Acoustic Neuroma

Acoustic Neuroma

Basics

Description

• Slow-growing benign tumor, most often arising from the vestibular division of 8th cranial nerve
• Originates from Schwann cells of the nerve sheath (“schwannoma”)
• Usually arises in the internal auditory canal near the cerebellopontine angle
• Often has extracanalicular portion into the cerebellopontine angle, but may also stay purely intracanalicular
• Most are unilateral; bilateral only seen in neurofibromatosis type II

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

Incidence

• 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).
• Smoking may decrease the risk (1).

Genetics

• Unknown for unilateral acoustic neuroma (AN)
• Neurofibromatosis type II: Bilateral ANs:
  • 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

Unknown

Commonly Associated Conditions

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

Diagnosis

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 (CN) V
  • Ataxia due to cerebellar or brainstem compression from very 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 & Interpretation

Lab

Initial lab tests

• Pure-tone and speech audiometry (asymmetrical, high-frequency sensorineural hearing loss)
• Speech discrimination
• Stacked auditory brainstem response (ABR): 95% sensitivity and 88% specificity (2). Can detect tumors <1 cm.
• Standard ABR: Can only detect tumors >1 cm

Imaging

Initial approach

• Magnetic resonance imaging (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
• Computed tomography (CT):
  • Detect tumors as small as 1 cm
  • Up to 37% false-negatives
  • Provides good information of surrounding bony structures of the tumor

Pathological Findings

• 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 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

Treatment

Medication

Chemotherapy has not yet been explored sufficiently.

Additional Treatment

General Measures

• Conservative management is suitable for elderly patients with contraindications to surgery and radiotherapy.
• Up to 57% of acoustic neuromas may show zero growth or shrinkage (3)[B].
• Up to 70% of extracanalicular tumors may never have growth rate exceeding 2 mm per year (4)[B].
• Growth rate of enlarging acoustic neuromas decreases over time:
  • From 4.9 mm/yr in the 1st year of detected growth to 0.75 mm in 4th year.
• Up to 20% of patients may eventually fail conservative management and require intervention.
• More likely to preserve hearing than radiotherapy or surgery (5)[C]
• 69% of patients with 100% speech discrimination at diagnosis have maintained good hearing even after 10 years of observation (6)[B]

Issues for Referral

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

Additional Therapies

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
  • No discernible significant difference between growth patterns of untreated tumors and those treated radiosurgically (6)[A]
  • Lower-dose radiation has lower complication rates, but evidence is insufficient on whether as effective as high-dose radiation in tumor control (7)[A]
  • Higher-dose radiation significantly influences hearing preservation rates (9)[C]
  • Complications include 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 (8)[A]
• Lowest rate of recurrence, with up to 97.5% complete tumor removal (8)[A]
• Intraoperative facial nerve monitoring is generally used.
• 3 standard approaches, all using operating microscopes:
  • Retromastoid/retrosigmoid: For any size, especially tumor 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.
• Endoscopic approach used in some centers
• Surgical complications:
  • Hearing loss
  • Cerebrospinal fluid leakage
  • Facial nerve injury
  • Headache
  • Meningitis

Ongoing Care

Follow-Up Recommendations

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

Complications

Due to pressure effect of a large tumor:

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