Schwannoma: Etiology, Epidemiology, Classification, Clinical Features, Diagnosis, Treatment, and Prognosis

Schwannoma (neurinoma, neurilemmoma) is a typically encapsulated benign tumor arising from Schwann cells in the peripheral and cranial nerves, most often involving the VIII pair (vestibular schwannoma, VS).

Most schwannomas grow slowly, have clear boundaries with the nerve (expansive growth), and very rarely become malignant.

3D model of a schwannoma:

Schwannoma of the spinal nerve (spinal schwannoma)

Schwannoma of the spinal nerve (spinal schwannoma)

Schwannoma of the spinal nerve (spinal schwannoma)

Etiology and molecular background

Sporadic schwannomas are caused by somatic spontaneous mutations in the NF2 gene on chromosome 22q, leading to the loss of merlin protein function, disrupting Schwann cell proliferation control. The pathogenesis of the disease corresponds to the “two-hit” model: the initial NF2 mutation in one allele followed by subsequent loss of heterozygosity or other inactivation of the second allele (LOH 22q, point mutations, mitotic recombination, promoter methylation).

In NF2-related schwannomatosis (formerly termed neurofibromatosis type 2), the mutant NF2 allele is present in the germline. Additional somatic events (“hits”) in NF2 and other genes on chromosome 22 lead to the development of multiple schwannomas (including bilateral vestibular ones), meningiomas, and ependymomas, etc.

In other forms of schwannomatosis (SMARCB1- and LZTR1-associated), mutations in the corresponding tumor suppressor genes on 22q are inherited; additional somatic inactivation of NF2 is necessary for tumor development.

Risk factors for schwannoma development

• Presence of NF2-related schwannomatosis: an autosomal dominant syndrome predisposing to schwannomas; pathogenic NF2 variant carriers nearly inevitably develop bilateral vestibular schwannomas and/or multiple other schwannomas.
• Presence of SMARCB1/LZTR1-swiftamatosis: heritable syndromes characterized by multiple predominantly non-vestibular schwannomas, in the absence of bilateral vestibular schwannomas.

Neurofibromatosis type 1 (NF1) is not associated with typical schwannomas (NF1 predominantly involves neurofibromas and malignant peripheral nerve sheath tumors, MPNSTs), and schwannomas are not considered a component of NF1.

Previous radiation therapy is a risk factor for the development of MPNSTs, but not of typical schwannomas.

Epidemiology

Schwannomas are the most common benign tumors of peripheral nerves.

Most schwannomas are solitary tumors; the presence of multiple tumors requires exclusion of NF2-related schwannomatosis and other forms of schwannomatosis.

Epidemiology of spinal and peripheral schwannomas

Among intradural extramedullary tumors of the spinal canal, nerve sheath tumors make up about 25%, with approximately 2/3 of these being predominantly sporadic schwannomas.

Only about 2% of all spinal schwannomas occur in patients with NF2; the rest are sporadic incidental findings or symptomatic solitary tumors.

In NF2-related schwannomatosis, spinal tumors (primarily dorsal root schwannomas, but also meningiomas and ependymomas) are identified in 90% of patients during their lifetime, making spinal schwannomas a key epidemiological feature of NF2-related schwannomatosis.

Peripheral (limb, plexus) schwannomas are rare in the general population. A large case series of hundreds of patients shows them as the main pathology among resected benign neoplasms of large peripheral nerve trunks.

Epidemiology of predisposition syndromes (in the context of schwannoma)

NF2-related schwannomatosis: population studies indicate an incidence rate of NF2-related schwannomatosis reaching 1:25000, with a significant proportion being mosaic* forms (at least half of de novo patients).

SMARCB1/LZTR1-related schwannomatosis: data from national neurofibromatosis study centers indicate that this is a much rarer condition than NF2-related schwannomatosis, but with a high individual “burden” of schwannomas in such patients. When analyzing the epidemiology of NF syndromes, schwannomatosis constitutes a minority compared to NF1 and NF2-related schwannomatosis.

Explanation:

*Mosaic forms of schwannomatosis are disease variants where the pathogenic gene variant (typically NF2, SMARCB1, or LZTR1) is present not in all cells of the organism but in some cell clones, arising post-fertilization (post-zygotically). In other words, it is a schwannomatosis caused by somatic mosaicism: some tissues carry the mutation, while others remain genetically normal.

Epidemiology of vestibular schwannomas (VS/acoustic neuroma/schwannoma of the cerebellopontine angle)

Vestibular schwannomas (VS) account for 80–90% of tumors in the cerebellopontine angle in adults; approximately 4–6% are associated with NF2-related schwannomatosis.

Up to 90–95% of patients with NF2 have bilateral vestibular schwannomas by the age of 30, while the vast majority of vestibular schwannomas in the general population are unilateral and sporadic.

In children and young adults with a solitary vestibular schwannoma, the proportion of patients with a germline NF2 mutation is significantly higher than in individuals over the age of 30, making age an important epidemiological marker of latent NF2-related schwannomatosis.

Clinical manifestations of schwannomas

Peripheral schwannomas (of peripheral nerves and plexuses)

Complaints:

• Slow-growing painless or painful nodule/tumor along the course of the nerve;
• Paresthesias;
• Local hypoesthesia or weakness in the nerve distribution area;
• Often a positive Tinel’s sign (tingling on tapping) is reported.

Objectively: a firm, elastic formation, mobile across the nerve axis but fixed along its course; neurological deficit is more often sensory, less often motor.

Spinal schwannomas

Spinal schwannomas often arise from dorsal (sensory) roots, remaining asymptomatic for a long time.

They are often an incidental finding during an MRI of the spine, less commonly during an X-ray of the chest.

Typical symptoms:

• Back pain;
• Radicular sensory disturbances;
• Motor disorders (muscle weakness below the level of spinal cord compression);
• Pelvic organ dysfunction.

3D model of a schwannoma:

Spinal schwannoma of the “hourglass/dumbbell” type with spinal cord compression

Spinal schwannoma of the “hourglass/dumbbell” type with spinal cord compression

Spinal schwannoma of the “hourglass/dumbbell” type with spinal cord compression

Schwannomatoses (NF2/SMARCB1/LZTR1-related schwannomatosis)

Multiple schwannomas are often accompanied by pain syndrome (often with disabling neuropathic pain in SMARCB1/LZTR1-related schwannomatosis), and in NF2-related schwannomatosis, the disease is often combined with meningiomas and spinal tumors.

Vestibular (acoustic) neuroma

The specific triad: unilateral (or bilateral in NF2-related schwannomatosis) sensorineural hearing loss, tinnitus (ringing in the ear), and balance disorder.

For larger tumors: headache, ataxia, symptoms of brainstem compression, hydrocephalus, and facial and trigeminal nerve involvement.

Classification of schwannomas

By localization

• Peripheral nerves (limbs, plexuses);
• Spinal schwannomas (intra/extradural extramedullary). Often (up to 15% of cases) there are “hourglass/dumbbell”-type schwannomas;
• Cranial nerves. The most common type is schwannoma of the eighth cranial nerve, also known as “vestibular schwannoma”/“acoustic schwannoma”/“schwannoma of the cerebellopontine angle.” Less commonly, schwannoma of the V (trigeminal) cranial nerve and others are reported.

By cause (based on clinical and genetic characteristics)

By histological type

• Conventional;
• Other types: cellular, plexiform, epithelioid, with degenerative changes (“ancient”).

All histological types of schwannomas mentioned above are benign.

Malignant peripheral nerve sheath tumor (MPNST) and malignant melanotic nerve sheath tumor (MMNST), formerly known as melanotic schwannoma, belong to separate classes of malignant nerve sheath tumors and are not “malignant schwannomas” per WHO 2021 terminology.

For vestibular schwannomas, separate anatomical classifications exist based on tumor localization and size (Koos grading system, Hannover/Samii classification), as well as functional classifications based on hearing (Gardner-Robertson) and facial nerve function (House-Brackmann).

Diagnosis of neurinoma

Clinical presentation and examination

Evaluation of tumor growth dynamics, pain, neurological manifestations; family history of NF1/NF2/schwannomatosis.

In vestibular schwannoma: pure tone audiometry, speech tests, and, if necessary, vestibular tests.

Imaging

MRI with contrast is the method of choice.

• Peripheral/spinal: hypointense/isointense on T1, hyperintense on T2, intensely and often homogeneously enhanced with contrast; often have a distinct capsule and eccentric positioning relative to the nerve;
• Vestibular: contrast-enhancing tumor within the internal auditory canal extending to the cerebellopontine angle (typical “ice-cream cone sign”).

CT — for assessment of bone structures (expansion of the bony canal or foramina; occasional vertebral destruction).

Morphological verification

For superficial and peripheral tumors, biopsy/total tumor removal with histological examination: Antoni A/B, Verocay bodies, diffuse S100-positivity.

For typical vestibular schwannoma and specific appearance on MRI in immunocompetent patients, biopsy is not required.

Genetic testing

Used not for verification of the tumor itself, but for identification/exclusion of tumor syndromes (NF2-schwannomatosis, SMARCB1/LZTR1-schwannomatosis), assessing family risk, and planning follow-up.

Indications:

• Multiple schwannomas;
• Young age;
• Bilateral vestibular schwannomas associated with another CNS tumor;
• Family history.

Differentiation between NF2-schwannomatosis and SMARCB1/LZTR1-schwannomatosis is essential, as it determines follow-up strategy and risk of other tumors.

For solitary sporadic schwannoma in an adult without a family history, testing is usually not required.

Differential diagnosis

Neurofibroma (often in NF1): intraneural, fusiform, non-expansive growth, lack of distinct capsule, presence of axons within the tumor, malignancy risk (MPNST).

Soft tissue tumors: lipoma, desmoid, ganglion, perineurioma, metastatic and other nerve lesions, paraganglioma, sarcoma.

Other intradural extramedullary tumors: meningioma, myxopapillary ependymoma, etc.

In vestibular schwannoma: meningioma of the cerebellopontine angle, epidermoid, metastasis, less commonly — glial tumor.

Treatment of schwannoma

The treatment strategy for schwannomas should be individualized, considering the following:

• Symptoms (pain, neurological deficit, spinal cord compression);
• Size and growth dynamics;
• Hearing status/facial nerve function (for vestibular schwannomas);
• Genetic background (NF2/SMARCB1/LZTR1);
• Age and concomitant diseases.

Options:

• Observation (wait-and-scan);
• Surgical therapy;
• Stereotactic radiosurgery/radiotherapy (SRS/SRT);
• Drug therapy (often targeted therapy with bevacizumab) — primarily for NF2/progressing inoperable vestibular schwannomas.

Surgical therapy

Peripheral and spinal schwannomas

Indications:

• Symptomatic tumors (pain, neurological manifestations);
• Rapid growth;
• Diagnostic uncertainty;
• Suspicion of malignancy.

The goal is a function-preserving total removal; subtotal resection is allowed if necessary to preserve nerve function.

Technique: microscope, intraoperative neuro-monitoring, and identification and preservation of functional roots; in dumbbell tumors — combined approach.

Outcomes: after total resection, recurrences are rare (less than 5%); with residual tumor, continued growth is noted in less than 30% of cases (more often in NF2-schwannomatosis), but the second surgery is required only for a minority.

Vestibular schwannoma

Current guidelines (EANO 2019, systematic reviews) emphasize a strategy based on size, symptoms, and hearing status.

1. For small/medium tumors with a hearing ear, three strategies are possible:

• Observation;
• SRS;
• Microsurgery with preservation of hearing and facial nerve function.

Systematic reviews show that for small vestibular schwannomas, SRS provides a high rate (>90%) of long-term tumor control and better preservation of facial and auditory nerves compared to surgical treatment.

2. For large vestibular schwannomas (usually >2.5–3 cm with brainstem compression), primary microneurosurgical resection with decompression of the brainstem is recommended; the retrosigmoid and translabyrinthine approaches are most often used.

The modern trend is “cranial nerve sparing,” that is, subtotal resection with subsequent early SRS on the residual tumor; this “hybrid” approach, according to multicenter series, provides high oncological control and better preservation of VII–VIII nerve function.

3. For NF2-associated vestibular schwannomas:

Surgical treatment of NF2-associated vestibular schwannomas is complicated by their multifocal nature and low vascularization, as well as a high risk of bilateral deafness.

Whenever possible, preference is given to less aggressive, hearing-preserving, or combined interventions, considering the potential effect of drug therapy (bevacizumab).

Radiotherapy

Stereotactic radiosurgery and radiotherapy (SRS&SRT)

Indications for peripheral/spinal schwannomas include:

• Recurrence/residual tumor after surgery;
• Inability to perform surgical treatment.

Stereotactic radiotherapy demonstrates high local control (>90%) and a low risk of complications in spinal schwannomas.

For vestibular schwannoma:

• SRS is an option for small/medium vestibular schwannomas, especially in patients where preserving facial and auditory nerve function is the primary goal with minimal intervention;
• Growth control >90% over a 10–15 year follow-up period with a single 11–13 Gy dose.

The risk of late hearing deterioration persists but is generally lower than after microsurgery in relevant cohorts.

Radiotherapy in NF2

Used more cautiously due to a higher risk of secondary neoplasms (MPNST, sarcomas, meningiomas) in patients with tumor syndromes.

In NF2, preference is given to surgery and/or targeted therapy (bevacizumab); radiotherapy is used if surgery is not feasible or seems dangerous.

Medication therapy

For sporadic isolated schwannomas (peripheral or spinal), there is no convincing evidence of the benefit of systemic therapy. The standard treatment is surgery/observation; radiotherapy is used if necessary.

The only agent with a solid evidence base and real clinical utility in NF2-schwannomatosis is bevacizumab for progressing, functionally significant vestibular schwannomas (and partially other NF2-tumors).

Bevacizumab

Anti-VEGF monoclonal antibody is the main proven systemic option in progressing NF2-associated vestibular schwannomas threatening hearing or brainstem compression.

A systematic meta-analysis of 8 cohorts (161 patients) showed a partial response in tumor volume in ≈41%, stabilization in 47%, and progression in 7%; hearing improvement in about 20% and stabilization (no worsening of hearing) in 69% of patients.

Limitations of bevacizumab therapy:

• Proteinuria;
• Hypertension;
• Risk of serious vascular complications (including rare intracranial hemorrhages).

Long-term safety and prolonged efficacy remain subjects of study.

Other targeted and experimental approaches

mTOR inhibitor everolimus: activity in NF2-associated vestibular schwannomas was found to be limited and inconsistent.

Tyrosine kinase inhibitors (lapatinib, EGFR/ErbB inhibitors) demonstrated single partial responses in small studies in patients with NF2.

Trials of the ALK inhibitor brigatinib and HDAC inhibitors (e.g., AR-42) are ongoing for NF2-associated tumors; currently, these are experimental options within clinical studies.

Prognosis of the disease

Sporadic benign schwannomas (peripheral, spinal, vestibular) with adequate treatment rarely relapse, practically do not affect overall survival, and allow preservation or improvement of neurological status in most patients.

After total resection of spinal schwannoma, recurrences are observed in less than 5%.

In NF2-schwannomatosis, the prognosis is determined by the multiplicity of tumors (vestibular schwannomas, meningiomas, ependymomas), progressive neurosensory deafness, balance disorders, and polyneuropathy. Despite improvements in diagnosis and therapy, life expectancy remains decreased compared to the general population.

Malignant transformation of a typical schwannoma is extremely rare. MPNST generally arises from plexiform neurofibromas in NF1 or after radiotherapy and has a significantly worse prognosis (5-year survival of 34–60%).

The “wait and scan” observation tactic for asymptomatic schwannomas:

• Justified and widely accepted for sporadic small vestibular and peripheral/spinal schwannomas in the absence of critical structure compression and good patient compliance;
• Requires a clear protocol of MRI monitoring (at least two early studies to determine growth type, then the interval is personalized);
• Must be accompanied by detailed patient information regarding the risk of growth and the possible necessity for subsequent interventions.

Schwannoma growth rate

The average growth of most schwannomas (including vestibular) is negligible (≈1 mm/year in diameter or modest volume increase), yet there is an infrequent subset of rapidly growing tumors that must be promptly identified.

Peripheral and spinal schwannomas

Most peripheral and spinal schwannomas grow slowly: about 5–10% of the volume per year.

In the study by El Sayed L et al. in a case series of 47 peripheral schwannomas (sporadic and schwannomatosis-related), the relative annual growth was approximately 34% per year; meanwhile, ≈19% of tumors did not grow or shrank, and ≈13% exhibited “rapid” growth (>2 cm³/year and >35%/year).

In the study by Lubelski D. et al. in a case series of 227 patients investigating the natural course of cervical plexus, peripheral, and spinal tumors, growth subtypes were identified: “slow” growth (~5–10% of the volume per year), “moderate,” and “rapid” (>80% per year), with growth type often determined after two follow-up evaluations.

Vestibular schwannomas (VS)

A systematic review by Yoshimoto Y. of natural history showed:

• Growth frequency (any positive increase) is ≈46% over an average follow-up period of 38 months;
• Regression: 8%;
• The average growth rate by maximum diameter is approximately 1.2 mm/year.

A modern review by Paldor I. et al. reports similar figures for VS growth: average growth of 0.99–1.11 mm/year; if the tumor has already shown growth by the first follow-up MRI, the expected growth rate is ≈3 mm/year, with rapid growth factors (>4 mm/year) including cysts, hemorrhages, and hormonal therapy.

The range of speeds is broad: from no growth or even reduction to 6–17 mm/year in individual case series.

Find more scientifically accurate content on our social media

Subscribe and don’t miss out the latest resources

Key practical focus for the physician

Any rapid change in size, pain character, or neurological status in a patient with a known schwannoma (especially with NF1) requires exclusion of MPNST (MRI with DWI, PET-CT, biopsy).

In young patients with vestibular schwannoma, NF2-related schwannomatosis should be actively excluded (MRI of the entire CNS, genetic testing).

For asymptomatic patients with small schwannomas and a stable clinical and radiographic picture, an observation strategy with regular MRI is safe.