Schwannomas are benign tumours of Schwann cell origin and are the most common tumour of peripheral nerves, and common posterior fossa masses.
This article provides a general overview of schwannomas. For a discussion of schwannomas located at specific sites, please refer to the relevant articles listed below.
Peak presentation is in the 5th-6th decades. When they occur in patients with neurofibromatosis type 2 (NF2), schwannomas usually present by the 3rd decade 4. There is no sex predilection.
Most schwannomas are solitary (90%) 2 and sporadic, however there is an association with NF2 (abnormality of chromosome 22). Multiple schwannomas are characteristic of NF2. Approximately 18% of solitary schwannomas occur in patients with NF2 4.
There is also schwannomatosis, which consists of multiple schwannomas without concomitant involvement of cranial nerve VIII.
Presentation depends on location of the tumour.
Schwannomas are benign encapsulated neoplasms of schwann cells (WHO grade I). They arise eccentrically from their parent nerve. They are composed of spindle cells of types: Antoni A and Antoni B 7.
Antoni type A neurilemoma pattern cells are densely packed and arranged in fascicles; acellular areas lie between opposing rows of parallel nuclei (Verocay bodies).
Antoni type B neurilemoma pattern cells are less compact and are prone to cystic degeneration.
Pathologic variants include 6:
- conventional (most common type): fibrous capsule, hyaline vessels, Antoni A and loose textured Antoni B areas (Verocay bodies)
- cellular schwannomas (predominantly Antoni A tissue without Verocay bodies)
- melanotic schwannomas (dense melanin pigment)
- plexiform schwannomas (do not undergo malignant change, unlike plexiform neurofibromas)
cranial nerves: any cranial nerve may be involved, except olfactory (CN I) and optic (CN II), which lack sheaths composed of Schwann cells. The most commonly involved cranial nerves are sensory nerves with pure motor nerves rarely involved; these include 7:
vestibulocochlear nerve (CN VIII): 90%
- most commonly the superior portion of vestibular division
- see acoustic neuroma
- trigeminal nerve (CN V): 2nd most common
- facial nerve (CN VII): 3rd most common
- cranial nerves of the jugular foramen (CN IX, X, XI)
oculomotor nerve (CN III) and trochlear nerve (CN IV): uncommon
- most commonly seen in patient with NF2 7
- hypoglossal nerve (CN XII): very rare
- vestibulocochlear nerve (CN VIII): 90%
- non-cranial nerve or intracerebral (very rare)
- cranial nerves: any cranial nerve may be involved, except olfactory (CN I) and optic (CN II), which lack sheaths composed of Schwann cells. The most commonly involved cranial nerves are sensory nerves with pure motor nerves rarely involved; these include 7:
- spinal schwannoma
- especially flexor surfaces (specifically ulnar and peroneal nerves)
General imaging features of schwannomas include:
- well circumscribed masses which displace adjacent structures without direct invasion
- cystic and fatty degeneration are common 4
- the larger a schwannoma, the more likely it is to show heterogeneity because of cystic degeneration or haemorrhage 3
- haemorrhage occurs in 5% of cases 3
- calcification is rare
CT is not as sensitive or specific for the diagnosis of schwannoma as MRI but is often the first investigation obtained. It is particularly useful in assessing bony changes adjacent to the tumour.
Imaging features include:
- low to intermediate attenuation
- intense contrast enhancement
- small tumours typically demonstrate homogeneous enhancement
- larger tumours may show heterogeneous enhancement
- adjacent bone remodeling with smooth corticated edges
Schwannomas have fairly predictable signal characteristics 7:
- T1: isointense or hypointense
- T1 C+ (Gd): intense enhancement
T2: heterogeneously hyperintense (Antoni A: relatively low, Antoni B: high)
- cystic degenerative areas may be present, espeically in larger tumours
- T2*: larger tumours often have areas of hemosiderin
A number of signs can also be useful:
- split-fat sign: thin peripheral rim of fat best seen on planes along long axis of the lesion in non fat suppressed sequences
- peripheral high T2 signal
- central low signal
- rarely seen intracranially 7
- fascicular sign: multiple small ring-like structures
Treatment and prognosis
Schwannomas are slow-growing lesions. Surgery is the treatment of choice. As schwannomas do not infiltrate the parent nerve, they can usually be separated from it. Recurrence is unusual, even after complete resection. They almost never undergo malignant change.
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