Vestibular Schwannoma and Acoustic Neuroma
Background
What is the cell of origin for vestibular schwannomas and acoustic neuromas (ANs)?
The Schwann cell of the myelin sheath is the cell of origin for ANs.
Which CN do ANs affect?
ANs affect CN VIII. Most affect the vestibular portion of the nerve.
In which anatomic region do ANs arise?
Most ANs are found in a region called the cerebellopontine angle (CPA).
Most people with symptomatic ANs will present between what ages?
Most symptomatic pts are 30–50 yo.
What proportion of ANs are sporadic?
The majority (90%) are sporadic as well as unilat.
What % of ANs are bilat, and with what genetic abnormality are they associated?
10% are bilat and associated with NF-2, the tumor suppressor gene on chromosome 22.
What protein is abnl in NF-2 pts?
Merlin or schwannomin (involved in actin cytoskeleton organization)
What is the name of the anatomic layer of CN VIII that gives rise to most ANs?
The Obersteiner-Redlich zone (the junction between the central and peripheral myelin) gives rise to most ANs.
Subclinical ANs are present in what % of the general population?
Up to 1% (autopsy series) of the general population harbor subclinical ANs.
ANs account for what % of intracranial tumors?
5%–8% of intracranial tumors are ANs.
Apart from NF-2, what are 2 other risk factors that predispose to the development of ANs?
Loud noise exposure (ORR 13) and parathyroid adenoma (ORR 3.4)
What are the Antoni A and Antoni B areas on histopathology?
Antoni A and B are zones of dense and sparse cellularity, respectively.
For what do ANs stain on immunohistochemistry?
Most ANs stain for S100.
How do bilat ANs fare after Tx when compared to unilat ANs?
Bilat ANs have similar failure rates to unilat lesions if treated adequately.
Workup/Staging
What tests are performed on physical exam for pts with CPA lesions?
Rinne test (mastoid bone, air conduction > bone conduction) and Weber test (occiput, vibratory sound louder on good side) to confirm sensorineural hearing loss; also need to check for other CN deficits (VII, hypesthesia, corneal twitching)
What CN is being tested when a pt is asked to tighten the ant neck muscles?
CN VII (innervates platysma muscle) can often be affected with large AN lesions.
Why are pts with CPA lesions often asked to march in place with their eyes closed on physical exam?
When the vestibular nerve is affected, pts will often veer to the side of the lesion.
What is the best initial screening test for ANs, and what does it usually show?
Audiometry (asymmetric sensorineural loss, more prominent at ↑ frequencies) is the best screening tool for ANs.
What is the avg growth rate per yr for ANs?
~1 mm/yr. Growth rates range from 0.5 mm/yr (slow-growing lesions) to 2 mm/yr (fast-growing lesions).
What % of ANs are stable (shrink/do not grow)?
~20%–40% of ANs are considered stable.
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