Acoustic Neuroma

Description

Acoustic neuromas (vestibular schwannomas) are slow-growing, benign, intracranial, extra-axial tumors originating from the vestibulocochlear nerve.

• Originated from Schwann cells of the nerve sheath
• Usually arise in the internal auditory canal near the cerebellopontine angle
• Most are unilateral; bilaterally seen in neurofibromatosis type 2 (NF2)

Epidemiology

• 6–10% of all intracranial tumors
• 80–90% of cerebellopontine angle tumors
• 95% of cases are unilateral.
• Present most commonly in the 5th to 6th decades, although signs and symptoms can occur earlier
• Female predominance
• 5% of acoustic neuromas are a result of NF2.
• Bilateral acoustic neuroma occurring in NF2 present before age 30 years

• 1/100,000 per year
• Asymptomatic lesions may be more common.

Prevalence
3,000 diagnosed annually in the United States

Etiology and Pathophysiology

• Compression of acoustic and facial nerve when located within internal acoustic canal
• Compression of brainstem, 4th ventricle, and trigeminal nerve when tumor is at the cerebellopontine angle
• Vestibular schwannomas are related to the NF2 gene and its product merlin. Merlin acts as a tumor suppressor and as a mediator of contact inhibition. Thus, deficiencies in both NF2 genes lead to vestibular schwannoma development.

• Unknown for unilateral acoustic neuroma
• NF2: bilateral acoustic neuromas
  • Autosomal dominant
  • Gene located on chromosome 22q11

Risk Factors

• High-dose ionizing radiation
• Pregnancy and epilepsy may increase risk.
• There is some literature suggesting that smoking cigarettes may be protective. However, further research must be conducted to support these claims.
• No proven increase in risk of acoustic neuroma with regular use of a mobile phone

Commonly Associated Conditions

• NF2
• Pregnancy may accelerate the growth of the tumor.

History

• Common
  • Sensorineural hearing loss (unilateral), often progressive
  • Sudden deafness
  • Loss of speech discrimination
  • Tinnitus
  • Disequilibrium is common, but vertigo is less common.
• Less common
  • Weakness/loss of facial muscle functions
  • Headache with hydrocephalus and increased intracranial pressure
  • Trigeminal nerve dysfunction when tumor is large and compressing on cranial nerve V (CN V)
  • Ataxia due to cerebellar or brainstem compression from large tumor

Physical Exam

• Exclude other causes of hearing loss (e.g., middle-ear effusion, infection, cerumen [earwax], cholesteatoma, or tympanic membrane perforation) with otoscopic exam.
• Neurologic exam of CNs
• Weber and Rinne tests to screen for sensorineural hearing loss versus conductive hearing loss
• Contralateral ear exam if you suspect NF2 (at risk for bilateral hearing loss)

Differential Diagnosis

• Cerebellopontine lesions
  • Meningioma
  • Glioma
  • Facial nerve schwannoma
  • Epidermoid cyst
  • Hemangioma
  • Arachnoid cyst
• Sensorineural hearing loss
  • Presbycusis (age related)
  • Ménière disease
  • Ototoxicity (i.e., medications)
  • Viral illness (i.e., labyrinthitis)
  • Autoimmune disease
  • Congenital
  • Cerebellar pathology

Diagnostic Tests & Interpretation

• MRI with gadolinium (gold standard)
  • Approaches 100% specificity; detects tumors starting at 2 mm
• Noncontrast T2-weighted fast spin echo MRI
  • Up to 98% specificity; less expensive than MRI with gadolinium
  • CT scan detects tumors as small as 1 cm; up to 37% false-negative findings; provides good information about surrounding bony structures of the tumor

• Audiogram
• Electrophysiologic testing
• Pure tone and speech audiometry (asymmetric, high-frequency sensorineural hearing loss)
• Speech discrimination
• Stacked auditory brainstem response (ABR): 95% sensitivity and 88% specificity; however, misses approximately up to 1/3 of tumors <1.5 cm
• Standard ABR: can only detect tumors >1 cm

• Well-demarcated and encapsulated mass attached to neural structures without direct invasion
• The mass may be dense or cystic.
• Diagnostic microscopy: densely packed spindle cells (Schwann cells) mixed in with myxoid and collagenous matrix
  • Antoni A will show zones of alternatively dense and sparse areas of tissue.
  • Antoni B will show a loose meshwork of cells with less cellular dense tissue.
  • Palisading nuclei (Verocay bodies)
  • S-100 antibody immunoreactivity

General Measures

• Treatment options include observation, stereotactic radiosurgery, fractionated radiotherapy, and microsurgery.
• Without intervention, 16–26% of patients require additional treatment, with 54–63% preserving functional hearing.
• With radiosurgery, only 2–4% of patients require additional treatment, and hearing preservation is accomplished in 44–66%.
• With fractionated radiotherapy, 3–7% of patients will require additional treatment; short-term hearing preservation is reported at 59–94% of patients, although long-term outcomes are not known.
• One study comparing observation to stereotactic radiosurgery demonstrated no differences in hearing preservation, hearing survival, and speech discrimination between the groups (1)[C].
• Up to 57% of acoustic neuromas may not grow or shrink without treatment.
• Up to 70% of extracanalicular tumors may never have a growth rate exceeding 2 mm/year.

ALERT
Patients with NF2-associated vestibular schwannomas tend to have higher rates of surgical complications, treatment costs, and hospital stays compared to sporadic cases.

Pediatric Considerations
Acoustic neuromas in children typically grow at slower rates compared to adults; this affects treatment outcomes.

Medication

A greater understanding of molecular tumorigenesis has yielded novel therapies. Suggested therapies:

• Bevacizumab may induce regression of progressive schwannomas by >40% and improves hearing.
• An inhibitor of vascular endothelial growth factor (VEGF) synthesis, PTC299, is currently in phase II trials as a potential agent to treat vestibular schwannoma.
• In vitro studies have shown that trastuzumab (an ErbB2 inhibitor) reduces vestibular schwannoma cell proliferation.

Issues For Referral

All tumors should at least be monitored, even if asymptomatic.

Geriatric Considerations
Observation is suitable for elderly patients with contraindications to surgery and radiotherapy: Observation is more likely to preserve hearing than radiotherapy or surgery. If the patient has dizziness related to the tumor, treatment may be indicated to prevent falls.

• Gamma Knife single-dose stereotactic radiosurgery
  • Performed on an outpatient basis
  • For smaller tumors (<3 cm) or contraindications to microsurgery
  • Shown to have better tumor control rates than observation in some studies
  • Lower dose radiation has fewer complications.
• Fractionated stereotactic radiosurgery
  • Delivers a higher dose of radiation and less damage to healthy tissue
  • Requires multiple treatments, and total radiation dose is higher than single-dose radiation
  • Suitable for all sizes of tumor

Surgery/Other Procedures

• Surgery is the recommended definitive treatment.
• Lowest rate of recurrence, with up to 97.5% complete tumor removal
  • Retromastoid/retrosigmoid: can be used for all acoustic tumors
  • Middle cranial fossa: for small tumors with aim of preserving hearing
  • Translabyrinthine: for all but large tumors
  • Transpetrous approaches are safe for acoustic neuroma removal, and the postoperative complication rate is low.
  • The retrolabyrinthine approach seems to be a good hearing preservative approach, regardless of tumor volume.
• In one study, 79.1% of surgeries involved a translabyrinthine approach, with the remaining combined retrosigmoid and transtemporal (10.4%), middle fossa (6.0%), and stereotactic radiosurgery (4.5%). The highest complication rate was seen in the combined retrosigmoid and transtemporal approach cohort (2)[B].
• The introduction of endoscopy for the middle fossa approach has resulted in improved preservation of hearing and facial nerve function.
• Treatment providers should be aware that radiation treatment carries a small risk of malignant transformation (0.3% in a large series).

Follow-up Recommendations

• Yearly MRI follow-up for slow-growing tumors is advised.
• MRI and audiometry
• In one study, composite quality of life (cQOL) score 0 to 5 years out of surgery was significantly highest among patients receiving stereotactic radiosurgery compared to microsurgery and observation. However, after 5 years, these differences were not seen.
• One study revealed that imbalance or disequilibrium may be indicative of tumor growth.
• Patients starting with serviceable hearing, average time to unserviceable hearing was 6.3 years (3)[B].
• In one study, 83% of patients who underwent acoustic neuroma resection still experienced persistent tinnitus.

Complications

• Mass effects: CN compression, hydrocephalus, brainstem compression, cerebellar tonsil herniation
• Surgical complications: hearing loss, CSF leakage, facial nerve injury, headaches, meningitis
• In one study, 22.4% of patients experienced postoperative complications; most commonly are facial weakness (13.4%), CSF leak (6.0%), and infection (3.0%) (2)[B].
• In a larger study, 28.2% of patients experienced complications; most commonly are nervous system complications not elsewhere specified (15.2%) and facial paralysis (8.7%). Other complications included hydrocephalus, CSF leak, dysphagia, among others (4)[A].
• CSF leak has been linked to increased patient BMI and translabyrinthine surgical approach (5)[B].
• Another major complication is dizziness. There is a strong association between headaches and dizziness (6)[B].

ICD-10

• D33.3 Benign neoplasm of cranial nerves

ICD-9

• 225.1 Benign neoplasm of cranial nerves

SNOMED

• 126949007 acoustic neuroma (disorder)