Schwannoma
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Shanshan Cen, M.D. [2]
Synonyms and keywords: Neurilemmoma, Neurinoma, Neurolemmoma, Schwann cell tumor, Schwannomatosis
Overview
Schwannomas are benign tumors of Schwann cell origin and are the most common tumor of peripheral nerves, including cranial nerves.
Historical Perspective
Classification
Pathophysiology
Causes
Differentiating Schwannoma from other Diseases
Epidemiology and Demographics
Risk Factors
Screening
Natural History, Complications and Prognosis
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Epidemiology
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.
Associations
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 the concomitant involvement of cranial nerve VIII.
Clinical presentation
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.
Pathology
Macroscopic appearance
Schwannomas are benign encapsulated neoplasms of Schwann cells (WHO grade I). They arise eccentrically from their parent nerve, with the nerve fibers splayed along their surface (as distinct to neurofibromas which arise within the nerve).
Microscopic appearance
Conventional schwannomas are composed of spindle cells which demonstrate two growth patterns: Antoni type A and Antoni type B 7,8.
Antoni type A pattern: elongated cells are densely packed and arranged in fascicles. Palisades are sometimes seen; when prominent these form Verocay bodies.
Antoni type B pattern cells are less compact and are prone to cystic degeneration.
Variants
Schwannoma variants include 6,8:
- ancient schwannoma
- cellular schwannoma
- predominantly composed of Antoni A tissue
- no Verocay bodies
- most commonly found in a paravertebral location, or trigeminal nerves (CN V)
- melanotic schwannoma: dense melanin pigment
- plexiform schwannoma
- usually arise from skin or subcutaneous tissues
- usually diagnosed at birth or childhood
- usually sporadic, but rarely associated with NF2
- should not be confused with plexiform neurofibromas
- associated with NF1
- may undergo malignant change
Locations
- intracranial schwannomas
- 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)
- spinal schwannoma
- arising from spinal nerve roots
- trunk
- intercostal nerves – see intercostal nerve schwannoma
- posterior mediastinum
- retroperitoneum
- gastrointestinal schwannoma
- limbs
- especially flexor surfaces (specifically ulnar and peroneal nerves)
Radiographic features
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 hemorrhage 3
- hemorrhage occurs in 5% of cases 3
- calcification is rare
CT
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 remodelling with smooth corticated edges
MRI
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, especially in larger tumors
- T2*: larger tumors 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
- target sign
- 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.
Overview
Schwannomatosis is one form of a genetic disorder called Neurofibromatosis (NF) that has only recently been recognized. It is a rare disorder, affecting only around 1:40,000 individuals.
Symptoms
- Schwannomas occur instead of Neurofibromas.
- Multiple Schwannomas occur.
- The Schwannomas develop on cranial, spinal and peripheral nerves.
- Chronic pain, and sometimes numbness, tingling and weakness.
- About 1/3 of patients have segmental Schwannomatosis, which means that the Schwannomas are limited to a single part of the body, such as an arm, a leg or the spine.
- There are several cases where people with Schwannomatosis have developed a vestibular Schwannoma (acoustic neuroma).
- Patients with Schwannomatosis do not have learning disabilities related to the disease.
- Symptoms are generally brought on by hormonal changes such as puberty and pregnancy.
Cause
- Like the NF1 and NF2 genes, the candidate Schwannomatosis gene, named INI1, is a tumor suppressor that regulates cell cycle, growth and differentiation. INI1 is also known as SMARCB1, hSNF5, or BAF47, and lies on human Chromosome 22. INI1 is also mutated in other tumors including malignant brain & kidney tumors in infants.
- Schwannomatosis is known to be a genetic disorder, however it can skip generations.
Treatment
- If feasible, the Schwannomas can be surgically removed. Any tumor-associated pain usually subsides after tumor removal. Damaged nerves and scar tissue can be a result of surgery and pain can be an ongoing problem.
- Sometimes, a tumor will reappear at the same site after surgery.
- If surgery is unfeasible, then pain management will have to be used. Schwannomatosis can sometimes cause severe, untreatable pain over time.
- Other than surgery and/or pain management, there are no other medical treatments available. There are no drugs available to treat Schwannomatosis.
- Gamma knife surgery can be performed on head tumors to help slow growth of a tumor, although there is no guarantee that it will work.
- Recently, many advances are being made in the treatment of schwannomas. Of interest is CyberKnife, manufactured by Accuray. Success rates, although limited in data, appear to be in the low to mid ninety percent range.
Diagnosis
Staging | History and Symptoms | Physical Examination | Laboratory Findings | X Ray | CT | MRI | Ultrasound | Other Imaging Findings | Other Diagnostic Studies | Biopsy
Treatment
Medical Therapy | Surgery | Primary Prevention | Secondary Prevention | Cost-Effectiveness of Therapy | Future or Investigational Therapies
Case Studies
Related Chapters
- Neurofibroma
- Vestibular schwannoma (Acoustic neuroma)