121 Vestibular Schwannoma
Vestibular schwannoma represents 8% of all intracranial tumours and 80% of cerebellopontine angle tumours. They arise from Schwann’s (neurilemmal) cells. The nerve of origin is usually the superior vestibular nerve. The term acoustic neuroma is still commonly used but is historic and factually incorrect and should be discarded.
121.1 Pathology
The medial portions of the cranial nerves are covered with glial stroma. Vestibular schwannomas originate at the junction of the glial and Schwann’s cells (Redlich–Obersteiner zone), which for the vestibular nerve is usually within the internal auditory meatus (IAM). The sexes are equally affected and, whatever the time of onset and rate of progression, the presentation is most often between the ages of 40 and 60. The annual incidence is approximately 1 in 50,000. Sixty to seventy percent of vestibular schwannomas do not grow on serial imaging and this is particularly likely to be the case in tumours of less than 1.5 cm. The average rate of change in growing tumours is 2 mm per year although a proportion grows at significantly greater rates. The majority of these tumours are unilateral, and the small proportion that is bilateral (5%) is seen in neurofibromatosis type 2 (NF2). This is an autosomal dominant disease due primarily to mutations on the long arm of chromosome 22. In NF2, vestibular schwannomas often present at an earlier age than those without the gene.
Macroscopically, the tumour appears as a firm yellowish encapsulated mass with the vestibular nerve splayed out on its surface. Histologically, the tumour consists of packed sheaves of connective tissue cells (fasciculated) or may be composed of a disorderly loose network of cells with intercellular vacuoles and cysts (reticular pattern). Haemorrhage can occur (particularly in the reticular type), leading to a sudden increase in size and therefore marked symptoms such as acute vertigo or sudden deafness although in reality this is rare.
121.2 Clinical Features
Clinically, two phases can be recognised: an oto-logical phase in which a small tumour compresses structures in the meatus, and a neurological phase as the tumour expands medially into the cerebellopontine angle.
1. Otological symptoms Gradual and progressive unilateral hearing loss is the usual presenting symptom (90%) and this is often associated with tinnitus (70%). Sudden-onset hearing loss can occur in 5 to 10%. A proportion of patients have normal hearing at presentation (5%). Associated significant vertigo is unusual as compensation for vestibular nerve damage usually keeps pace with the slow rate of loss of peripheral vestibular function. Many patients will, however, note previous episodes of short-lived vertigo or a longer period of imbalance (as if they were on a deck of a ship).
2. Trigeminal nerve symptoms Facial pain, numbness and paraesthesia may all occur.
3. Headache Discomfort and dull aching around the ear and mastoid area are probably caused by posterior fossa dura irritation.
4. Late symptoms Like most motor nerves, the facial nerve is resistant to slow pressure deformity and symptoms of facial weakness or hemifacial spasm are uncommon. Ataxia and unsteadiness develop with progressive brainstem displacement and cerebellar involvement. Diplopia due to pressure on the sixth cranial nerve, and hoarseness with dysphagia due to involvement of the ninth and tenth nerves is rare.
5. Terminal symptoms The raised cerebrospinal fluid (CSF) pressure causes failing vision due to papilloedema, headache, alteration in conscious level and eventually coma.
In the otological phase, general examination will usually reveal no abnormalities. The patient may have a unilateral sensorineural hearing loss. Hypo-aesthesia of the posterior external ear canal on the side of the hearing loss may be present (Hitselberger’s sign). Loss of the corneal reflex is an early sign of trigeminal nerve impairment. Nystagmus when present may be vestibular or cerebellar in nature. Facial nerve impairment is usually of the sensory element and can be elicited as a lack of taste on electrogustometry or loss of lacrimation on Schirmer’s test, although these tests are now rarely undertaken. Slight facial weakness may show as a delay in the blink reflex. Neurological signs of the other cranial nerve palsies and ataxia may eventually become apparent.
121.3 Investigations
1. Radiological investigations The most accurate means of identifying small intracanalicular tumours is magnetic resonance imaging (MRI) of the brain and internal acoustic meatus. Standard practice in the United Kingdom is to perform a T2-weighted non-contrast MRI of the IAMs and brain. A T1-weighted MRI with gadolinium contrast is used for abnormal or equivocal scans. Gadolinium contrast is the gold standard in ongoing tumour surveillance. Computerised tomography (CT) scanning with high-definition and enhancement techniques will accurately diagnose and delineate most tumours above 1 cm in diameter and can be used when MRI is unavailable.
2. Audiometry
a. A unilateral or asymmetrical sensorineural hearing loss can usually be demonstrated by a pure-tone audiogram. The hearing loss is classically a neural lesion with no loudness recruitment, abnormally rapid adaptation and disproportionately poor speech discrimination (rollover effect on speech audiogram). If the pure-tone threshold hearing loss is greater than 70 dB, the accuracy of audiometric testing is poor and these factors often restrict future rehabilitation with hearing aids.
b. Stapedial reflex decay can be measured using impedance audiometry, and this gives a low false-negative rate of around 5%. Brainstem electric response audiometry has only a 3% false-negative rate. It demonstrates a retrocochlear lesion by an increased latency between N1 and N5 waves.
3. Vestibular investigations Caloric responses are usually reduced in or absent from the affected side, but there is no abnormality in some patients with small tumours. Special audio-metric and vestibular testing are now rarely, if ever, used for the diagnosis of a vestibular schwannoma.
121.4 Differential Diagnosis of a Tumour at the Cerebellopontine Angle
1. Vestibular schwannomas (constitute 80% of cerebellopontine angle tumours).
2. Meningioma.
3. Neuroma of the seventh nerve.
4. Congenital cholesteatoma.
5. Aneurysm of the basilar or vertebral arteries.
6. Cholesterol granuloma of the petrous apex.
7. Cerebellar tumour.
121.5 Management
It is essential that these patients are assessed by a skull base multi-disciplinary team in determining optimum management. Careful consideration of the age and general condition of the patient and the size, hearing, site and rate of growth of the lesion are all factors to be considered. In view of tumour size contributing to the management of vestibular schwannomas, the British Skullbase Society has defined tumour size as meaning size in millimetres at the cerebellopontine angle and categorised the size as follows:
• Intracanalicular—0 mm.
• Small—1–15 mm.
• Medium—16–29 mm.
• Large—30–40 mm.
• Giant—> 40 mm.
1. Conservative management Small, slow-growing tumours in elderly patients can be watched by carrying out CT or MRI scanning at regular intervals to gauge the rate of growth. Following U.K. consensus, the largest extrameatal tumour diameter on axial MR imaging is recorded as the measurement of tumour size. Annual MRI shows that around 60% of vestibular schwannomas are not growing. Hearing rehabilitation in this patient group can be challenging, but it is important to offer support. Single-sided hearing loss may be helped with CROS aiding or bone conduction implants.
2. Radiosurgery and radiotherapy Stereotactic radiosurgery (Gamma Knife): Non-invasive treatment providing single fraction of high-dose ionising gamma radiation to the tumour with the intention to arrest future tumour growth. The patient’s head is fixed in a stereo-tactic frame, but it is performed as a ‘single visit’ relatively pain-free day-case procedure. The documented ‘control’ rate for tumours less than 2 cm is approximately 95%. Additionally, it may reduce the rate of hearing loss in some patients.
Traditional hyperfractionated radiotherapy (including more recently CyberKnife) is also used to treat vestibular schwannomas. This offers similar results in terms of tumour control yet requires multiple visits for treatment.
Radiosurgery may make future microsurgery more difficult and there is a theoretical risk of inducing future malignant change. It is used to control small tumours. It is associated with facial pain in approximately 3% of cases and facial nerve weakness in less than 1%. Initial swelling may be evident on post-treatment imaging prior to regression.
3. Microsurgery Removal of a vestibular schwannoma often results in hearing loss and carries a risk to cranial nerves IV to XI. This risk is determined by the size of the tumour and the expertise and experience of the team undertaking the surgery. Patients with tumours more than 25 to 30 mm or those with neurological signs will usually be advised to have microsurgery, even if they have excellent hearing in the affected ear. The morbidity and mortality from both tumour growth and its operative removal increase with large tumours. A small tumour can be extracted from the meatus with negligible risk and with preservation of the facial nerve, although such tumours are often managed conservatively.
There are three surgical approaches to the cerebellopontine angle and the choice depends on the position and size of the tumour and pre-operative assessment by the neuro-otologist and neurosurgeon.
1. Translabyrinthine approach This is one of the commonest approaches, but all residual hearing is lost. The approach has the highest rate of facial nerve preservation as the nerve is identified and controlled laterally, at the fundus of the IAM, early in the operation and tends to be used for patients with a severe sensorineural hearing loss (< 70% speech discrimination) and more lateral tumours.
2. Retrosigmoid approach This route provides wide exposure of the cerebellopontine angle and is often employed in larger tumours. The patient’s hearing and facial nerve can be preserved, so this approach is also used for tumours when there is good hearing. The effect of cerebellar retraction is a potential concern, however.
3. Middle fossa route This gives limited access and was previously used for small intrameatal tumours as both hearing and the facial nerve can be preserved. It is now rarely used in the United Kingdom as such tumours are manged conservatively or with radiotherapy. There is a 15% risk of the patient developing epilepsy and in the United Kingdom, driving is banned for 1 year following this procedure.
121.6 Complications of Surgery
• Major/catastrophic: Death or stroke—1%
• Serious—CSF leak through the wound or nose via the often down the eustachian tube occurs in 2% in the senior author’s experience although meningitis is rare. Deep vein thrombosis (DVT), pulmonary embolism (PE) and lower cranial nerve injury are rare too at about 1%.
• Facial nerve injury. This risk is proportionate to tumour size and the skill/experience of the surgical team. It is important that each surgeon knows his or her complication rate to provide accurate informed consent. As a guide, complication rates for skilled/experienced surgeons will be about in patients with up to a 20-mm tumour, 95% will have normal or near-normal facial nerve function post-operatively. Those with tumour size 20 to 30 mm have about an 80% chance of normal/near-normal facial nerve function, and those with a tumour more than 30 mm have a 70% chance of having normal/near-normal facial nerve function post-operatively.
Fig. 121.1 shows the algorithm used by the senior author in the decision-making process of managing vestibular schwannoma.
