Herbert B. Newton, MD, FAAN
DESCRIPTION 
- Acoustic schwannomas are extra-axial benign neoplasms that arise from the vestibular branch of the 8th cranial nerve in the cerebellopontine angle (CPA) region, near the porus acusticus. They are typically slow growing, with a growth rate of 1 to 2 mm/year. In some cases, especially older patients, the tumor may remain dormant. There are three stages of growth: canalicular, cisternal, and brainstem compressive. In late stages of growth, the seventh cranial nerve becomes draped over the mass, as it grows into the cistern toward the brainstem.
EPIDEMIOLOGY
Incidence/Prevalence
- Schwannomas account for 6 to 8% of all primary brain tumors. The majority of schwannomas (85 to 90%) are of the acoustic type and are usually unilateral (95%). The annual incidence is 1 case in 100,000 persons. Bilateral tumors occur in patients with Neurofibromatosis (NF) type II.
- All races and ethnic groups equally affected. Typical presentation is between 44 and 64 years of age. Females have a higher incidence than males: 1.5:1.
RISK FACTORS
- There are no known definite risk factors for acoustic schwannomas except for NF types I and II. Prior cranial radiation may be causally related in rare cases.
Genetics
- The majority of acoustic schwannomas are sporadic and unilateral. Approximately, 5% can develop in association with NF type I or II. In all NF-related tumors and between 65% and 70% of sporadic tumors, there are mutations of a tumor suppressor gene located at 22q12, the NF2 gene. NF2 codes for a protein, schwannomin, that interacts with cytoskeletal proteins involved in regulation of cell adhesion and proliferation.
GENERAL PREVENTION
- No preventive measures are known.
PATHOPHYSIOLOGY/ETIOLOGY
- The cells of origin of acoustic schwannomas are transformed Schwann cells from the 8th cranial nerve. In most cases, the initial genesis of transformation is unknown. The tumor appears as a discrete, rounded, encapsulated mass of a milky white color, arising from a nerve fascicle.
COMMONLY ASSOCIATED CONDITIONS
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HISTORY
- Common symptoms include unilateral sensory hearing loss (96%), unsteadiness (77%), tinnitus (71%), headache (29%), mastoid pain or otalgia (28%), facial numbness, diplopia, and vertigo. The mean time from onset of symptoms to diagnosis is 3.7 years. The loss of hearing and balance is slow and gradual in most cases. Tinnitus is typically unilateral, mild, and constant.
PHYSICAL EXAM
- Common neurological signs include unilateral sensorineural hearing loss in 90 to 95% of patients. Preserved hearing suggests the tumor will be less than 1.5 cm in size. In 50% of patients at presentation, hearing loss is the solitary neurological sign. Gait is either normal or only mildly affected. Large tumors (i.e., greater than 3 cm) can cause gait ataxia, dysmetria, nystagmus, facial hypesthesia, and papilledema.
DIAGNOSTIC TESTS AND INTERPRETATION
Lab
Initial Lab Tests
- Regular lab testing is not indicated, unless for pre-surgical screening.
Imaging
Initial Approach
- MRI, with and without gadolinium contrast, is the most critical diagnostic test. Axial and coronal enhanced images should be obtained. MRI is more sensitive than CT for small intracanalicular tumors and vascular structures, although both modalities properly visualize large masses. On T1 images, the tumor is usually isointense to brain while on T2 images it is hyperintense. Schwannomas enhance densely after administration of gadolinium. An MRI negative for an enhancing mass in the internal auditory canal rules out an acoustic schwannoma.
Diagnostic Procedures/Other
- Screening tests that are sometimes used before MRI or CT in patients with hearing loss include pure tone audiometry, speech discrimination assessment, and auditory-evoked brainstem responses (AEBR). In 60 to 70% of patients, high-frequency loss is present on audiometry. Speech discrimination is abnormal in 45 to 80% of cases. AEBR is the most sensitive non-imaging test and shows delayed latency or loss of wave V in approximately 95% of patients.
Pathological Findings
- Microscopically, schwannomas have biphasic architecture, with Antoni A and B regions. Antoni A is most common, with features of dense, compact rows of elongated spindle-shaped cells. Antoni B regions demonstrate loosely organized areas of stellate cells, lipid, and microcystic change. Mitoses and nuclear pleomorphism may be seen. Abnormalities of chromosome 22 are common, including monosomy and alterations of the long arm (i.e., deletions, inversions, translocations).
DIFFERENTIAL DIAGNOSIS
- Acoustic schwannomas account for 80% of tumors in the CPA region. The differential diagnosis includes other masses or processes that can cause a progressive syndrome in the CPA: meningioma, epidermoid cyst, exophytic brainstem glioma, ependymoma, choroid plexus papilloma, schwannomas of other cranial nerves (V, VII, IX, X, XI), jugular foramen paraganglioma, metastatic tumor, vascular processes (aneurysm, arteriovenous malformation), and abscess.
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MEDICATION
First Line
- Dexamethasone (8–16 mg/d) may be of benefit to reduce edema and swelling for patients in the brainstem compressive stage of growth. It may also improve transient symptoms of pressure and swelling after radiotherapy (RT) or radiosurgery.
ADDITIONAL TREATMENT
General Measures
- In certain patient cohorts, tumors are followed conservatively after diagnosis, including those with poor health, elderly patients with small lesions (less than 10 mm) or who are reluctant to proceed to surgery, and any patient with poor hearing in the contralateral ear. Tumors are likely to remain quiescent if they remain stable during the initial observation period (usually 6 months). Conservative approaches are unjustified in most young patients due to accelerated growth rates.
Issues for Referral
- Evaluation for hearing aids in patients with residual hearing; physical therapy as needed (e.g., dysmetria, ataxia); facial nerve grafting and reconstruction can be of benefit in selected patients.
Additional Therapies
- Conventional external beam RT and stereotactic radiosurgery (linear accelerator, proton beam, Gamma Knife) can be adjunctive or alternative forms of treatment in selected patients. RT (30 to 50 Gy) should be considered in patients with large residual tumors after surgery, large recurrent tumors, and patients with large tumors that are poor surgical candidates. RT can lengthen progression-free survival. Patients most appropriate for radiosurgery include those that are medically unstable, elderly (>65 years of age), contralaterally deaf, have failed previous surgery, or refuse surgical intervention. Tumors <3 cm in diameter are most suitable. Dosing is usually between 16 and 18 Gy in a single fraction to the 50% isodense line. Local control rates range from 90 to 95%, with variable amounts of tumor shrinkage. Complications of radiosurgery include hearing loss, nausea and emesis, headaches, and delayed facial neuropathy.
- Recent data suggest that molecular chemotherapy can be of benefit in the treatment of selected patients with acoustic schwannomas. Bevacizumab is a monoclonal antibody with activity against vascular endothelial growth factor (VEGF). VEGF appears to be active in progressive acoustic schwannomas. Treatment of some tumors will result in modest tumor shrinkage, and the potential for improved hearing.
SURGERY/OTHER PROCEDURES
- Complete surgical resection is the treatment of choice in most patients. Tumors <1 cm in diameter are most likely to be completely resected while preserving cranial nerve function. Three surgical approaches are commonly used, the choice between techniques depends on tumor size, depth of internal auditory canal penetration, hearing status, exposure of the facial nerve, and patient age. The suboccipital or retrosigmoid approach allows for excellent exposure of the tumor and the CPA, and hearing may be preserved; this approach is excellent for large tumors. The translabyrinthine or the anterosigmoid approach allows for good exposure of the internal auditory canal, CPA, and course of the facial nerve; although postoperative complications are reduced (especially facial nerve paralysis), hearing is abolished. The middle fossa or subtemporal approach does not give good exposure of the CPA, but does allow for removal of intracanalicular or small cisternal tumors, while sparing hearing and minimizing complications. Traction of the cerebellum during the suboccipital approach can cause dysmetria. Traction of the temporal lobe during the middle fossa approach can cause epilepsy or dysphasia.
- Intraoperative monitoring of cranial nerves V, VII, and XI during surgical resection is an excellent method for reducing morbidity of these nerves. Monitoring of cranial nerve VIII remains controversial, it may reduce morbidity during resection of tumors <2 cm in size.
IN-PATIENT CONSIDERATIONS
Initial Stabilization
- Admissions are unusual, except in the setting of surgical resection of the tumor.
Admission Criteria
- Admission is generally reserved for pre-surgical evaluation and surgical resection; angiography may be included in the work-up to assess regional vascular anatomy and rule out aneurysms and vascular malformations of the CPA; patients with brainstem compression might benefit from admission for intravenous dexamethasone.
Discharge Criteria
- Appropriate after recovery from surgery.
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ICD9
225.1 Benign neoplasm of cranial nerves
