, atraumatic bimanual microdissection can be accomplished using two microforceps to disconnect the fine arachnoid bands between the tumor and the encasing sheaths of the nerves and pia.
Bimanual microdissection provides a great advantage without the use of bipolar electrocautery that can place the facial nerve at risk. However, the sitting position requires nonergonomic posture of the surgeon’s arm, which leads to early fatigue. Most anesthesiologists are not comfortable with the sitting position, and therefore its use is limited in the United States.
I avoid the supine position due to the risk of neck stiffness associated with long operative sessions. I prefer the park-bench position. For technical nuances of patient positioning, exposure and dural opening, refer to the chapter on Extended Retrosigmoid Craniotomy.
A large retromastoid craniotomy/ectomy extends along the edges of the transverse and sigmoid sinuses. Smaller craniotomy/ectomy may not provide adequate decompression if intraoperative cerebellar swelling is encountered. Bone removal for large tumors extends to the level of the posterior fossa floor, but does not involve unroofing the foramen magnum.
The petrous-tentorial junction is identified to orient the surgeon regarding the route of the intradural trajectory. Inadvertent supracerebellar dissection can lead to avulsion of the bridging veins and torrential venous bleeding.
A small lateral portion of the cerebellum may be excised to allow adequate tumor exposure without aggressive cerebellar retraction (this maneuver may be necessary only in younger patients with a “full” cerebellum, otherwise cisternal opening and CSF drainage is adequate). Next, the arachnoid bands over the posterior aspect of the tumor capsule are released. Stimulation of the posterior/inferior surface of the capsule will exclude an aberrant posterior/inferior location of the facial nerve.
Following devascularization of the tumor by coagulating the feeders from the dura over the porus acusticus, microdissection proceeds inferiorly. If the tumor has substantial inferior extension, the enucleated capsule is mobilized away from the lower cranial nerves and out of the jugular foramen. The decompressed tumor is mobilized away from the nerves rather than vice versa.
Once the inferior pole is resected and shrunken, the tumor is internally debulked further along its superior pole and rostral dissection proceeds, mobilizing the capsule away from the tentorium and CN V. The facial nerve may be draped over this pole and adjacent to CN V. This portion of the capsule should be carefully mapped with the stimulator probe before its dissection.
Contraction of the temporalis muscle caused by stimulation of the motor fibers of the trigeminal nerve should not be mistaken for localization of the facial nerve. Mapping of an attenuated/atrophied facial nerve in a patient with a giant tumor can be difficult, especially in the presence of preoperative facial weakness. Repetitive mapping at slightly higher stimulation parameters may be necessary to completely exclude the presence of the facial nerve in the region.
The capsule is sharply dissected away from the trigeminal nerve root entry zone and is rolled from the medial to lateral direction. The trochlear nerve and superior cerebellar artery are usually encased within thick arachnoid bands and are readily mobilized.
Next, the thoroughly cored out tumor capsule is rolled laterally away from the middle cerebellar peduncle and brainstem toward the porus. Meticulous hemostasis will allow the operator to appreciate the most important factor in safe resection of these challenging tumors: microdissection along the arachnoid membranes and protection of the brainstem pial surfaces.
The capsule is gently pulled upon in the lateral direction via tumor forceps as extra-arachnoid dissection is performed using microforceps to detach the arachnoid membranes from the tumor. Periodic irrigation by the assistant clears the field.
Direct placement of the suction device over the brainstem and cranial nerves is avoided. The veins over the surface of the brainstem are engorged and prone to avulsion, leading to blood loss and interference with adequate visualization of the dissection planes. Gentle pressure/tamponade using a cotton ball over the site of the hemorrhage followed by coagulation of the vein along its more proximal segment away from the brainstem is a reasonable strategy.
Again, stepwise strategic internal tumor debulking using an ultrasonic aspirator followed by tumor mobilization is an efficient and safe maneuver to avoid inadvertent injury by undue traction on the surrounding structures.
Further mobilization of the inferior pole of the tumor should protect the posterior inferior cerebellar artery and its branches. En passage vessels are microsurgically mobilized using sharp dissection, while small tumor-feeding vessels are carefully coagulated and cut. Blunt dissection of the perforators and their subsequent avulsion is avoided.
CN VIII is encountered during dissection of the inferior pole of the tumor from the brainstem. Stimulation mapping avoids inadvertent injury to CN VII that may be near CN VIII. Preservation of CN VIII in patients with large and giant tumors is almost impossible and not advisable if preoperative hearing was deemed nonfunctional. This nerve is isolated and sharply cut in large tumors. Inadvertent traction on the nerve is prevented.
In the presence of preoperative middle cerebellar peduncle and/or brainstem edema, violation of the pial membranes is likely during resection of large and giant tumors. If such violation occurs, a small piece of cottonoid may be used to mobilize (peel away/”wipe away”) the brainstem from the tumor capsule without placing the former at risk of injury by the suction apparatus. Most of the edema is usually within the middle cerebellar peduncle.
As the tumor is mobilized away from the brainstem along its superior pole and midsection, stimulation mapping will localize the facial nerve at the tumor capsule or at its exit zone near the brainstem. The most reliable maneuver to expose the nerve safely is to peel the tumor laterally and identify the nerve at its root exit zone near the brainstem.
Removal of the tumor at the depth of the cerebellopontine cleft/fissure may require the greatest amount of cerebellar retraction; changing the angle of the microscope’s view and intermittent dynamic retraction using the handheld suction device will minimize the required persistent force on the neural structures. During this phase of the operation, any changes in vital signs, significant violation of the pial membranes, or worrisome facial nerve recordings should lead the operator to adjust his or her operative strategy or even consider staging the operation.
After localization of the facial nerve along the superior half of the capsule, I remove the inferior half of the tumor guided by stimulation mapping. CN VI may be adherent to a giant tumor.
As the superior pole of the tumor is peeled off, I expose the root entry zone of CN V thoroughly. This part of the nerve is adherent; meticulous dissection will minimize the risk of postoperative trigeminal neuropathy and resultant corneal anesthesia.
Bleeding can be a nuisance, but blind bipolar coagulation should be avoided. Some minor oozing is tolerated as long as microdissection can proceed with reasonable visualization of the operative field.
At this stage of the procedure, I take a break while my neuro-otology colleague removes the tumor within the IAC.
Removal of the Intracanalicular Component of the Tumor
The inferior wall of the IAC is drilled to the level of the fundus after the dura is dissected from the petrous bone. Cutting and diamond burrs remove the bone using generous irrigation fluid to avoid heat injury to the nerves within the canal. A large piece of Telfa is used to cover the cerebrovascular structures and the tumor within the CP cisterns to prevent bone dust from covering the operative field.
Ultimately, bone removal allows identification of the distal segments of CNs VII/VIII complex free of the tumor within the canal. Air cells are often entered and should be carefully waxed at the end of this stage.
The facial nerve becomes attenuated and most adherent to the tumor along the junction of its cisternal and intracanalicular segments at the porus. The operator must use a combination of gentle blunt and sharp microdissection techniques within both the cisternal and canalicular planes to avoid facial nerve injury.
Removal of the tumor within the IAC and exposure of the facial nerve at the porus will provide important anatomic information regarding the turn of the nerve around the tumor capsule at the porus and the route of the facial nerve. This information can estimate the course of the nerve from its already identified root exit zone near the brainstem to the area of the porus. If the facial nerve is significantly attenuated, its anatomic preservation may not be possible in patients with giant tumors.
Gross total tumor resection is attempted. However, if the tumor is very adherent to the facial nerve near the porus, I leave a small sheet of the tumor over the nerve to optimize facial function. This residual small piece of the tumor can be managed postoperatively through surveillance imaging and treated with radiosurgery if its growth is detected.
The surgeon may use the blunt-tipped stimulator as a dissecting instrument to peel away the nerve from the residual capsule. Significant tension on the nerve is avoided and sharp dissection is used when possible. Shear injury to the nerve as a result of excessive traction is irreversible.
The anterior aspect of the tumor capsule contains small arteries and, more commonly, veins near the facial nerve. Bleeding from these vessels can be a significant source of annoyance. Gentle irrigation, application of tamponade using pieces of thrombin-soaked cotton, and patience are needed to avoid aggressive coagulation and suction that can easily injure the nerve.
After the tumor has been completely removed or subtotally resected, the surgical field is inspected. Response of the facial nerve to stimulation of 0.07 mA at its exit zone near the brainstem is a good indicator of acceptable facial function after surgery.
The airs cells at the porus are rewaxed. Meticulous hemostasis is followed by watertight dural closure and generous application of bone wax to the mastoid air cells. If cerebellar swelling is apparent, the bone flap should not be replaced and a suboccipital decompressive craniectomy and duraplasty are performed.
A number of different facial nerve pathways around the tumor capsule have been described. These pathways are summarized below.
Resection of small VSs presents less technical challenge.
After surgery, patients are observed in the intensive care unit for any signs of neurologic deterioration, sudden hypertension, or ventilatory difficulty. Appropriate eye care is provided. The combination of trigeminal neuropathy and facial nerve paralysis places the eye at significant risk and appropriate precautions are taken to minimize the risk of corneal ulceration.
If dysphagia or respiratory insufficiency is encountered, early percutaneous gastrostomy and tracheostomy tubes are necessary to avoid aspiration pneumonia and hypoxemia.
The most feared complications are intracerebellar hemorrhage and progressive edema leading to symptomatic brainstem compression. Avoidance of significant cerebellar retraction, meticulous microdissection along the arachnoid and pial membranes, and prompt management of postoperative hypertension will minimize these unfortunate events.
Watertight dural closure minimizes the risk of pseudomeningocele formation. If rhinorrhea is encountered, I ask my otology colleagues to perform a mastoidectomy and obliterate the air cells with fat graft. In other words, the operative field is not re-explored.
The main neurologic morbidity of acoustic neuroma surgery is facial nerve palsy, which is both functionally and psychologically damaging to the patient and should be avoided through radical subtotal tumor removal. The size of the tumor is the main predictive factor of postoperative facial weakness.
Despite anatomic preservation of the facial nerve, some patients suffer complete facial paralysis after surgery. These patients should be reassured that reasonable recovery of facial function is possible. In the first few days after surgery, the eye is covered with a protective shield and viscous eyedrops are prescribed. Shortly thereafter, gold weight placement within the eyelid and/or tarsorrhaphy are performed. The patient is asked to protect the eye and wear protective spectacles.
At least 6 months of expectant management for the facial nerve palsy is provided before any further treatment is recommended. If permanent facial paralysis is evident, a faciohypoglossal anastomosis is recommended after the first postoperative year. This maneuver is also considered earlier if the nerve was anatomically noncontinuous at the conclusion of surgery.
If a staged operation is planned, the interval between operations may be 2 to 4 weeks, based on the patient’s recovery process from the first operation. Some surgeons advocate for a longer interval to allow further recovery of the cranial nerves and brainstem. If poor facial function is present from the first operation, the second stage is delayed until the nerve achieves a good functional recovery so it can be monitored via stimulation mapping during the second stage.
Pearls and Pitfalls
- Radical subtotal tumor resection is advised to avoid facial nerve palsy. The results of radiosurgery for treatment of growing residual tumors are very encouraging.
- Peeling the tumor from the medial to lateral direction has the best chance of protecting the facial nerve.
- Operative efficiency avoids surgeon’s fatigue and leads to improved outcomes.
Contributor: Andrew R. Conger, MD, MS
*Redrawn with permission from Tew JM, van Loveren HR, Keller JT. Atlas of Operative Microneurosurgery, WB Saunders, 2001. © Mayfield Clinic
For additional illustrations of facial nerve anatomy, please refer to the Jackler Atlas by clicking on the image below:
For additional illustrations of facial nerve repair, please refer to the Jackler Atlas by clicking on the image below:
For additional illustrations of the hypoglossal-facial anastomosis, please refer to the Jackler Atlas by clicking on the image below:
For additional illustrations of a retrosigmoid approach to the internal auditory canal and cerebellopontine angle, please refer to the Jackler Atlas by clicking on the image below:
For additional illustrations of tumor growth patterns, please refer to the Jackler Atlas by clicking on the image below:
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