Chapter 61. Vestibular Schwannoma (Acoustic Neuroma)

• Asymmetric (unilateral) sensorineural hearing loss, tinnitus, and disequilibrium
• Disproportionately diminished speech discrimination score relative to deterioration in pure-tone average
• Facial and trigeminal nerve symptoms with larger tumors.

Vestibular schwannomas (VS) (acoustic neuromas) are nerve sheath tumors of the superior and inferior vestibular nerves (cranial nerve VIII). They arise in the medial internal auditory canal (IAC) or lateral cerebellopontine angle (CPA) and cause clinical symptoms by displacing, distorting, or compressing adjacent structures in the CPA.

VS are by far the most common tumors involving the CPA. VS make up 80% of CPA tumors and 8% of all intracranial tumors. Various epidemiology studies have shown an incidence of 10 per 1 million individuals each year. This figure correlates with 2000–3000 individuals diagnosed with VS each year in the United States. There is no gender bias and the age of presentation is between 40 and 60 y of age. Ninety-five percent of VS occur in a sporadic fashion. The remaining 5% of patients have neurofibromatosis type 2 (NF2) or familial VS. The age of presentation is earlier in nonsporadic VS and patients usually present in the second or third decades of life.

The CPA consists of a potential cerebrospinal fluid (CSF)-filled space in the posterior cranial fossa bounded by the temporal bone, cerebellum, and brainstem. The CPA is a roughly triangular-shaped structure in the axial plane and is filled with CSF (Figure 61–1). The superior boundary is the tentorium and the inferior boundary is the cerebellar tonsil and medullary olives. The anterior border is the posterior dural surface of petrous bone and clivus, and the posterior border is the ventral surface of the pons and cerebellum. The medial border is the cisterns of the pons and medulla and the apex is the region of the lateral recess of the fourth ventricle. The lateral opening of the fourth ventricle, the foramen of Luschka, opens into the CPA. Cranial nerves V–XI traverse the cephalic and caudal extent of the CPA. The central structures crossing the CPA to and from the IAC are the facial (CN VII) and vestibulocochlear nerves (CN VIII), respectively.

Cranial nerves VII and VIII are covered with central myelin provided by neuroglial cells as they cross the CPA and carry a sleeve of posterior fossa dura into the IAC. The transition to peripheral myelin made by the Schwann cells occurs at the medial opening of the IAC. The vestibulocochlear nerve divides into three nerves: (1) the cochlear nerve and (2 and 3) the superior and inferior vestibular nerves in the lateral extent of the CPA or medial IAC. The IAC is divided into four quadrants by a vertical crest, called Bill's bar, and a transverse crest. CN VII comes to lie in the anterosuperior quadrant and is anterior to the superior vestibular nerve and superior to the cochlear nerve, whereas the inferior vestibular nerve lies in the posteroinferior quadrant and is inferior to the superior vestibular nerve and posterior to the cochlear nerve (see Figure 61–1). The anteroinferior cerebellar artery (AICA) is the main artery in the CPA and is the source of the labyrinthine artery. The labyrinthine artery via the IAC is an end artery for the hearing and balance organs. The AICA has a variable relationship to cranial nerves VII and VIII and to the IAC.

Pathogenesis

VS originate in the Schwann cells of the superior or inferior vestibular nerves at the transition zone (Obersteiner–Redlich zone) of the peripheral and central myelin. This transition zone occurs in the lateral CPA or medial IAC. Therefore, VS most often arise in the IAC and occasionally in the CPA. These schwannomas rarely arise from the cochlear nerve and are rarely malignant. The propensity to develop from the vestibular nerves may be due to the vestibular ganglion in the IAC having the highest concentration of Schwann cells.

Recent studies have improved our molecular understanding of VS. VS occur as a result of mutations in a tumor suppressor protein, merlin, located on chromosome 22ql2. Merlin is a cytoskeletal protein encoded by the NF2 gene that is necessary for the maintenance of contact inhibition of cellular growth. The formation of VS requires mutations of both copies of the NF2 gene. One functioning merlin protein prevents the formation of VS. Somatic mutations in both copies of the NF2 gene result in sporadic VS. The probabilities of two spontaneous, independent mutations at one locus predict a unilateral VS presenting in the fourth to sixth decades of life.

In contrast, familial VS occurring in NF2 only requires one occurrence of somatic mutation. People with NF2 inherit one mutated merlin protein and one normal merlin protein. A mutation in the normal allele leads to bilateral VS by age 20. Therefore, NF2 is an autosomal recessive mutation at the gene level since disease expression requires mutations in both alleles of the gene, but the inheritance is autosomal dominant (pseudodominant) since inheritance of one mutated allele often leads to a disease state. NF2 is a central form of neurofibromatosis, with affected patients having central nervous system tumors, including schwannomas, meningiomas, and gliomas. Most of these patients develop bilateral VS. In comparison, patients with NF type l (von Recklinghausen disease) have intra- and extracranial tumors, and <5% of these patients develop unilateral VS. Genetic screens for the NF2 mutation have been developed and offer genetic counseling for family members of patients with NF2. The severity of mutation involving the merlin gene in NF2 can predict the severity of disease manifestation.

Clinical Findings

Symptoms and Signs

Hearing Loss

Hearing loss is present in 95% of patients with VS. Conversely, 5% of patients have normal hearing; therefore, unilateral vestibular or facial complaints without hearing loss do not rule out retrocochlear disease. Of patients with hearing loss, most have slowly progressive hearing loss with noise distortion. Twenty percent have an episode of sudden hearing loss. The improvement of hearing loss with or without treatment does not rule out retrocochlear disease. The level of hearing loss is not a clear predictor of tumor size.

Tinnitus and Disequilibrium

Tinnitus is present in 65% of patients. The tinnitus is most often constant with a high buzzing pitch. This symptom is often not reported by patients because of the focus on the accompanying hearing loss. Similarly, owing to the central compensation for the slowly evolving vestibular injury, patients tolerate and adapt well to the disequilibrium they experience. The majority of patients have self-limiting episodes of vertigo. The disequilibrium is initially mild and constant and often does not prompt a medical visit. Disequilibrium is present in 60% of patients.

Facial and Trigeminal Nerve Dysfunction

Facial and trigeminal nerve dysfunction occurs after the auditory and vestibular impairments. The patients usually have midface (V2) numbness and also often have an absent corneal reflex. The motor supply of the trigeminal nerve of the muscles of mastication is rarely affected. The sensory component of the facial nerve is first affected and causes numbness of the posterior external auditory canal and is referred to as Hitselberger sign. Facial weakness or spasm occurs in 17% of patients and usually leads to a diagnosis of VS within 6 months.

Other Symptoms

Patients with large VS tumors or tumors that have undergone rapid expansion have visual complaints of decreased visual acuity and diplopia due to compromise of CN II, IV, or VI. Hydrocephalus leads to complaints of headache, altered mental status, nausea, and vomiting and, on examination, increased intracranial pressure and papilledema. Compression of the lower cranial nerves IX and X causes dysphagia, aspiration, and hoarseness, and examination reveals a poor gag reflex and vocal cord paralysis.

Imaging Studies

Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) with gadolinium contrast is the gold standard for the diagnosis or exclusion of VS. An MRI scan also allows for surgical planning. The various lesions within the CPA may be differentiated based on their varying imaging and enhancing characteristics. The MRI characteristics of a VS include a hypointense globular mass centered over the IAC on a T1-weighted image with enhancement when gadolinium is added. VS are iso- to hypointense on T2-weighted images (Figure 62–2).

Computed Tomography Scanning

When MRI scans cannot be used or are not accessible, a computed tomography (CT) scan with iodine contrast or an auditory brainstem response (ABR) offers a reasonable alternate screening modality. CT scanning with contrast provides consistent identification of CPA tumors that are larger than 1.5 cm or have at least a 5-mm CPA component. VS appear as ovoid masses centered over the IAC with nonhomogeneous enhancement. CT scans with contrast can miss intracanalicular tumors unless there is bony expansion of the IAC.

Special Tests

Audiology

The average patient requires 4 y from the onset of symptoms to the diagnosis of VS. The diagnostic dilemma lies in choosing the appropriate patient to undergo audiometric and imaging studies. Most patients present with complaints of unilateral hearing loss or hearing distortion, unilateral tinnitus, vertigo or disequilibrium, and facial numbness, weakness, or spasm. Patients with unilateral auditory, vestibular, and facial complaints need to undergo careful evaluation to rule out retrocochlear disease. The initial step in the evaluation includes an audiology exam. If the audiology exam suggests a retrocochlear lesion, then imaging of the CPA is performed to rule out a retrocochlear lesion. Vestibular testing lacks specificity in diagnosing VS.

The standard auditory evaluation should include pure-tone audiometry, a word recognition score (WRS), acoustic reflex thresholds, and acoustic reflex decay. Pure-tone audiometry of patients with VS shows asymmetric, down-sloping, high-frequency sensorineural hearing loss in almost 70% of patients. The hearing may also be normal, may involve only the low frequency, or may be a flat hearing loss or a trough or peak hearing loss. A retrocochlear-based hearing loss causes WRS scores to be lower than that predicted by the pure-tone thresholds. This out-of-proportion depression of speech intelligibility is further accentuated when retested at a higher speech intensity. This phenomenon is called rollover. A poor WRS is present in about 50% of patients with VS. An abnormal WRS should trigger an imaging evaluation, but a normal WRS does not rule out a VS. A loss of acoustic reflexes or the presence of acoustic reflex decay is present in most cases of VS, but normal acoustic reflexes do not preclude VS.

Vestibular Testing

Vestibular testing does not provide a sensitive or specific means of diagnosing VS. The most common test ordered to evaluate vestibular complaints includes an electronystagmogram (ENG). An ENG in a patient with VS will show a reduced caloric response in the problematic ear. The extent of the vestibular function predicts the amount of postoperative vertigo. The location of the VS on the inferior or superior vestibular nerve may also be predicted by the ENG because the ENG primarily evaluates the lateral semicircular canal innervated by the superior vestibular nerve.

Auditory Brainstem Response

An ABR is the measured electrical response of the cochlea and its brainstem pathway to short-duration, broadband clicks. The evoked response is a characteristic waveform with five identifiable peaks (I–V). The absolute latency or timing of each wave is recorded. In patients with VS, the ABR is fully or partially absent, or there is a delay in the latency of Wave V in the affected ear. The delay may be an absolute delay based on normative data or a delay compared with the latency of Wave V in the opposite ear. An interaural delay of Wave V latency >0.2 ms is considered abnormal. Overall, ABR has a sensitivity >90% and a specificity of 90% in detecting VS. However, when considering only small intracanalicular tumors, 18–33% of tumors are missed. As the detection limits and costs of imaging studies have improved, the role of ABR in diagnosis of VS has dramatically declined.

Differential Diagnosis

The three most common tumors of the CPA includet schwannomas, meningiomas, and epidermoids (Table 61–1). Each of these tumors has a similar clinical presentation and each is primarily differentiated by its imaging characteristics. Other CPA lesions include congenital rest lesions (epidermoids, arachnoid cyst, and lipoma), schwannomas of other cranial nerves, intraaxial tumors, metastasis, vascular lesions (paraganglioma and hemangioma), and lesions extending from the skull base (cholesterol granulomas and chordoma).

The natural history of VS includes a slow rate of growth in the IAC and then into the cistern of the CPA. Studies show that periods of growth are intermixed with periods of quiescence. The average growth rate is 1.8 mm/y. This slow growth causes progressive and often insidious symptoms and signs since there is displacement, distortion, and compression of the structures first in the IAC and then in the CPA. This slow growth via cellular proliferation provides a predictable progression of symptoms and signs. Occasionally, the tumor may undergo rapid expansion due to cystic degeneration or hemorrhage into the tumor. A rapid expansion causes rapid movement along the subsequent phases of VS symptoms and may cause rapid neurological deterioration.

The initial intracanalicular growth affects the vestibulocochlear nerve in the rigid IAC and causes unilateral hearing loss, tinnitus, and vertigo or disequilibrium. These three symptoms are the typical presenting complaint of not only patients with VS but also patients with other lesions of the CPA. It is interesting that the motor component of the facial nerve is resistant to injury during this phase of growth and patients have normal facial function. The tumor then grows into the CPA cistern and grows freely without causing significant new symptoms because structures in the CPA are initially displaced without injury (see Figure 61–2A). As the tumor approaches 3 cm, it abuts on the boundaries of the CPA and results in a new set of symptoms and signs. Compression of the CN V causes corneal and midface numbness or pain. Further distortion of CN VIII and now CN VII causes further hearing loss and disequilibrium and also facial weakness or spasms. Brainstem distortion leads to narrowing of the fourth ventricle (see Figure 61–2B).

Further growth leads to the final clinical spectrum of CPA syndrome. The patient develops cerebellar signs due to compression of the flocculus and cerebellar peduncle. The patient also develops obstructive hydrocephalus due to closure of the fourth ventricle. The increasing intracranial pressure manifests in ocular changes, headache, mental status changes, nausea, and vomiting. If the VS continues to grow without intervention, death occurs from respiratory compromise.

Treatment

The treatment of patients with CPA tumors includes surgical removal, observation, and irradiation. Surgical removal has historically been the primary management modality for VS. More recently, observation with serial MRI and radiation therapy have increased in popularity. The size of the tumor, age of the patient, residual auditory function, vestibular dysfunction, and cranial neuropathies are some of the important factors in determining optimal intervention. Observation is reasonable in older patient or in whom the tumor is not growing. Gamma knife is preferentially used in older patient, those who cannot tolerate a surgical procedure or who have a limited life expectancy.

Surgical Measures

The surgical approaches to the CPA include translabyrinthine, retrosigmoidal, and middle fossa craniotomies (Figure 61–3A). The appropriate approach for a particular patient is based on the hearing status, the size of the tumor, the extent of IAC involvement, and the experience of the surgeon (Table 61–2). The approaches are either hearing preserving or hearing ablating. The retrosigmoidal and middle fossa approaches are hearing preserving. However, they have limitations of exposure to all aspects of the CPA and IAC. The middle fossa approach is well suited for patients with good hearing and a tumor that is <1.5 cm in the CPA. The retrosigmoidal approach is well suited for patients with good hearing and a tumor <4 cm and not involving the lateral IAC. In the retrosigmoid approach, the lateral IAC is usually only directly accessible following the removal of the posterior semicircular canal; the violation of the posterior semicircular canal leads to hearing loss. The translabyrinthine approach causes total hearing loss and so is well suited for patients with poor hearing (pure-tone average >50) or patients with good hearing and tumors not accessible by the hearing-preserving approaches. Generally, hearing preservation is poor with tumors >2 cm and those that involve the lateral IAC.

Three critical issues inherent in all three techniques are the extent of exposure of the IAC and CPA, the identification and preservation of the facial nerve, and the extent of brain retraction. These operations use electrophysiological monitoring of CN VII and an ABR in hearing preservation approaches.

Translabyrinthine Approach

The primary approach for removal of VS is the translabyrinthine approach. The boundaries of the approach include the mastoid facial nerve and cochlear aqueduct anteriorly, middle fossa dura superiorly, posterior fossa dura posteriorly, and jugular foramen inferiorly (see Figure 61–3A). These boundaries are approached via the familiar postauricular incision. A complete canal mastoidectomy is accomplished with identification of the incus, tegmen, sigmoid sinus, and facial nerve. A compete labyrinthectomy is then performed with medial skeletonization of the middle and posterior fossa dura and decompression of the sigmoid sinus to the jugular foramen. After bony skeletonization of the IAC, the dura of the IAC is opened, and the facial nerve is identified medial to the transverse crest (Bill's bar). Once the facial nerve is identified in the fundus or lateral aspect of the IAC, tumor removal occurs from a lateral to medial direction along the IAC. In large tumors, the tumor is debulked internally and then the tumor capsule is removed from the surrounding structures, including the facial nerve. After tumor removal, abdominal fat is placed into the defect.

The three advantages of the translabyrinthine approach are the ability to remove tumors of all sizes, minimal retraction of the brain, and the ability to directly visualize and preserve the facial nerve. The rate of facial nerve preservation is 97%. The rate of CSF leakage presenting under the incision or draining through the nose via the eustachian tube is 5–8%. The majority of these CSF leaks resolve with conservative management that includes mastoid dressing and fluid restriction. A minimal risk of meningitis is associated with a CSF leak.

Retrosigmoidal Approach

The retrosigmoidal approach is a modification of the traditional suboccipital approach used by neurosurgeons to address most posterior fossa lesions. The retrosigmoidal approach is a versatile approach with a panoramic view of the CPA from the foramen magnum inferiorly to the tentorium superiorly (Figure 61–3B). The medial two-thirds of the IAC are also accessible without violating the inner ear, therefore preserve hearing.

The surgical technique starts with a curvilinear skin incision 6 cm behind the ear over the retromastoid region. The soft tissue and posterior nuchal musculature are elevated to expose the mastoid and retromastoid bone. A 5 × 5 cm craniotomy is performed with the sigmoid as the anterior boundary and the transverse sinus as the superior boundary. An elevation of a bone plate is technically difficult, so the bone may be removed by drilling. The bone fragments are collected and are replaced during closure. The bone fragments will reform a bone plate and prevent adherence of the musculature to the dura. If decompression of the sigmoid sinus is needed for exposure, a mastoidectomy may also be performed. The dura is then opened along the sigmoid sinus and the cerebellum is seen. The CSF from the cisterna magnum needs to be released prior to retracting the cerebellum. Medial retraction of the cerebellum allows visualization of the CPA. To address the IAC component of the tumor, the posterior IAC bone needs to be removed. The bone dust created is carefully confined and removed to prevent meningeal irritation. The extent of IAC skeletonization is limited by the proximity to the inner ear. The endolymphatic duct and sac serve as landmarks to the proximity of the posterior semicircular canal and allow preservation of the inner ear and hearing. The facial nerve is normally anterior to the tumor or its position is ascertained with facial nerve monitoring. The tumor removal is as previously described. After tumor removal and hemostasis, air cells along the IAC and mastoid are closed with bone wax or bone cement to eliminate paths for CSF leak. A fat or muscle graft may also be placed into the petrosal defect to prevent CSF leak. The dura is closed and the bone plate or bone pate is replaced. The musculature and soft tissue are meticulously closed.

The primary advantage of the retrosigmoidal approach relative to the translabyrinthine approach is the ability for hearing preservation in properly selected tumors. If hearing preservation is not an issue, the retrosigmoidal approach allows a versatile approach to the CPA and IAC. The relative disadvantages compared with the translabyrinthine approach include persistent postoperative headache, increased difficulty in resolving CSF leaks, the need for cerebellar retraction, and the inability to have direct access to the facial nerve. The combination of intradural drilling leading to meningeal irritation by bone dust and dissection of suboccipital musculature causes nearly 10% of patients to have a persistent, severe, postoperative headache. In the case of extensive pneumatization of the IAC and mastoid, the air cells may be difficult to completely seal, and the inability to address the aditus-ad-antrum or the eustachian tube causes CSF leaks to be persistent, despite conservative treatment. The extent of cerebellar retraction is minimal in small tumors, but the amount of retraction increases with larger tumors. The surgical control of the facial nerve is adequate in the retrosigmoidal approach, but the exposure of the facial nerve is superior in the translabyrinthine approach.

Middle Fossa Approach

The middle fossa approach provides a hearing-preserving approach to intracanalicular tumors with a <1.5 cm cisternal component. The surgical technique involves an inverted U-shaped incision centered over the ear. The temporal muscle is reflected inferiorly to expose the squamous portion of the temporal bone. A 5 × 5 cm temporal craniotomy is performed and is centered over the zygomatic root. Extradural elevation of the temporal lobe is accomplished to reveal the floor of the temporal bone. The greater superficial petrosal nerve leading to the geniculate ganglion reveals the anterior, lateral boundary of the IAC, and the arcuate eminence reveals the posterior boundary of the IAC (Figure 61–3C). These landmarks may be difficult to identify, and the IAC dura may have to be identified medially by drilling toward the porus acousticus. Once the IAC is identified and well skeletonized medially, the bone removal continues laterally. However, the extent of IAC skeletonization laterally is limited by the basal turn of the cochlea anteriorly and the superior semicircular canal posteriorly. The IAC dura is opened posteriorly to avoid injury to the facial nerve. The tumor is dissected free of the facial nerve and removed in a medial to lateral direction. Any air cells are sealed and the dural defect is covered with a fat or muscle plug. The craniotomy bone flap is replaced and the incision is closed.

The middle fossa approach is unique compared with the posterior fossa craniotomies because the entire IAC is accessible without violating the inner ear; direct visualization of the medial IAC is difficult even with the middle fossa approach. This exposure allows for the removal of intracanalicular tumors while maintaining hearing preservation. The limitations of the middle fossa approach include tumors with a >1.5 cm cisternal component. In situations of hearing preservation, an extended middle fossa approach with further removal of bone around the IAC as well as the elevation or division of the superior petrosal sinus and tentorium allow improved exposure into the CPA. The relative merits of the procedure with increased temporal lobe retraction and limited access to the posterior fossa in the event of bleeding relative to a retrosigmoidal approach continues to be defined. The disadvantages of the middle fossa approach include temporal lobe retraction and poor surgical position of the facial nerve relative to the tumor. Temporal lobe retraction may cause transient speech and memory disturbances and auditory hallucinations. The facial nerve, especially if the tumor originates from the inferior vestibular nerve, will be between the surgeon and the tumor. The increased manipulation of the facial nerve during tumor removal increases the risk of transient facial paresis.

Nonsurgical Measures

Observation

The predictable correlation between VS size and significant neurological symptoms and the relative slow growth of VS allow observation to be a management option for VS. Patients may be observed if their life expectancy is shorter than the growth time required for the VS to cause significant neurological symptoms. The growth pattern of the VS should be assessed in these patients with a second radiological evaluation in 6 months and then in yearly radiological evaluations. Studies have shown that 15–24% of patients undergoing conservative management require surgery or stereotactic radiation. If the growth rate in the first year exceeds 2–3 mm, then the patient is likely to need the treatment for the VS. The patient should understand that the initial conservative management, rather than immediate surgical intervention, may necessitate a resection of a larger tumor that is less amenable to hearing preservation or stereotactic radiation (or both) if intervention becomes necessary in the future.

Stereotactic Radiation

The goal of stereotactic radiation is to prevent further growth of VS while preserving hearing and facial nerve function. This goal directly differs from the goal of complete tumor removal in microsurgical therapy. The mechanism of stereotactic radiation relies on delivering radiation to a specific intracranial target by using several precisely collimated beams of ionizing radiation. The beams take various pathways to the target tissue, therefore creating a sharp dose gradient between the target tissue and the surrounding tissue. The ionizing radiation causes necrosis and vascular fibrosis, and the time course of the effect is >1–2 y. There is an expected transient swelling of the tumor in the short term with modest shrinkage over time. The ionizing radiation is most commonly delivered using a 201-source cobalt-60 gamma knife system. The standard linear accelerator can also be adapted to deliver stereotactic radiation. The practical aspects include the patients wearing a stereotactic head frame, computer-assisted radiation planning using an MRI scan, and a single treatment for delivery of the radiation Figure 61–4.

The success of stereotactic radiation in arresting tumor growth depends on the dose of radiation delivered. However, the rate of cranial nerve neuropathies, including hearing loss, is decreased by lowering the radiation dose. The current trend has been to lower the marginal radiation dose, and the long-term tumor control with these current dosing plans is under investigation. Since VS have a slow growth rate, these studies require 5- to 10-y follow-ups to provide reliable data about tumor control. Studies have shown control rates from 85% to more than 95%. The hearing preservation rate decreases each year after radiation and stabilizes after 3 y in 50% of patients. The rate of facial nerve dysfunction varies from 5% to 20% based on the radiation dose at the margin of the tumor and the length of the facial nerve in the radiation field. Approximately 25% of patients have trigeminal nerve neuropathy. The persistence and extent of these neuropathies with the lower dosing protocols continue to be studied. Hydrocephalus is also a complication of radiation.

As the long-term effectiveness and sequelae of stereotactic radiation are further defined, the indications for radiation therapy will become further refined. Radiation therapy is useful in patients in whom the arrest of tumor growth is acceptable. These patients have either short-life expectancies or a high surgical risk. Compared with microsurgery, stereotactic radiation may allow improved hearing preservation in patients with 2–3 cm VS. Radiation therapy in large tumors (>3 cm) or tumors causing brain compression will exacerbate symptoms because of initial tumor swelling.

Operative Complications

The intraoperative complications for all three approaches include vascular injury, air embolisms, parenchymal brain injury, and cranial nerve injury. The anteroinferior cerebellar artery originates from the basilar artery and supplies the labyrinthine artery as well as the lower portion of the cerebellum and the vein of Labbé, which can be the only venous drainage of the temporal lobe. The anteroinferior cerebellar artery and vein of Labbé are vulnerable to injury during VS surgery. In the event of an air embolism via an open vein, the patient should be placed into a left lateral and Trendelenburg position to trap the air in the right ventricle; the air can then be aspirated via a central venous catheter. The cerebellum during a retrosigmoidal craniotomy and the temporal lobe during a middle fossa craniotomy are at risk from retraction injury.

Postoperative Complications

Postoperative complications include hemorrhage, stroke, venous thromboembolism, the syndrome of inappropriate antidiuretic hormone, CSF leak, and meningitis. Postoperative hemorrhage manifests as neurological and cardiovascular deterioration and requires evacuation. Studies have shown that, postoperatively, low-molecular-weight heparin in addition to compression stockings and intermittent pneumatic compression devices may further reduce the risk of thromboembolism in high-risk patients (eg, elderly and obese patients) without increasing the risk of intracranial bleeding. The most common complication is CSF leak. CSF leak occurs in 5–10% of cases either via the wound or via a pneumatic pathway to the eustachian tube. Most of these leaks resolve with conservative care, which includes placing wound sutures at the leak site, replacing the mastoid dressing, and decreasing intracranial pressure with acetazolamide (Diamox), fluid restriction, and bed rest. Some patients also require a lumbar subarachnoid drain, and a very few patients need surgical reexploration.

A related complication is meningitis. Meningitis occurs in 2–10% of patients and may be aseptic, bacterial, or due to CSF leak and arachnoid irritation from the fat graft (lipoid). The distinction between aseptic and bacterial meningitis is necessary because the treatment for aseptic meningitis is a steroid taper and antibiotics for bacterial meningitis. Delayed meningitis should be considered bacterial and likely due to a CSF leak.

Operative Prognosis and Rehabilitation

The most concerning issues to patients are deafness, imbalance, and facial nerve weakness. The most important factors for hearing preservation are the tumor size and the preoperative hearing level. Hearing preservation ranges from 20% to 70%. Patients with contralateral hearing tolerate the unilateral loss but may benefit from bone-anchored hearing aids. Patients with poor contralateral hearing may be rehabilitated with a CROS (contralateral routing of signal) hearing aid or a cochlear implant if the cochlear nerve fibers are preserved. Almost half of the patients will have vertigo or imbalance beyond the postoperative period, but these symptoms have a minimal impact on daily activities.

The rapidity of vestibular compensation after unilateral vestibular loss is determined by the patient's efforts to exercise and challenge the vestibular system. Patients who continue to have disequilibrium in the extended postoperative period are referred for vestibular rehabilitation therapy. Facial nerve function is also best predicted by tumor size. In smaller tumors, more than 90% of patients are found to have House-Brackmann Grade 1 or 2 function (Grade 1 is normal and Grade 6 is complete paralysis). If all tumor sizes are considered, then approximately 80% of patients have Grade 1 or 2 function.

The rehabilitation of facial nerve injury is based on the general principles of nerve injury, recovery, and rehabilitation. If the nerve is transected intraoperatively, the nerve should be repaired primarily, if possible, or with a greater auricular interposition graft. The postoperative function may be predicted in an anatomically intact nerve by the intraoperative stimulability of the nerve. If the nerve stimulates at <0.2 V, then there is a >85% chance of Grade 1 or 2 function at 1 y. The lack of facial function (Grade 6) at 1 y and no reinnervation potentials on electromyograph should lead to a hypoglossal–facial transposition, an interposition nerve graft, or a crossfacial graft. If facial rehabilitation has been delayed and there is electrical “silence” of the facial muscle on electromyograph, muscle transpositions with the temporalis or masseter muscle to the lip give improved tone and symmetry to the lower face. The upper face can be rehabilitated with a brow lift and gold weight for the eye. The eye must be protected with lubrication, ointment, and an eye bubble if there is either incomplete eye closure or a lack of sensation to the cornea due to trigeminal nerve involvement. The lack of eye care leads to corneal injury and blindness.