Acoustic schwannoma

Acoustic schwannomas are a relatively common type of tumour which arise from the vestibulocochlear nerve (CN VIII), and when bilateral are strongly suggestive of (NF2).

Demographics and clinical presentation

They account for 7 - 8% of all primary intracranial tumours ² and 75 - 90% of cerebellopontine angle masses ^1,2,8. The typical presentation is with sensorineural hearing loss or tinnitus. 

Pathology

Acoustic schwannomas are benign tumours, which usually arise from the intracanalicular segment of the vestibular portion of the the vestibulocochlear nerve (CN VIII) ^2,4, near the transition point between glial and Schwann cells (Obersteiner-Redlich zone) ⁸ . In over 90% of cases these tumours arise from the inferior division of the vestibular nerve ⁸. Bilateral vestibular schwannomas are highly suggestive of neurofibromatosis type 2 (NF2), although bilateral tumours are encountered in the familial form of acoustic schwannomas in the absence of other stigmata of NF2 ⁵. 

Well circumscribed encapsulated masses, which unlike neuromas, arise from but are separate from nerve fibers ⁷, which they usually splay and displace rather than incorporated. 

They can display two types of growth pattern:

1. Antoni A
   • elongated cells with cytoplasmic processes arranged in fascicles ⁷
   • little stromal matrix
   • Verocay bodies : nuclear free zones of processes lying between regions of nuclear palisading
2. Antoni B
   • loose meshwork of cells
   • less densely cellular
   • microcysts and myxoid change

Radiographic features

Most vestibular schwannomas have an intracanalicular component, and often result in widening of the porus acousticus resulting in the trumpet IAM sign, which is present in up to 90% of cases ⁵. In a minorty of cases ( ~20%) they are purely extracanalicular, only abutting the porus acousticus ^1,5. Occasionally these tumours with grow laterally through the cochlear or vestibule into the middle ear. Rarely they are small and confined to the labyrinth ⁴.

Small tumours tend to be solid whereas cystic degeneration seen commonly in larger tumours ². Calcification and haemorrhagic areas are also seen.

MRI

• T1
  • slightly hypointense c.f. adjacent brain: 63% ²
  • isointense c.f adjacent brain: 37% ²
  • hypointense cystic areas
• C+ (GAD) : contrast enhancement is vivid, but heterogeneous in larger tumours
• T2
  • heterogeneously hyperintense c.f to adjacent brain ⁵
  • cystic areas fluid intensity
  • may have associated peri-tumoural arachnoid cysts ³

What the surgeon wants to know

In addition to general remarks about the size and location of the tumour, important findings that influence surgical management include ⁸:

• how close to the fundus of the IAC the tumour reaches
• anatomical variations
  • high-riding or dehiscent jugular bulb
  • aberrant ICA
  • aberrant course of the facial nerve
  • pneumatisation of the temporal lobe
  • size and location of the sigmoid sinus

Treatment

There is a great variability in the rate of growth of these tumours, and as such the decision to treat requires consideration of patient's age and co-morbidities. The options include ⁶ :

• sterotactic radiosurgery
• microsurgery : number of approaches are possible, including ⁸:
  • retrosigmoid (suboccipital)
    • able preserve hearing
    • can be used for large tumours
    • limited view of the IAC
  • middle cranial fossa
    • best for small intracanalicular tumours
    • able to preserve hearing
  • translabyrinthine
    • careful skeletonisation of the facial nerve required
    • loss of hearing guaranteed

Overall there tumour recurrence is low, ranging between 1 and 9% ⁸.

Differential diagnoses

The most frequent differential to be considered are:

• meningioma
  • significant signal heterogeneity with cystic or haemorrhagic areas is more typical of vestibular schwannoma than meningiomas (although cystic meningiomas do occur) 
  • meningiomas tend to have a broad dural base
  • usually lack trumpet IAM sign
• metastasis
• ependymoma

See also

• cerebellopontine angle masses