Vestibular Schwannoma (Acoustic Neuroma)

Definition and epidemiology

Vestibular schwannoma is a benign tumour arising from the Schwann cells that sheath the vestibular division (superior or inferior vestibular nerve) of the eighth cranial nerve. It usually starts in the internal auditory canal and extends into the cerebellopontine angle as it grows. The widely used name “acoustic neuroma” is misleading: the tumour arises from the vestibular rather than the cochlear nerve, and from Schwann cells rather than neurons (Carlson and Link, 2021).

Most cases are sporadic and unilateral. Bilateral vestibular schwannomas are the hallmark of NF2-related schwannomatosis and appear at a younger age; this entry covers the sporadic tumour (Goldbrunner et al., 2020). By location, tumours are divided into intracanalicular (confined to the internal auditory canal) and extracanalicular; the Koos classification, widely used in surgical planning, grades tumours in four stages according to their relation to the brainstem (Koos et al., 1998).

A systematic review of population-based studies found an annual incidence of 3.0-5.2 per 100,000 person-years; incidence rises to 20.6 per 100,000 in people over 70, the median age at diagnosis is 60, and 48-72% of tumours are intracanalicular at diagnosis. Lifetime prevalence is estimated to exceed 1 in 500. The rise in recent decades is largely explained by wider use of MRI and earlier detection of small tumours (Marinelli et al., 2022).

Symptoms and signs

The commonest presenting complaint is sensorineural hearing loss in one ear that progresses slowly over years. Patients often notice that they move the telephone to the other ear or struggle to follow conversation in a crowd. Tinnitus in the same ear is common and is sometimes the only complaint. In some patients the tumour presents as sudden sensorineural hearing loss, which is why every adult with sudden loss is investigated for a retrocochlear cause (Carlson and Link, 2021).

Because the tumour grows slowly, the brain usually compensates for the loss of balance function over time, so mild unsteadiness or wobbliness with quick head movements is more typical than severe rotational vertigo. A growing tumour can compress the trigeminal nerve and cause facial numbness, press on the brainstem and cerebellum and cause gait disturbance, or obstruct cerebrospinal fluid flow and cause headache and hydrocephalus. Although the facial nerve lies very close to the tumour, facial palsy at presentation is rare.

Symptom severity does not always match tumour size: a small intracanalicular tumour can cause marked loss, while hearing may be relatively preserved with a large tumour. Some tumours are found incidentally on MRI performed for another reason (Marinelli et al., 2022).

Causes and risk factors

Sporadic vestibular schwannoma results from loss of function of both copies of the NF2 gene, which encodes the protein merlin, in Schwann cells. In a sporadic tumour this change is confined to tumour cells and is not inherited; in NF2-related schwannomatosis one copy is altered from birth and bilateral tumours develop (Carlson and Link, 2021).

Hearing loss is not caused by mechanical compression alone. Proposed mechanisms include compression of the cochlear nerve, impaired flow in the labyrinthine artery supplying the inner ear, and substances released by the tumour altering the composition of inner ear fluids. The cochlea can therefore become involved over time, and audiological findings may shift from a purely retrocochlear pattern to a mixed cochlear and retrocochlear pattern.

The natural history is variable. In a Danish series of 552 patients followed with MRI, growth was found in 17% of intracanalicular and 28.9% of extracanalicular tumours, mostly within the first five years after diagnosis (Stangerup et al., 2006). The fact that many tumours do not grow for years supports surveillance as the first approach for small tumours.

Audiological and clinical assessment

The audiologist is often the first link in the diagnostic chain. NICE recommends considering MRI of the internal auditory meati in adults with sensorineural hearing loss and no localising signs if pure tone audiometry shows an asymmetry of 15 dB or more at any two adjacent test frequencies among 0.5, 1, 2, 4 and 8 kHz (National Institute for Health and Care Excellence [NICE], 2018). Even with a symmetrical audiogram, persistent unilateral tinnitus may warrant medical referral to look for a retrocochlear cause. The expected audiological findings are:

• Pure tone audiometry: sensorineural loss in the affected ear, often worse in the high frequencies, although flat or low-frequency patterns also occur. There is no air-bone gap.
• Speech audiometry: word recognition is poorer than expected from the pure tone thresholds; a fall in score at high presentation levels (rollover) suggests a retrocochlear lesion.
• Acoustic reflexes: with stimulation of the affected ear, reflexes may be elevated or absent, and reflex decay may be positive. The reflex pattern helps localise the lesion.
• Otoacoustic emissions (OAE): may be preserved despite the hearing loss if the cochlea is not involved, which is a strong pointer to a retrocochlear lesion.
• Auditory brainstem response (ABR): a prolonged I-V interpeak latency, an interaural wave V latency difference or absent waves are expected. In a meta-analysis ABR sensitivity was 93.4% overall and 85.8% for tumours under 1 cm, with a specificity of 82% (Koors et al., 2013). A normal ABR does not exclude a small tumour, and waves may not be assessable in severe loss.
• Vestibular tests: a reduced caloric response on the affected side, reduced gain with catch-up saccades on the video head impulse test (vHIT) and reduced amplitude of vestibular evoked myogenic potentials (VEMP) may be seen; the findings depend on which nerve is involved.

MRI is the definitive test; gadolinium-enhanced T1-weighted or high-resolution T2-weighted sequences show small tumours in the internal auditory canal (Goldbrunner et al., 2020). ABR remains useful when MRI is not possible (for example with a pacemaker or severe claustrophobia) or not readily available. The differential diagnosis includes meningioma, epidermoid cyst, facial nerve schwannoma, vestibular neuritis, Ménière’s disease and other asymmetric cochlear losses.

Treatment and audiological rehabilitation

Treatment decisions rest with the neurotology and neurosurgery team, taking into account tumour size, growth rate, hearing level, age, general health and patient preference. The options are MRI surveillance (wait and scan), microsurgery, stereotactic radiosurgery and fractionated radiotherapy. According to the EANO guideline, observation and radiosurgery are weighed against each other for small tumours, whereas surgical decompression is mandatory for large tumours and may be followed by radiotherapy or radiosurgery. Drug treatment has no role in sporadic tumours (Goldbrunner et al., 2020).

During surveillance the tumour is monitored with MRI and hearing with audiograms and speech tests at regular intervals; active treatment is considered if growth or worsening symptoms are found. In microsurgery the translabyrinthine approach sacrifices hearing, while the retrosigmoid and middle fossa approaches aim to preserve it in selected small tumours. Radiosurgery does not remove the tumour but aims to stop its growth. With all three approaches, hearing in the affected ear may decline over time (Carlson and Link, 2021).

Hearing rehabilitation should be planned whatever treatment is chosen. If word recognition in the affected ear is adequate, a hearing aid can be fitted to that ear. Once the ear is no longer usable, the options used in unilateral hearing loss apply: a CROS aid routes sound from the affected side to the better ear, and bone conduction implants transmit sound through the skull to the cochlea of the better ear. Both reduce the head shadow effect but do not restore sound localisation or binaural summation.

If the cochlear nerve is preserved anatomically and functionally, a cochlear implant (CI) can be placed at the time of tumour removal, or during radiosurgery follow-up or surveillance. A systematic review reported a median AzBio sentence score of 70% in CI users with sporadic tumours, with poorer results in larger tumours (Tadokoro et al., 2021). In unilateral loss a CI is the only option that can restore some degree of hearing with two ears; electrical stimulation testing and detailed counselling are needed beforehand. In Türkiye, cochlear implantation for unilateral profound hearing loss is not covered by the national social security institution (SGK), and for a CROS system only one component is reimbursed as a hearing aid; patients should be informed about these options.

Impact on quality of life and advice

The word “tumour” causes considerable anxiety for many patients. Explaining clearly that vestibular schwannoma is benign, does not spread and usually grows slowly is the first step in counselling. During MRI surveillance in particular, anxiety often rises before each scan; agreeing at the outset on the follow-up schedule and on which findings would change management helps reduce it.

Unilateral hearing loss makes it harder to understand speech in noise, locate sounds and follow group conversations, and increases listening effort and fatigue. It helps to keep your better ear towards the speaker or the sound source in class or meetings, choose seats away from background noise and use the telephone on the better ear. If tinnitus is troublesome, sound therapy and counselling can be discussed with your team.

Unsteadiness after treatment usually settles over weeks, and vestibular rehabilitation speeds this up. If facial nerve function is affected after surgery, eye protection and physiotherapy become important. Self-report questionnaires such as the Spatial Hearing Questionnaire, the Speech, Spatial and Qualities of Hearing Scale and the Tinnitus Handicap Inventory can be used to track hearing and its impact.