Vestibular Schwannoma

Vestibular Schwannoma (Acoustic Neuroma, neurilemmoma or eighth nerve tumour)

Vestibular Schwannoma, also commonly referred to as Acoustic Neuroma, is a benign, encapsulated tumour that originates from the Schwann cells covering the vestibular nerve, a component of the eighth cranial nerve. The term “Acoustic Neuroma” is technically a misnomer, as these tumours arise from the vestibular rather than the acoustic division of the nerve, and they do not originate from neuronal tissue but from the nerve’s myelin sheath. Vestibular schwannomas are characterized by their slow growth, often extending over several years. Malignant transformation of these tumours is exceedingly rare.

Vestibular schwannomas account for 80-90% of all cerebellopontine angle (CPA) tumours, with the remaining 10-20% comprising meningiomas, epidermoids, and other rare neoplasms. CPA tumours make up approximately 10% of all intracranial tumours. Bilateral vestibular schwannomas are a hallmark of Neurofibromatosis Type 2 (NF2), a genetic disorder characterized by the development of multiple schwannomas, meningiomas, and other types of tumours.

 

Aetiology

Vestibular schwannomas can occur due to several etiological factors, including:

1. Idiopathic: Most cases of vestibular schwannoma occur sporadically without a known cause. The incidence is highest in individuals aged 40-60 years, with no significant difference in prevalence between males and females.
2. Genetic: Vestibular schwannomas can be associated with a genetic defect on chromosome 22q12, which is responsible for encoding a tumour suppressor protein known as merlin (or schwannomin). This protein normally regulates Schwann cell growth. A mutation in this gene leads to uncontrolled proliferation of Schwann cells, resulting in tumour formation. This genetic mutation is also implicated in Neurofibromatosis Type 2 (NF2), a condition that typically presents with bilateral vestibular schwannomas.
3. Environmental: Exposure to high-dose ionizing radiation has been identified as a risk factor for the development of vestibular schwannomas, though this is relatively uncommon.

Pathology

Vestibular schwannomas most commonly arise from the Schwann cells of the superior or inferior vestibular nerve, which is a part of the vestibulocochlear nerve (cranial nerve VIII). The predominant site of origin is the inferior vestibular nerve, although historically, the superior vestibular nerve was believed to be more commonly involved. The tumour typically originates at the Obersteiner-Redlich zone, the transitional area between central and peripheral myelin at the junction of the vestibular nerve and the glial cells within the internal auditory canal (IAC). Schwann cells are abundant in this region. As the tumour develops in the nerve sheath, it compresses rather than invades the nerve on which it arises.

Bill’s bar is named after William House (nickname Dr Bill). It separates the facial nerve from the superior vestibular nerve at the fundus of the IAC. Identification of Bill’s bar allows early identification of the facial nerve. Facial nerve is mostly not involved with pathology (ie, a vestibular schwannoma arising from a vestibular nerve).

Anatomy of Internal canal:

The internal auditory canal (IAC) is a critical structure in the pathogenesis and progression of vestibular schwannomas. It is divided into four quadrants by two anatomical landmarks: the crista falciformis (a horizontal bony ridge) and Bill’s bar (a vertical bony crest). The contents of these quadrants are as follows:

• Anterior Superior Quadrant: Contains the motor root of the facial nerve and the nervus intermedius.
• Posterior Superior Quadrant: Contains the superior vestibular nerve.
• Anterior Inferior Quadrant: Contains the cochlear nerve.
• Posterior Inferior Quadrant: Contains the inferior vestibular nerve.

There is a close anatomical relationship between the motor root of the facial nerve, nervus intermedius and vestibulocochlear nerve. This explains the disturbances in lacrimation, taste, salivary flow, hearing, balance or facial motor control that may result from lesions in either the cerebellopontine angle or internal acoustic meatus.

Boundaries of Cerebellopontine angle:

The cerebellopontine angle (CPA) is a triangular space located in the posterior cranial fossa. It is bordered by the following structures:

• Posteriorly: The cerebellum
• Anteromedially: The pons
• Laterally: The petrous part of the temporal bone

This space is a common site for the development of vestibular schwannomas and other CPA tumours, which can compress the adjacent cranial nerves and brainstem, leading to a variety of neurological deficits.

Tumor Origin and Growth

The growth of the tumour is difficult to interpret. It is extremely slow-growing and there may be a history of several years. The growth pattern of the tumour determines the pattern of symptoms. The tumour may grow continuously or only to a certain size, followed by stagnation or even shrinkage. The growth pattern of vestibular schwannoma is typically slow, with rates categorized as:

• Slow growth: 0.02 cm per year.
• Medium growth: 0.20 cm per year.
• Fast growth: 1.00 cm per year.

Tumors confined to the internal auditory canal may be asymptomatic initially but can cause symptoms as they enlarge, potentially widening and eroding the canal. Progressive growth into the CPA may result in the compression of cranial nerves (V, IX, X, XI). Further progressive growth in the cerebellopontine angle inferiorly will eventually lead to compression of the brain stem and/or the cerebellum, occlusion of the fourth ventricle. It leads gradually to hydrocephalus followed by death if untreated.

 

Gross and Microscopic Appearance

Grossly, vestibular schwannomas are well-circumscribed, benign, encapsulated tumors. Small tumors are pink to yellow and rubbery, while larger tumors are more yellow, mottled, and may contain areas of hemorrhage, fibrosis, and cystic changes.

Microscopically, vestibular schwannomas are composed of two distinct histological patterns:

• Antoni A fibers: These are cellular, compact fibers with spindle-shaped cells and nuclei arranged in whorling or palisading patterns (Verocay bodies).
• Antoni B fibers: These are less cellular, loosely packed, and have a spongy appearance.

Most tumors display a predominance of Antoni A fibers with interspersed Antoni B areas.

Classification according to the size of the tumour

The classification scheme as proposed and recommended at the consensus meeting on “Reporting Systems of Vestibular Schwannoma” in 2003. 

Intrameatal – The tumour is within the meatus and does not extend beyond porus (medial end of IAC). Extrameatal size of such a tumour is zero.

Alternatively, another commonly used classification includes:

• Intracanalicular: Tumor limited to the internal auditory canal.
• Small size: Tumor up to 1.5 cm.
• Medium size: Tumor between 1.5–4 cm.
• Large size: Tumor over 4 cm.

Clinical Features

The clinical presentation of vestibular schwannoma is largely dependent on the size and location of the tumor. Early symptoms are often subtle and may be attributed to other conditions, leading to delays in diagnosis.

1. Vestibulocochlear Symptoms:

   • Hearing Loss: The most common initial symptom of vestibular schwannoma is unilateral sensorineural hearing loss, which is typically slow and progressive. In some cases, however, hearing loss may occur suddenly. The hearing loss is often associated with a disproportionate decrease in speech discrimination ability compared to the degree of pure-tone hearing loss. In rare instances, patients may present with normal hearing despite the presence of a tumor.
   • Tinnitus: Patients frequently experience high-pitched, continuous tinnitus in the affected ear. This symptom may be particularly bothersome in individuals with concurrent hearing loss.
   • Vertigo and Imbalance: Patients may experience episodic vertigo or a sense of imbalance, particularly with head movements. However, because the contralateral vestibular system often compensates for the slow loss of vestibular function, symptoms may be mild or absent.

2. Cranial Nerve Involvement:

   • Eighth Cranial Nerve: Tinnitus, hearing loss, and vestibular symptoms are indicative of eighth nerve involvement. Audiological testing typically reveals sensorineural hearing loss.
   • Seventh Cranial Nerve (Facial Nerve): Sensory fibers of the facial nerve may be affected before the motor fibers, leading to hypesthesia (reduced sensation) or pain in the external auditory canal (Hitzelberger’s sign), loss of taste on the anterior two-thirds of the tongue, delayed blink reflex, and reduced lacrimation.
   • Fifth Cranial Nerve (Trigeminal Nerve): Involvement of the trigeminal nerve is common in larger tumors, particularly those greater than 2.5 cm in diameter. Symptoms may include diminished corneal reflex, facial paraesthesia, and numbness. Muscle involvement is rare but possible in advanced cases.
   • Other Cranial Nerves: Although less common, large vestibular schwannomas may affect additional cranial nerves, including the glossopharyngeal (IX), vagus (X), accessory (XI), and hypoglossal (XII) nerves, leading to dysphagia, nasal regurgitation, vocal changes, and other deficits. In rare cases, the oculomotor (III), trochlear (IV), and abducens (VI) nerves may also be involved.

3. Brainstem and Cerebellar Dysfunction:

   • Large vestibular schwannomas may compress the brainstem, leading to symptoms such as ataxic gait, weakness, and sensory disturbances in the extremities, and exaggerated tendon reflexes.
   • Cerebellar dysfunction may manifest as dysmetria (inability to control the distance, speed, and power of a movement), dysdiadochokinesia (inability to perform rapidly alternating movements), and ataxic gait. Patients may also have difficulty walking in a straight line and may tend to fall towards the side of the lesion.

4. Raised Intracranial Pressure:

   • In advanced cases, the tumor may obstruct cerebrospinal fluid pathways, leading to hydrocephalus and raised intracranial pressure. Symptoms of increased intracranial pressure include headache, nausea, vomiting, diplopia (double vision), and papilledema (swelling of the optic disc).

Diagnosis and Investigations

Early diagnosis of vestibular schwannoma is crucial for optimal management and preservation of neurological function. A thorough evaluation of patients presenting with unilateral sensorineural hearing loss, tinnitus, or imbalance is necessary to detect the tumor at an early stage.

1. Neurological Examination:

   • A detailed neurological examination should assess cranial nerve function, vestibular function, cerebellar function, and signs of brainstem involvement. Fundus is examined to see blurring of disc margins or papilloedema caused by raised intracranial pressure and optic nerve compression.

2. Vestibular Function Tests:

   • Romberg Test: The patient may demonstrate a tendency to fall towards the side of the lesion when standing with feet together and eyes closed.
   • Unterberger Step Test: The patient may rotate towards the side of the lesion when marching in place with eyes closed.
   • Caloric Testing: There may be a diminished or absent response on caloric testing, indicating a loss of vestibular function on the affected side.

3. Audiological Tests:

   • Pure Tone Audiometry (PTA): Asymmetrical sensorineural hearing loss, particularly in the high frequencies, with poor speech discrimination, is suggestive of vestibular schwannoma.
   • Speech Audiometry: Disproportionate speech discrimination loss relative to pure-tone thresholds and the presence of a rollover phenomenon (decrease in discrimination score with increasing loudness) are indicative of retrocochlear pathology.
   • Short Increment Sensitivity Index (SISI) test shows a retrocochlear type of lesion (score of 0–20%). Threshold tone decay test shows a retrocochlear type of lesion. Stapedial reflex delay present. It is not done routinely as more sensitive and specific tests are available. Recruitment phenomenon is absent as it is in cochlear lesions.
   • Brainstem Auditory Evoked Response (BAER) or Auditory Brainstem Response (ABR): This test assesses the conduction of electrical impulses along the auditory nerve pathways. In vestibular schwannoma, there may be a prolonged latency in waves III and V, and an increased interpeak latency between waves I and V. However, this test is less sensitive for small tumors and is largely supplanted by MRI.

4. Imaging Studies:

   • Plain X-rays (transorbital, Stenver’s, Towne’s and submentovertical views). It can detect about 80% of tumours except for intracanalicular tumours.
   • Magnetic Resonance Imaging (MRI): MRI with gadolinium contrast enhancement is the gold standard for diagnosing vestibular schwannoma. It provides detailed visualization of the tumor, including its size, extent, and involvement of surrounding structures. The tumor typically appears as an enhancing lesion in the internal auditory canal and may extend into the cerebellopontine angle.
   • Computed Tomography (CT) Scan: CT scanning may be useful in certain situations, such as when MRI is contraindicated. CT can detect bone erosion or widening of the internal auditory canal, but it is less sensitive than MRI for detecting soft tissue tumors.
   • Vertebral angiography. 

Treatment

The management of vestibular schwannoma depends on various factors, including tumour size, location, patient age, and overall health. Treatment options include observation, surgical resection, and radiation therapy.

1. Observation: Small, asymptomatic tumours, or those in elderly or high-risk patients, may be managed with a watchful waiting approach, involving regular MRI scans to monitor tumour growth.

2. Surgical Resection: Surgical removal of the tumour is indicated in symptomatic patients or when the tumour shows significant growth on serial imaging. Surgical approach depends on the size of tumour. The various surgical approaches to the cerebellopontine angle are:

   • Translabyrinthine Approach: Most favoured approach. Early identification of the facial nerve and avoidance of cerebellar retraction or resection is its advantage. It involves the destruction of the inner ear and so is not an option for hearing preservation surgery. It has a higher incidence of cerebrospinal fluid (CSF) fistula than other approaches. Usually done in CPA tumours < 3 cm in size. 
   • Retrosigmoid (suboccipital) Approach:  Preferred for removal of all posterior fossa tumours but particularly for large ones. But it gives a limited view of the internal auditory canal and the chances of residual tumour are greater. Usually done in large CPA tumours> 3 cm in size.
   • Middle Fossa Approach:  The middle fossa approach is preferred in hearing preservation surgery and small intracanalicular tumours. However, the approach is somewhat cramped. Usually done in intracanalicular tumours < 1 cm in size.
   • Combined translabyrinthine-suboccipital approach.
     Although surgery has a role for those with large tumours (>3 cm maximal intracranial diameter), which the role of stereotactic radiotherapy is considered minimal. For smaller tumours, a conservative approach is commonly advised.

3. Stereotactic Radiosurgery (SRS): For small to medium-sized tumours, radiosurgery using Gamma Knife or CyberKnife can be effective in controlling tumour growth while minimizing damage to surrounding tissues. SRS is particularly useful in patients who are not candidates for surgery. Cyberknife is more accurate. It uses real-time image guidance technology through computer-controlled robotics.

4. Radiation Therapy: Fractionated stereotactic radiotherapy (FSRT) may be used for larger tumours or those in sensitive locations where surgery is risky. It involves delivering multiple small doses of radiation over several weeks. It can also be done in patients who are not willing for surgery have contraindications to surgery or have a residual tumour. Radiotherapy arrests the growth (and hence avoids surgery) of the tumour and preserves auditory function. 

COMPLICATIONS OF SURGERY:

1. 7th, 5th, 6th, 4th Cranial nerve palsy
2. CSF fluid leakage (Otorrhea, Rhinorrhoea)
3. Intracranial infection (Meningitis)
4. Intraoperative bleeding
5. Brain oedema
6. Venous air embolism
7. Cardiac arrhythmia
8. Brain herniation
9. Death, stroke, haemorrhage, brain injury

DIFFERENTIAL DIAGNOSIS:

Acoustic neuroma should be differentiated from other three most common cerebellopontine angle tumours, e.g. meningioma, primary cholesteatoma and arachnoidal cyst. One should also rule out Meniere’s disease.

TUMOURS OF CEREBELLOPONTINE ANGLE:

• Acoustic neuroma
• Meningioma
• Epidermoid (cholesteatoma)
• Arachnoid cyst
• Schwannoma of other cranial nerves (e.g. CN V >VII > IX, X, XI)
• Aneurysm
• Glomus tumour
• Metastasis

———- End of the chapter ———–

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Learning resources.

• Scott-Brown, Textbook of Otorhinolaryngology Head and Neck Surgery.
• Michael M Paparella, Textbook of Otolaryngology: Principles & Practice.
• Glasscock-Shambaugh, Textbook of  Surgery of the Ear.
• Logan Turner, Textbook of Diseases of The Nose, Throat and Ear Head And Neck Surgery.
• Rob and smith, Textbook of Operative surgery.
• P L Dhingra, Textbook of Diseases of Ear, Nose and Throat.

Author:

Dr. Rahul Bagla
MBBS (MAMC, Delhi) MS ENT (UCMS, Delhi)
Fellow Rhinoplasty & Facial Plastic Surgery.
Renowned Teaching Faculty
Mail: msrahulbagla@gmail.com
India

Please read. Glomus Tumour. https://www.entlecture.com/glomus-tumour/

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