Acoustic schwannomas, also known as vestibular schwannomas, are relatively common tumours that arise from the vestibulocochlear nerve (CN VIII) and represent ~80% of cerebellopontine angle masses. Bilateral acoustic schwannomas are strongly suggestive of neurofibromatosis type 2 (NF2).
On imaging, they classically present as a solid nodular mass with an intracanalicular component that often results in widening of the porus acusticus. They usually show intense contrast enhancement and, when larger, cystic degeneration can be present. Haemorrhagic areas may also be seen, but calcification is typically not present.
Acoustic schwannomas account for 7-8% of all primary intracranial tumours 2 and 75-90% of cerebellopontine angle masses 1,2,8. The vast majority (95%) of solitary lesions are sporadic. 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.
The typical presentation is with adult-onset sensorineural hearing loss or tinnitus. In some patients, this goes unnoticed, and presentation is delayed until the lesion is much larger and presents with symptoms related to mass effect. Possibilities include cerebellar and brainstem symptoms (e.g. cranial nerve dysfunction other than vestibulocochlear), or hydrocephalus due to the effacement of the fourth ventricle.
Acoustic schwannomas are benign tumours (WHO grade 1), which usually arise from the intracanalicular segment of the vestibular portion of 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.
They are well-circumscribed encapsulated masses which, unlike neuromas, arise from but are separate from nerve fibres 7, which they usually splay and displace rather than engulf.
They can display two types of growth pattern:
- 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
- loose meshwork of cells
- less densely cellular
- microcysts and myxoid change
- most have an intracanalicular component, often widening the porus acusticus (trumpeted IAM sign) (up to 90%) 5
- a small "CSF cap" typically remains separating intracanalicular tumour from the cochlea; however, grow laterally through the cochlea (transmodiolar) or vestibule (transmacular) into the middle ear may occasionally occur
- extracanalicular extension may result in an "ice cream cone" appearance, presumably representing tumour growth inwards along a path of least resistance
- a minority are purely extracanalicular, merely abutting the porus acusticus (~20%) 1,5
- rarely, small tumours may be confined to the labyrinth (see intralabyrinthine schwannoma) 4
- small tumours tend to be solid, but larger tumour commonly demonstrate cystic degeneration 2
- +/- haemorrhagic areas
- typically without calcification
May show erosion and widening of the internal acoustic canal. The density of these tumours on non-contrast imaging is variable, and often they are hard to see, especially on account of beam hardening and streak artefact from the adjacent petrous temporal bone.
Contrast enhancement is present but can be underwhelming, especially in larger lesions with cystic components.
- slightly hypointense to the adjacent brain (63%) 2
- isointense to the adjacent brain (37%) 2
- may contain hypointense cystic areas
- heterogeneously hyperintense to adjacent brain 5
- fluid intensity cystic areas
- may have associated peritumoral arachnoid cysts 3
T1 C+ (Gd)
- contrast enhancement is intense
- however, heterogeneous in larger tumours
Linear enhancement may not indicate a tumour, but if there is a nodular enhancement, suspect tumour recurrence (requires follow-up MRI).
Treatment and prognosis
There is 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 :
- observation and follow-up
- stereotactic radiosurgery
- microsurgery: number of approaches are possible, including 8:
- retrosigmoid (transmeatal) (suboccipital)
- able to preserve hearing
- can be used for large tumours
- limited view of the internal auditory canal
- has a greater chance of residual tumour (in the lateral aspect of internal auditory canal)
middle cranial fossa
- best for small intracanalicular tumours
- able to preserve hearing
- careful skeletonisation of the facial nerve required (i.e. intraoperative facial nerve monitoring by needle electromyography with continuous stimulation)
- loss of hearing guaranteed
- retrosigmoid (transmeatal) (suboccipital)
Overall tumour recurrence is low, ranging between 1 and 9% 8.
The most frequent differentials to be considered are:
- usually more homogeneous in appearance: 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 the trumpeted internal acoustic meatus sign
- calcification more common
- no enhancing component
- very high signal on DWI
- does not widen the internal auditory canal
- usually does not remodel the internal auditory canal, as metastases are usually present for only a short time
- centred on the fourth ventricle
- does not extend into the internal auditory canal
- usually younger patients
What the surgeon wants to know
In addition to general remarks about the size and location of the tumour, significant findings that influence surgical management include 8:
- how close to the fundus of the IAC the tumour reaches
- anatomical variations
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