ACOUSTIC NEUROMA

ACOUSTIC NEUROMA - Sam Kim, MD, MBBS, MedSc; Phillip Chang, MD
BASICS
DESCRIPTION
• Slow-growing benign Schwannoma, most often arising from the vestibular division of 8th cranial nerve
• Originates from Schwann cells of the nerve sheath
• Usually arise in the internal auditory canal near the cerebellopontine angle
• Most are unilateral; bilateral only seen in Neurofibromatos 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 of 30
Incidence
• 1/100,000 per year
• Asymptomatic lesions may be more common
Prevalence
3,000 diagnosed annually in the US
RISK FACTORS
Unknown
Genetics
• Unknown for unilateral acoustic neuroma
• Neurofibromatosis type II: Bilateral ANs
- Autosomal domiant
- Gene located on chromosome 22q1
PATHOPHYSIOLOGY
Exerts pressure on the surrounding structures
• Compression of acoustic and facial nerve when within internal acoustic canal
• Compression of brainstem, 4th ventricle and trigeminal nerve when at the cerebellar pontine angle
ETIOLOGY
Unknown
ASSOCIATED CONDITIONS
• Neurofibromatosis type II
• Pregnancy may accelerate the growth of the tumor


DIAGNOSIS
SIGNS AND SYMPTOMS
• Common
- Sensorineural hearing loss (unilateral)
- 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 CN V
- Ataxia due to cerebellar or brainstem compression from very large tumor
History
• Hearing loss is often progressive
• Loss of speech discrimination
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 neurological examination 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
TESTS
• Pure-tone and speech audiometry (asymmetrical, high-frequency sensorineural hearing loss)
• Speech discrimination
• Stacked auditory brainstem response (ABR): 95% sensitivity and 88% specificity (1). Can detect tumors 1 cm
• Standard ABR: Can only detect tumors >1 cm.
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
• 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
- Meniere's disease
- Ototoxicity
- Presbycusis
- Cerebellar pathology
TREATMENT
SPECIAL THERAPY
Conservative management:
• Suitable for elderly patients contraindicated to surgery and radiotherapy
• Up to 57% of acoustic neuromas may show no growth (2)[A]
• Average growth rate is 1.9 mm per year (2)[A]
• Up to 20% of patients may eventually fail conservative management.
Radiotherapy
Stereotactic radiosurgery
• Gamma knife stereotactic radiosurgery
- Performed on an outpatient basis
- Alternative for those with smaller tumor (3 cm) or contraindicated to microsurgery
- Have shown to suppress tumor growth and provide good tumor control (3)[B]
- Complications include trigeminal and/or facial nerve neuropathy from radiation damage.
• Fractionated stereotactic radiosurgery
- Conformal radiation delivers a higher dose radiation within the tumor and less damage to surrounding healthy tissue.
- Requires multiple treatment
MEDICATION (DRUGS)
Chemotherapy has not yet been explored sufficiently.
SURGERY
• Recommended definitive treatment (A)
• Lowest rate of recurrence, with up to 97.5% complete tumor removal (4)[A]
• Intraoperative facial nerve monitoring is generally used.
• 3 standard approaches
- Retromastoid/retrosigmoid: For any size
- Middle cranial fossa: For small tumors with aim of preserving hearing
- Translabirinthe: For larger tumors. Hearing not preserved. Completely exposes the distal internal auditory canal
• Surgical complications
- Hearing loss
- CSF leakage
- Facial nerve injury
- Headache
- Meningitis
FOLLOW-UP
COMPLICATIONS
Due to pressure effect of a large tumor
• Cranial nerve compression
• Hydrocephalus
• Brainstem compression
• Cerebellar tonsil herniation
REFERENCES
1. Don M, et al.The stacked ABR: a sensitive and specific screening tool for detecting small acoustic tumors. Audiol Neurootol. 2005;274-290.
2. Smouha EE, et al. Conservative management of acoustic neuroma: A meta-analysis and proposed treatment algorithm. Laryngoscope. 2005;450-454.
3. Lunsford LD, Niranjan A, Flickinger JC, et al. Radiosurgery of vestibular Schwannomas: Summary of experience in 829 cases. J Neurosurg. 2005;102 Suppl:195.
4. Kaylie DM, et al. A meta-analysis comparing outcomes of microsurgery and gamma knife radiosurgery. Laryngoscope. 2000;1850-1856.