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Schwannomas, less commonly called neurinomas or neurilemmomas, are benign tumors of Schwann cell origin and are the most common tumor of peripheral nerves, including cranial nerves.
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.
The vast majority of schwannomas are sporadic, with a peak presentation in the 5th to 6th decades. There is no sex predilection 9.
When they occur in patients with neurofibromatosis type 2 (NF2), schwannomas usually present by the 3rd decade 4.
Most schwannomas are solitary (90%) 2,9 and sporadic, however, there is an association with neurofibromatosis type 2 (NF2) (abnormality of chromosome 22). Multiple schwannomas are characteristic of neurofibromatosis type 2. Approximately 18% of solitary schwannomas occur in patients with neurofibromatosis type 2 4.
There is also schwannomatosis, which consists of multiple schwannomas without the concomitant involvement of cranial nerve VIII.
Presentation depends on the location of the tumor (see below) but generally, symptoms are due to local mass effect or dysfunction of the nerve they arise from.
- cranial nerves: although almost any cranial nerve may be involved, except olfactory nerves and optic nerves which lack sheaths composed of Schwann cells, by far the most commonly involved nerve is the vestibulocochlear nerve (CN VIII)
- non-cranial nerve or intracerebral (very rare)
- intraorbital schwannoma: commonly arise from supraorbital and supratrochlear nerves in the upper anterior orbital cavity 10
- arising from spinal nerve roots
- especially flexor surfaces (specifically ulnar and peroneal nerves)
Schwannomas are benign encapsulated neoplasms of Schwann cells (WHO grade 1 9). They arise eccentrically from their parent nerve, with the nerve fibers splayed along their surface (as distinct to neurofibromas which arise within the nerve).
Conventional schwannomas are composed of spindle cells that demonstrate two growth patterns: Antoni type A and Antoni type B 7-9.
Antoni type A pattern: elongated cells are thickly bundled and organized in fascicles. Palisades are occasionally observed; when they are prominent they constitute Verocay bodies. The cells also have fusiform nuclei and eosinophilic cytoplasm. Antoni type A is a strongly Periodic acid-Schiff (PAS) positive and immunoperoxidase assay for laminin 10.
Antoni type B pattern cells are less compact and are prone to cystic degeneration, and haphazardly distributed cells with distinct cytoplasmic margins in a loose myxoid matrix 10.
Several schwannoma subtypes are recognized 6,8,9:
- degenerative changes with hyalinization, ischemic change, and atypical appearing nuclei
- cellular schwannoma
- predominantly composed of Antoni type A tissue
- no Verocay bodies
- most commonly found arising from large nerves, brachial and lumbosacral plexus and thus often in a paravertebral location
- epithelioid schwannoma
- usually sporadic but may be associated with schwannomatosis
- microcystic/reticular schwannoma
- rarest subtype
- typically arise from viscera, especially the gastrointestinal tract 9
- plexiform schwannoma
- neuroblastoma type 10
- schwannoma with neuromelanin accumulation
- not a distinct subtype
- not to be confused with PRKAR1A-associated malignant melanotic nerve sheath tumor (a.k.a. melanotic schwannoma) seen in patients with Carney complex 9
General imaging features of schwannomas include:
- well-circumscribed masses which displace adjacent structures without direct invasion
- cystic and fatty degeneration is common 4
- the larger a schwannoma, the more likely it is to show heterogeneity because of cystic degeneration or hemorrhage 3
- hemorrhage 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 tumor.
Imaging features include:
- low to intermediate attenuation
- intense contrast enhancement
- small tumors typically demonstrate homogeneous enhancement
- larger tumors 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 type A: relatively low; Antoni type B: high)
- cystic degenerative areas may be present, especially in larger tumors
- T2*: larger tumors often have areas of hemosiderin
Several signs can also be useful:
- split-fat sign: the thin peripheral rim of fat best seen on planes along the 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 after complete resection. They rarely undergo malignant change.