Definition: A benign cerebellopontine angle tumour that grows from the superior vestibular component of the vestibulocochlear nerve, usually presenting with unilateral sensorineural hearing loss.
Vestibular schwannomas are also called acoustic neuromas, and the two terms may be used interchangeably. However, acoustic neuroma is a misnomer, because the tumour actually grows on the vestibular component and is pathologically a schwannoma.
Tumours can enlarge significantly to cause cerebellar findings and obstructive hydrocephalus, and may become life threatening as a result.
The annual incidence is 0.2 to 1.7 out of 100,000 with a 3:2 female-to-male ratio.
Increased numbers of smaller tumours are detected due to prevalence of magnetic resonance imaging.
Vestibular schwannomas comprise 6% of all intracranial tumours and 70% to 80% of all cerebellopontine angle tumours. All races and ethnicities are affected equally.
Vestibular schwannomas are occasionally associated with the genetic disorder neurofibromatosis type 2, with a reported incidence of 1.4 to 4 out of 100,000 in people with this disorder.
How to diagnose Vestibular schwannoma?
Patients often present with minimal symptoms that tend not to correlate with tumour size.
The most common symptom is unilateral sensorineural hearing loss. Magnetic resonance imaging (MRI) scanning is considered the definitive test. Tumours may also be found on routine hearing examinations for occupational or educational screening.
History
Patients may have minimal symptoms, as tumours grow slowly. Signs and symptoms may not always correlate with tumour size: patients may have very large tumours (>3 cm) but may present with minimal symptoms.
Conversely, patients with very small (<1 cm) tumours may present with significant troublesome symptoms. Symptoms may also be intermittent.
Younger patients may compensate for the sensorineural deficits well and present only with a larger tumour. In young pregnant women, symptoms can present during or shortly after pregnancy.
Unilateral sensorineural hearing loss is common. Unlike noise-induced or age-related hearing loss, loss is typically asymmetrical and hearing aids do not improve speech discrimination. Unilateral facial numbness and progressive episodes of dizziness are also common.
Other, less common, symptoms include unilateral facial weakness, tinnitus, metallic or reduced taste (on the affected side), increased or decreased tears (on the affected side), headache, diplopia on lateral gaze, swallowing difficulties, and poor sound localisation.
A co-existing diagnosis of neurofibromatosis type 2 increases the prevalence of vestibular schwannoma.
Physical examination
Signs include slower blink reflexes, poor balance, or nystagmus; however, these are relatively uncommon.
Large tumours can cause significant problems with increased intracranial pressure, gait disturbance, and coordination difficulties. Papilloedema may present in large tumours with increased intracranial pressures; these patients may also develop hydrocephalus.
Investigations
Patients with unilateral hearing loss (especially if progressive) not responding to local ear care warrant referral for audiology testing, auditory brainstem reflexes and/or ear, nose, and throat consultation.
All patients with sensorineural or retrocochlear findings on audiometry, especially if asymmetrical, are recommended for a gadolinium-enhanced MRI or computed tomography (CT) of the head (depending on what is available).
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| Vestibular schwannoma as seen on MRI |
MRI scanning is considered the definitive test, as it is more sensitive than CT scanning.
Differential diagnosis
- Meningioma
- Epidermoid
- Facial nerve schwannoma
- Trigeminal schwannoma
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Aetiology & Pathophysiology
Tumour suppressor gene abnormalities on chromosome p22 (merlin or schwannomin protein) are thought to cause tumour growth.
Tumours occur at the junction of the central and peripheral Schwann cells or Obersteiner-Redlich zone. Most growths are sporadic without a known underlying cause.
There is a familial autosomal dominant form in which patients have bilateral tumours in addition to other intracranial tumours (neurofibromatosis type 2).
Because the tumours grow on the vestibulocochlear nerve (vestibular component), patients present with decreased hearing and episodes of dizziness or vertigo.
The amount of hearing loss appears unrelated to the size of tumour at presentation. In the case of slow-growing tumours (average of 2 to 3 mm/year), symptoms may be minimal or fluctuating as the brain is able to compensate for the slow-growing tumour.
Up to 60% of tumours do not seem to grow very much on scans. Because of the unilateral hearing loss, patients have difficulty localising sound and find it difficult to home in on a voice in a crowd.
As the tumours enlarge other nerves become affected, and the symptom that is most common, after those related to hearing and balance, is facial numbness.
Patients may have difficulty swallowing caused by large tumours affecting the lower cranial nerves and brainstem. True cerebellar symptoms are usually a late finding found in larger tumours with significant compression of the brainstem/ipsilateral middle cerebellar peduncle.
Nystagmus and gait difficulties are also found in larger tumours. Facial nerve function is affected late, resulting in a slower blink, altered taste, and altered tearing.
Larger masses can also distort and compress the fourth ventricle, causing obstructive hydrocephalus and related symptoms.
Rarely, non-obstructive hydrocephalus can occur, presumably due to increased cerebrospinal fluid protein and decreased cerebrospinal fluid absorption.
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Management approach
Patient management is influenced by local experience in focused radiation and surgical approach. Specific recommendations to patients, tumour size limits, and age and hearing level limits will vary greatly depending on local experience.
The rates of facial nerve, hearing, and quality of life preservation for the different treatments also vary due to local experience.
Management decisions
Management includes observation as well as treatment.
Smaller tumours in patients with minimal symptoms (e.g., mild hearing loss) may be followed up with serial scanning, especially if there is no documented growth.
Care should be individualised with patient preference taken into consideration.
Important issues to consider include:
- 40% to 60% of smaller tumours may not enlarge.
- Progression of a patient's symptoms is an indicator of growth.
- Smaller tumours are easier to treat (fewer risks to facial nerve and hearing).
- Younger patients with fewer co-morbidities may respond better to treatment.
- Intervention is unlikely to improve hearing.
Depending on the tumour size and configuration, level of hearing loss, and experience of treating physicians, hearing preservation rates can vary greatly.
Because the hearing loss rate with treatment is significant, hearing preservation should be considered an added bonus.
Thus, treatment decisions should weigh the fact that hearing preservation is more successful with good pre-procedure hearing and there is a risk of a slow loss of hearing without treatment, and also the fact that there is a risk of hearing loss over a short period of time (immediate with surgery) with treatment.
If there is radiological evidence of tumour growth or strong patient preference towards intervention, treatment is recommended.
Older patients with slow-growing tumours have an option of continued observation only.
There is not enough evidence to conclude that stereotactic radiation has better long-term hearing preservation when compared with conservative management.
At 6 months from presentation, patients who are being observed only should have an MRI, followed by yearly scans.
Radiation and surgery
Treatment choices include focused radiation (stereotactic radiotherapy and stereotactic radiosurgery) and surgery. Chemotherapy is ineffective.
Stereotactic radiosurgery is performed as a single fraction treatment.
A cranial fixation frame is often attached to the patient's head by screws placed into the skull under local anaesthetic.
Stereotactic radiotherapy refers to the use of multiple small doses of radiation, which affect the tumour to a greater degree than surrounding normal tissue.
Stereotactic radiotherapy is completed with a relocalisable face mask and does not require frame fixation.
Doses are given daily over a period of weeks.
Surgery is completed with a number of approaches tailored to the patient's tumour size and degree of hearing loss, and to the surgeon's experience.
In general, a middle fossa approach is completed through an incision above the affected ear for smaller tumours confined to the internal acoustic meatus.
This is a hearing-preserving operation. The translabyrinthine approach is performed through an incision behind the ear, working through the temporal bone.
This procedure does not preserve hearing. The retrosigmoid approach is completed through an incision behind the ear and can preserve hearing.
Intraoperative neurophysiological monitoring of cranial nerves and tracks in the brainstem is used in most centres. Monitoring of cranial nerves, in particular the facial nerve, has changed the nature and success of surgery in terms of cranial nerve morbidity.
In a small proportion of patients who opt for radiotherapy, tumours may grow despite treatment and may require salvage surgery.
In those patients selected for surgical treatment, tumour recurrence may occur postoperatively and then may require follow-up salvage radiation.
Tumour <1 to 1.5 cm
Stable:
- Clinical observation is recommended if no growth appears on serial scans.
- If the patient prefers more aggressive treatment and is young and healthy, either surgery or radiation may be a favourable option.
- If the patient is deemed unfit for surgery (i.e., an older person, unwell, on anticoagulants), observation should continue unless tumour growth ensues. The choice depends on local physician expertise and patient preference.
- Progression of symptoms without change in tumour size in patients treated conservatively may also indicate moving on to radiation or surgery.
Growing:
- If tumour growth is apparent on serial scans, focused radiation (stereotactic radiotherapy and stereotactic radiosurgery) or surgery is recommended.
- The choice depends on local physician expertise and patient preference.
- If the patient has significant comorbidities, is an older person, or is on anticoagulation medication, observation is still an option. In a small proportion of patients who opt for radiotherapy, tumours may grow despite treatment and may require salvage surgery.
- In those patients selected for surgical treatment, very rarely, tumour recurrence may occur postoperatively and may then require follow-up salvage radiation.
Tumour 1.5 to 3 cm
Stable:
- Observation is recommended if no growth is apparent on serial scans; however, if the patient is young and healthy, intervention is preferable because facial nerve preservation rates decrease over time for these tumours.
- If symptoms worsen and tumour size remains stable in patients treated conservatively, progression to radiation or surgery is indicated.
- If the patient is an older person and/or in ill health, the risk of intervention may be higher than the benefits gained; however, the decision is based upon patient choice and surgeon expertise. In a small proportion of patients who opt for radiotherapy, tumours may grow despite treatment and may require salvage surgery.
- In those patients selected for surgical treatment, tumour recurrence may occur postoperatively and may then require follow-up salvage radiation.
Growing:
- If tumour growth is apparent on serial scans, treatment with focused radiation or surgery is recommended. The choice depends on local physician expertise and patient preference.
- If the patient has significant comorbidities, is an older person, or is on anticoagulants, observation is still an option.
- In a small proportion of patients who opt for radiotherapy, tumours may grow despite treatment and may require salvage surgery.
- In those patients selected for surgical treatment, tumour recurrence may occur postoperatively and may require follow-up salvage radiation.
Tumour >3 cm
- With or without tumour growth on serial scans, surgery is recommended owing to the large size and potential damage to adjacent structures and resulting functional deficits.
- Surgery is recommended in most centres in preference to radiation, because of improvement in mass effect and symptoms offered by surgery over radiation.
- Surgery to preserve hearing is not offered by most surgeons as the mass effect of such a large tumour has often already caused damage to hearing structures.
- If the patient is an older person, is unwell, or is on anticoagulants, observation may be preferable owing to increased risks. Patient preference is involved in treatment decision.
Management of tumours in the only hearing ear
In the rare situation where the patient has a tumour in the only hearing ear, considerations include: preparing and counselling the patient for the likelihood of complete hearing loss; manoeuvres for improving hearing in the contralateral ear (including cochlear implants); brainstem auditory implants at the time of surgery in the affected last hearing ear; and cochlear implants in the ear with the tumour after radiation treatments or surgery in those with intact promontory testing.
Detailed discussion of the management of these patients is not within the scope of this topic.