Acoustic Neuroma (Vestibular Schwannoma)

• A vestibular schwannoma is the most common cerebellopontine angle tumour, accounting for roughly 85% of lesions there and 8% of all intracranial tumours.

• Both sporadic and NF2-related tumours are rare, benign, slow-growing lesions originating from the vestibular divisions of the eighth cranial nerve.

The cerebellopontine angle (CPA), found within the space of the posterior fossa, it's relations are:

1. Laterally: Petrous bone

2. Medially: Cerebellum and pons

3. Inferiorly: Cerebellar tonsil

4. Superior: Tentorium

The CPA contains cranial nerves V-XII and the anterior inferior cerebellar artery (AICA), structures at risk as a vestibular schwannoma enlarges.

• NF2 is an autosomal dominant disorder associated with a mutation on chromosome 22q12.2.

• Bilateral vestibular schwannomas are almost pathognomonic of neurofibromatosis type 2.

History

• Common: Hearing loss, tinnitus or balance disturbance.

• Symptoms depend on the size of the tumour and it's location:

  • There is no correlation between tumour size and the degree of hearing loss at presentation, whereas other non-audiological symptoms are associated with larger tumour size.

Examination

• Full Neurotological examination:

  • Hitzelberger sign (reduced sensation of posterior ear canal wall)

  • Loss of taste

  • Schirmers's test (reduced lacrimation)

  • Cerebellar signs, Gait disturbance, intention tremor, nystagmus (Brun's), hypotonia

Audiometry

Rollover phenomenon: word recognition (speech discrimination) score paradoxically falls as presentation intensity is increased, a sign of retrocochlear pathology.

Phonemic regression: speech discrimination is disproportionately poor relative to the pure-tone thresholds.

Tone decay (auditory fatigue): auditory perception of a sustained pure-tone stimulus fades over time.

ABR

Prolonged Interaural latency of wave V (only sensitive to tumours >1.5 cm) or an increased I–V latency.

• Wave I : Generated by the distal portion of the auditory nerve before it enters the internal auditory canal.

• Wave V: originates in the upper brainstem (inferior colliculus). A delay in one ear compared to the other points to a conduction problem along the auditory pathway.

• Retrocochlear pathologies cause a selective delay or absence of Wave V while leaving Wave I relatively unaffected. This pattern disproportionately prolongs the I–V interpeak latency, which serves as a primary marker for lesions along the auditory nerve and brainstem.

  Diagnostic Marker	Normal Finding	Retrocochlear Finding
  I–V Interpeak Latency	~3.8 to 4.0 ms	Prolonged (> 4.4 ms)
  Interaural Wave V Difference	< 0.2 to 0.3 ms	Prolonged (> 0.3 ms asymmetry)
  Wave V Morphology	Clear, sharp peak	Degraded, delayed, or entirely absent

Imaging

Contrast-enhanced MRI is the diagnostic modality of choice and must be performed whenever there are unilateral or asymmetric audiovestibular symptoms of unknown cause.

T1/T2 hypointense but bright on T1 with contrast.

Differential Diagnosis

Meningeal pathology: Meningioma, arachnoid cyst , and metastases.

Cranial nerve pathology: facial (CN VII) schwannoma, or malignant peripheral nerve sheath tumour.

Skull base pathology: chordoma, chondrosarcoma and cholesterol granuloma.

Cisternal: Epidermoid cyst, lipoma, arachnoid cyst.

• MDT approach

• Growth in first year predicts further tumour growth.

  • Average growth rate of 1 mm/year

  • 1/3 will show growth in within the first 3 years

  • 1/2 will show growth within 5 years.

• Watch and wait: Small/Slow growing( <2cm in size or growth <1mm per year)

  • Year 1: Scan every 6 months

  • Years 2-4: Scan yearly

  • Years 4+ every other year, then every 3 years

  • Consider discharge if no growth for 5 years.

• Medium-sized (<2.5 cm in size) or small but growing tumours (growth >2 mm per year) may be treated with either surgical resection or radiotherapy.

• Large tumours (size > 3 cm) are generally managed with surgical resection.

Surgery

• Surgery is the treatment of choice for large tumours or following radiotherapy failure, and is also a viable option for growing small- and medium-sized tumours.

Translabyrinthine

• Highest rate of facial nerve preservation, wide operative view, minimal brain retraction; sacrifices hearing.

Middle fossa

• Indicated for small intracanalicular tumours (CPA component <1 cm) with serviceable hearing that is worth preserving.

• Temporal lobe retraction carries an epilepsy risk, mandating anti-epileptics and therefore a 12-month UK driving ban; facial nerve injury rates are higher than translabyrinthine.

Retrosigmoid

• Good hearing preservation, wide view suitable for any tumour size, good facial nerve preservation.

  • Requires cerebellar retraction.

  • Preferred for the largest tumours.

Complications:

• General anaesthetic risks, CSF leak, meningitis, SIADH, plus the approach-specific issues above.

Radiotherapy

• Aims to stabilise size, and stop further growth.

  • Stereotactic: Gamma knife surgery, cyberknife or fractionated

• Radiotherapy achieves a high tumour control rate of 92–98% with low short-term complication rates.

  • Hearing loss progresses faster than it would without treatment, and there is a small long-term risk of inducing new tumours or malignant transformation within the treated tumour.

Medical

• In neurofibromatosis type 2, systemic drug therapies such as bevacizumab(Avastin) have shown significant efficacy in controlling tumour growth.

• After radiotherapy, hearing loss progresses more than it otherwise would, and there is a small long-term risk of inducing new tumours or malignant change within the treated tumour.

• Surgical resection carries a risk of facial nerve palsy, which increases with increasing tumour size.

• Standardised reporting of vestibular schwannoma outcomes follows the Kanzaki consensus reporting systems established at the consensus meeting on systems for reporting results in vestibular schwannoma.