Description

• Acoustic neuromas (vestibular schwannomas) are slow-growing, benign intracranial tumors originating from the vestibulocochlear nerve:
  • Originate from Schwann cells of the nerve sheath
  • Usually arise in the internal auditory canal near the cerebellopontine angle
  • Most are unilateral; bilateral seen in neurofibromatosis type II
• Patients present with hearing loss, tinnitus, and balance dysfunction. Rarely, they can result in facial paralysis, brainstem compression, and even death.

Epidemiology

• 6–10% of all intracranial tumors
• 80–90% of cerebellopontine angle tumors
• 95% of cases are unilateral
• Present most commonly in the 5th–6th decade
• Female predominance
• Bilateral acoustic neuroma occurring in neurofibromatosis II present before age 30

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

Prevalence
3,000 diagnosed annually in the US

Risk Factors

• Pregnancy and epilepsy may increase risk (1)[C].
• Smoking may decrease the risk (1)[C].
• There is no proven increase in risk of acoustic neuroma with regular use of a mobile phone (2)[C],(3)[A].

• Unknown for unilateral acoustic neuroma
• Neurofibromatosis type II: Bilateral acoustic neuromas:
  • Autosomal dominant
  • Gene located on chromosome 22q1

Pathophysiology

• Exerts pressure on the surrounding structures
• Compression of acoustic and facial nerve when located within internal acoustic canal
• Compression of brainstem, 4th ventricle, and trigeminal nerve when tumor at the cerebellar pontine angle

Etiology

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 (4).

Commonly Associated Conditions

• Neurofibromatosis type II
• Pregnancy may accelerate the growth of the tumor

History

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

Physical Exam

• Examination with otoscope to exclude other causes of hearing loss (e.g., middle-ear effusion, infection, wax, cholesteatoma, or tympanic membrane rupture)
• Detailed neurologic exam concentrating on the cranial nerves
• Weber and Rinne tests to confirm sensorineural hearing loss
• Evaluation of the contralateral ear in patients <30 years; suspect neurofibromatosis type II

Diagnostic Tests and Interpretation

• MRI with gadolinium (gold standard):
  • 100% specificity
  • Detects tumors starting at 2 mm
  • Tumor has marked enhancement with gadolinium
• Noncontrast T2-weighted fast spin-echo MRI:
  • 98% specificity
  • Cheaper than MRI with gadolinium
• CT:
  • Detect tumors as small as 1 cm
  • Up to 37% false negatives
  • Provides good information about surrounding bony structures of the tumor

• Pure-tone and speech audiometry (asymmetrical, high-frequency sensorineural hearing loss)
• Speech discrimination
• Stacked auditory brainstem response (ABR): 95% sensitivity and 88% specificity. Can detect tumors <1 cm.
• Standard ABR: Can only detect tumors >1 cm

• Well-demarcated and encapsulated mass attached to neural structures without direct invasion
• Can be dense or cystic
• Microscopic: Densely packed spindle cells (Schwann cells) mixed in with myxoid and collagenous matrix:
  • Zones of alternatively dense and sparse areas of Antoni type A and B

Differential Diagnosis

• Cerebellopontine lesions:
  • Meningioma
  • Glioma
  • Facial nerve schwannoma
  • Epidermoid
  • Hemangioma
  • Arachnoid cyst
• Sensorineural hearing loss:
  • Ménière disease
  • Ototoxicity
  • Presbycusis
  • Cerebellar pathology

Medication (Drugs)

A greater understanding of the molecular mechanisms behind vestibular schwannoma tumorigenesis has begun to yield novel therapies. With further research, it may be possible to significantly reduce morbidity and mortality rates by decreasing tumor burden, tumor volume, hearing loss, and cranial nerve deficits seen in vestibular schwannomas. The following have been suggested therapies (4)[C]:

• Avastin induces regression of progressive schwannomas by over 40% and improves hearing.
• An inhibitor of 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.

Additional Treatment

• Treatment options include observation, stereotactic radiosurgery, fractionated radiotherapy, and microsurgery (5)[B].
• Without intervention 16–26% of patients require additional treatment, with 54–63% preserving functional hearing.
• With radiosurgery, only 2–4% require additional treatment and hearing preservation is accomplished in 44–66%.
• With fractionated radiotherapy 3–7% will require additional treatment and hearing preservation is reported at 59–94% of patients, although long-term outcomes are not known.
• Up to 57% of acoustic neuromas may not grow or shrink without treatment (6)[B].
• Up to 70% of extracanalicular tumors may never have a growth rate exceeding 2 mm per year (7)[B].

• Conservative therapy is more likely to preserve hearing than radiotherapy or surgery (8)[C].
• 69% of patients with 100% speech discrimination at diagnosis have maintained good hearing, even after 10 years of observation (9)[B]

• Yearly MRI follow-up for slow-growing tumors is advised.
• If an asymptomatic tumor becomes symptomatic, this is often indication for intervention.

Stereotactic radiosurgery:

• Gamma knife single-dose stereotactic radiosurgery:
  • Performed on an outpatient basis
  • Alternative for those with smaller tumor (<3 cm) or contraindications to microsurgery
  • Stereotactic radiation is shown to have better tumor control rates than conservative management (10)[A].
  • Lower-dose radiation has lower complication rates, but evidence is insufficient whether this is as effective as high-dose radiation in tumor control (11)[A].
  • Higher-dose radiation significantly influences hearing preservation rates (12)[C].
  • Complications include hydrocephalus, trigeminal, and/or facial nerve neuropathy from radiation damage.
• Fractionated stereotactic radiosurgery:
  • Conformal radiation delivers a higher dose of radiation within the tumor and less damage to surrounding healthy tissue.
  • Requires multiple treatments and the total dose of radiation is higher compared to the single-dose radiation.
  • Suitable for all sizes of tumor.

Surgery/Other Procedures

• Recommended definitive treatment (13)[A].
• Lowest rate of recurrence, with up to 97.5% complete tumor removal (13)[A]
• Intraoperative facial nerve monitoring is generally used.
• 3 standard approaches, all using operating microscopes:
  • Retromastoid/Retrosigmoid: For any size, especially tumors located mostly outside the internal auditory canal and adjacent to the brainstem. May require retraction of cerebellum.
  • Middle cranial fossa: For small tumors with aim of preserving hearing. Involves retraction of temporal lobe and has higher risk of facial nerve injury.
  • Translabyrinthine: For larger tumors. Hearing not preserved. Completely exposes the distal internal auditory canal and has more favorable facial nerve results.
  • Transpetrous approaches are safe for acoustic neuroma removal and the postoperative complication rate is low (14)[B].
  • The retrolabyrinthine approach seems to be a good hearing preservative approach, regardless of tumor volume (14).
• Endoscopic approach used in some centers
• Surgical complications:
  • Hearing loss
  • CSF leakage
  • Facial nerve injury
  • Headache
  • Meningitis

Follow-Up Recommendations

MRI and audiometric follow-up for those treated by radiotherapy and conservative management

• Preoperative practice of physical activity promotes the neuroplasticity of neural networks involved in motor learning, which allows to benefit of physical therapy more rapidly and efficiently (15).
• Physical activity allows the implementation of new sensorimotor and behavioral strategies leading to an improvement of balance control.

Complications

Due to pressure effect of a large tumor:

• Cranial nerve compression
• Hydrocephalus
• Brainstem compression
• Cerebellar tonsil herniation

ICD-9

• 225.1 Benign neoplasm of cranial nerves
• 237.72 Neurofibromatosis, type 2 [acoustic neurofibromatosis]

ICD-10

• D33.3 Benign neoplasm of cranial nerves
• Q85.02 Neurofibromatosis, type 2

SNOMED

• 126949007 acoustic neuroma (disorder)
• 92503002 neurofibromatosis, type 2 (disorder)