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|Vestibulocochlear Nerve
🧠 Acoustic neuroma (vestibular schwannoma) is a benign tumour arising from Schwann cells of the vestibulocochlear nerve (CN VIII).
📈 Accounts for ~8–10% of primary intracranial tumours and ~80–90% of cerebellopontine angle (CPA) masses.
💡 Always suspect in unilateral sensorineural hearing loss (USNHL) ± tinnitus until excluded (NICE NG98/NG155 recommend MRI IAM in such cases with asymmetry or localising signs).
📖 About
- Benign, slow-growing schwannoma of the vestibular portion of CN VIII (cochlear division less commonly involved initially).
- Incidence ~1–3 per 100,000/year; sporadic in >95%; bilateral in NF2 (autosomal dominant, NF2 gene/merlin mutation).
- ~80–90% of CPA lesions; most common CPA tumour.
- Slow growth (~1–2 mm/year average; many stable or regress); morbidity from local compression of CN VIII, VII, V, brainstem, cerebellum.
🧬 Aetiology
- Proliferation of Schwann cells (peripheral nerve myelination).
- Sporadic (unilateral, majority); hereditary in NF2 (bilateral VS in >90% of cases; other schwannomas/meningiomas).
- Rare associations: prior childhood cranial irradiation; no strong link to mobile phone use (large studies negative).
⚠️ Risk Factors
- Peak age 40–60 years; slight female predominance in sporadic cases.
- NF2 or family history (genetic counselling essential).
- High-dose cranial irradiation in childhood (e.g., for other cancers).
📉 Effects of Tumour Expansion
- CN VIII → progressive unilateral SNHL (high-frequency first), tinnitus, imbalance/vertigo/disequilibrium.
- CN V → facial hypoesthesia, reduced corneal reflex (late).
- CN VII → facial weakness (House-Brackmann grade), taste disturbance (rare early).
- Cerebellum → ipsilateral ataxia, dysmetria, unsteady gait.
- Brainstem/hydrocephalus → cranial nerve deficits, long-tract signs, raised ICP (headache, nausea, vomiting, papilloedema).
🩺 Clinical Features
- Unilateral SNHL (~90–95%; insidious, high-frequency loss).
- Tinnitus (often high-pitched, continuous).
- Balance issues: disequilibrium, unsteadiness (true vertigo less common).
- Late: facial numbness/weakness, headache, ataxia.
- Bilateral → suspect NF2 (screen for other tumours).
💡 Exam Pearl: Unilateral/asymmetrical SNHL ± tinnitus → urgent MRI IAM to exclude VS (NICE NG98: asymmetry ≥15 dB at ≥2 frequencies or localising signs; NG155: unilateral non-pulsatile tinnitus with signs).
🔎 Investigations
- Audiometry: Pure-tone confirms asymmetric SNHL; speech discrimination often reduced disproportionately.
- MRI brain/IAM with gadolinium: Gold standard (detects tumours ≥1–2 mm; classic "ice-cream cone" in IAC/CPA).
- CT head: If MRI contraindicated (e.g., pacemaker); shows IAC widening/erosion in larger tumours.
- BAEP: Prolonged latencies (supportive, not routine).
- Other: Vestibular testing (calorics/VNG); genetic testing/MRI screening if NF2 suspected.
🧾 Differential Diagnosis
- CPA meningioma (dural tail on MRI).
- Epidermoid cyst (hyperintense on DWI).
- Other: trigeminal schwannoma, metastases, arachnoid cyst, MS (demyelination mimic).
🛠️ Management (Individualised; CNS/EANO Consensus-Aligned)
- Observation ("wait and scan"): Preferred for small (<1.5–2 cm), asymptomatic/slow-growing/incidental tumours; elderly/poor surgical candidates. Serial MRI every 6–12 months initially, then longer intervals if stable.
- Microsurgery: For large (>2.5–3 cm) tumours with brainstem compression or rapid growth. Approaches: retrosigmoid (hearing preservation possible), translabyrinthine (hearing sacrifice but good facial outcomes), middle fossa (small intracanalicular). Goal: gross total resection with facial nerve preservation (House-Brackmann I–II in experienced centres).
- Stereotactic radiosurgery (SRS) (Gamma Knife/CyberKnife): Excellent control (>95% at 5–10 years) for small-medium (<2.5–3 cm) tumours; high hearing preservation rates in selected cases; lower cranial nerve morbidity vs surgery for small lesions.
- Fractionated stereotactic radiotherapy: For larger tumours unsuitable for single-fraction SRS.
- Supportive/Rehab: Hearing aids/CROS devices, vestibular physiotherapy, facial nerve rehab if weakness; tinnitus management.
- MDT: Neurosurgery, ENT/skull base, neuroradiology, audiology, oncology, genetics (NF2).
📊 Prognosis
- Excellent long-term survival (>95% 10-year); benign tumour.
- Hearing preservation: ~50–70% with SRS/small tumours; lower with surgery (approach-dependent).
- Facial nerve function: Good in >90% with modern techniques; temporary weakness common.
- Recurrence: ~5% after gross total resection; higher after subtotal; long-term MRI follow-up essential.
- Untreated progressive growth → brainstem compression, hydrocephalus, death (rare in monitored cases).
🔒 Prevention & Screening
- No prevention for sporadic cases.
- NF2: Genetic counselling, annual/biennial MRI from age 10–15; early intervention for growing tumours.
📚 References (Current as of March 2026)
