Acoustic Neuroma (Vestibular Schwannoma)

A benign Schwann-cell tumour of the vestibular division of the eighth cranial nerve, classically arising at the internal acoustic meatus. It is the commonest tumour of the cerebellopontine angle (CPA) and a perennial NEET PG favourite for its triad of unilateral progressive sensorineural hearing loss (SNHL), poor speech discrimination, and the NF2 (bilateral) association.

Definition and nomenclature

"Acoustic neuroma" is a historical misnomer retained by tradition. The tumour is:

• Histologically a schwannoma (not a neuroma, not arising from neurons).
• Anatomically arising not from the acoustic (cochlear) nerve but from the vestibular division of CN VIII — hence the preferred term vestibular schwannoma.
• Most often from the superior vestibular nerve at the glial–Schwann cell junction (Obersteiner–Redlich zone) within the internal auditory meatus (IAM)/internal auditory canal (IAC).

High-yield: It is a benign, encapsulated, slow-growing tumour of Schwann cells, arising from the vestibular (usually superior vestibular) division of CN VIII, most commonly at the internal acoustic meatus. Despite the name, the parent cell is NOT a neuron.

It accounts for roughly 80% of CPA tumours and about 6–8% of all intracranial tumours. Peak presentation is in the 4th–6th decades; unilateral tumours are typically sporadic.

Histopathology

Two classic microscopic patterns are described (Verocay being the most testable):

Feature	Antoni type A	Antoni type B
Cellularity	Dense, compact, organised	Loose, hypocellular, myxoid
Cell arrangement	Palisading nuclei → Verocay bodies	Disorganised, vacuolated
Stroma	Little	Abundant, oedematous/microcystic
Significance	Classic "neoplastic" pattern	Degenerative areas

• Verocay bodies = nuclear-free zones flanked by two rows of palisading nuclei (pathognomonic of schwannoma).
• Immunohistochemistry: strongly and diffusely S-100 positive (neural crest origin). This distinguishes it from a meningioma (EMA positive).
• Antoni A and B usually coexist in the same tumour.

High-yield: S-100 positive + Verocay bodies + Antoni A/B = vestibular schwannoma. Meningioma (the chief CPA differential) is EMA positive, S-100 negative.

Etiology and pathophysiology

• The driver is loss of the NF2 tumour-suppressor gene on chromosome 22q12, which encodes merlin (schwannomin). Loss of merlin removes contact-inhibition of Schwann-cell proliferation.
• Sporadic unilateral tumours show somatic NF2 mutation/22q loss.
• Bilateral tumours indicate Neurofibromatosis type 2 (NF2) — germline NF2 mutation, autosomal dominant.

Mechanism of symptoms — stepwise growth:

1. Intracanalicular phase → tumour confined to the IAC. Compresses CN VIII fibres + labyrinthine artery → unilateral SNHL, tinnitus, vague imbalance. (Hearing loss is the earliest and commonest symptom.)
2. Cisternal phase → tumour reaches the CPA cistern; CN V involvement → corneal reflex loss, facial hypoaesthesia.
3. Brainstem compression phase → flattening of pons/cerebellar peduncle → ataxia, nystagmus.
4. Hydrocephalic phase → obstruction of CSF flow (4th ventricle) → raised intracranial pressure (ICP), headache, papilloedema, and ultimately tonsillar herniation.

High-yield: Facial nerve (CN VII) runs with CN VIII in the IAC yet is remarkably resistant — facial weakness is a late and uncommon presenting feature. Early facial palsy with a CPA mass should make you reconsider the diagnosis (think facial nerve schwannoma or malignancy).

Clinical features

The classic evolution follows the anatomy described above.

• Unilateral, slowly progressive SNHL — the hallmark; present in ~95%. Patients often complain of difficulty using the phone on the affected ear.
• Poor speech discrimination out of proportion to the pure-tone loss — a retrocochlear signature (rollover phenomenon).
• Tinnitus — high-pitched, unilateral; second commonest symptom.
• Disequilibrium / unsteadiness rather than true rotatory vertigo (slow growth allows central compensation).
• Sudden SNHL in ~10–20% (from labyrinthine artery compromise) — hence every unilateral/asymmetric SNHL deserves a retrocochlear workup.
• Reduced/absent corneal reflex (CN V) — earliest sign of CPA extension (Hitselberger sign is reduced sensation over the posterior-superior EAC wall = CN VII sensory branch).
• Cerebellar signs — ataxia, dysdiadochokinesia, intention tremor in large tumours.
• Raised ICP — headache, vomiting, papilloedema (late).

Cranial nerve involvement sequence (mnemonic, by anatomical proximity): VIII → V → VII → IX, X as the tumour enlarges from canal to CPA to jugular foramen region.

High-yield: Speech discrimination score (SDS) is disproportionately poor relative to the pure-tone average — the single most useful audiometric clue to a retrocochlear lesion.

Audiology — localising to retrocochlear

Test	Cochlear (e.g. Meniere)	Retrocochlear (acoustic neuroma)
Recruitment	Present (positive)	Absent
SISI score	High (>70%)	Low (<30%)
Tone decay (Carhart)	Absent/minimal	Marked (>30 dB)
Speech discrimination	Relatively preserved	Disproportionately poor / rollover
Stapedial reflex decay	Negative	Positive (reflex decay)
OAEs	Absent (cochlear damage)	Often present (cochlea intact)
BERA	Normal IPLs	Prolonged I–V interpeak latency

Investigation of choice

Screening / functional: BERA (Brainstem Evoked Response Audiometry / ABR) is the most sensitive audiological test (~95% sensitivity for tumours >1 cm).

• Wave V latency prolongation and an increased wave I–V interpeak latency (IPL).
• An interaural wave V latency difference >0.2 ms between ears is the classic abnormal cut-off.
• Less sensitive for small intracanalicular (<1 cm) tumours — these may be missed.

High-yield (BERA): Acoustic neuroma → increased I–V interpeak latency and an interaural V latency difference >0.2 ms. BERA reflects neural conduction delay along CN VIII into the brainstem.

Gold standard / investigation of choice: MRI of the IAM with gadolinium contrast.

• Detects tumours as small as 1–2 mm; the definitive test.
• Appearance: lesion centred on the IAC with an "ice-cream cone" configuration (intracanalicular cone + CPA globe), avid enhancement, and widening/flaring of the porus acusticus.
• T1: isointense; T2 (e.g. CISS/FIESTA): the tumour is dark against bright CSF, outlining the IAC contents.
• CT shows a funnel-shaped widened/eroded IAC and is reserved for when MRI is contraindicated or for bony/surgical planning.

High-yield: Gadolinium-enhanced MRI of the IAM is the investigation of choice / gold standard. The "ice-cream cone" sign (scoop in the canal, globe in the CPA) is the classic image. A funnel-shaped widened porus acusticus is the radiological hallmark.

Diagnostic approach in one line:

Asymmetric/unilateral SNHL → PTA + speech audiometry (poor SDS, tone decay) → BERA (prolonged I–V IPL) → Gd-MRI IAM (ice-cream cone, confirms diagnosis).

Staging by size

Stage	Size	Typical features
Intracanalicular	Confined to IAC	SNHL, tinnitus only
Small	<1.5 cm (in CPA)	+ early imbalance
Medium	1.5–2.5 cm	CN V signs, touches brainstem
Large	2.5–4 cm	Brainstem compression
Giant	>4 cm	Hydrocephalus, raised ICP

Management / treatment of choice

Three broad options; choice depends on tumour size, growth rate, age, hearing status, and patient preference.

1. Observation ("wait and scan")

• For small (<1.5–2 cm), asymptomatic or minimally symptomatic tumours, elderly/comorbid patients.
• Serial annual Gd-MRI. Many tumours grow slowly (~1–2 mm/year) or not at all.

2. Stereotactic radiosurgery (Gamma Knife / LINAC)

• For small–medium tumours (<3 cm), poor surgical candidates, residual tumour after surgery.
• Aim: arrest growth (high tumour-control rates) and preserve hearing/facial nerve better than open surgery in selected cases.

3. Microsurgical excision — the definitive treatment of choice for large/growing/symptomatic tumours, especially with brainstem compression.

Approach	Best for	Hearing	Notes
Translabyrinthine	Any size, non-serviceable hearing	Sacrificed	Best facial nerve identification; no cerebellar retraction
Retrosigmoid (suboccipital)	Large tumours, hearing preservation attempt	May be preserved	Wide exposure; risk of cerebellar retraction, headache
Middle cranial fossa	Small intracanalicular tumours, hearing preservation	Best chance to preserve	Limited to small tumours; temporal lobe retraction

High-yield: Translabyrinthine approach always sacrifices hearing but gives the surest facial-nerve preservation and needs no cerebellar retraction. Middle fossa is chosen for small intracanalicular tumours when hearing preservation is the goal.

• Surgical priorities (in order): life > facial nerve function > hearing preservation > complete tumour removal.
• Hydrocephalus with raised ICP may need a CSF shunt/EVD before or alongside definitive treatment.

Complications

• Of the tumour: progressive deafness, brainstem compression, obstructive hydrocephalus → raised ICP → coning (life-threatening), cerebellar dysfunction.
• Of surgery: facial nerve palsy (commonest cranial nerve injury), total hearing loss, CSF leak/rhinorrhoea, meningitis, corneal exposure keratitis (combined V + VII deficit → loss of sensation and blink), lower cranial nerve palsies, cerebellar injury, vascular injury (AICA).
• Of radiosurgery: delayed facial/trigeminal neuropathy, hearing deterioration, rarely malignant transformation, transient post-radiation tumour swelling.

High-yield: A patient with both CN V (corneal anaesthesia) and CN VII (no blink) deficits is at high risk of exposure keratitis — protect the cornea (lubricants, tarsorrhaphy). This is a classic post-CPA-surgery complication question.

Key differentials (CPA mass)

Lesion	Distinguishing clue
Meningioma (2nd commonest CPA tumour)	Broad dural base, dural tail sign, EMA+/S-100−, calcifies, hyperostosis; eccentric to IAC
Epidermoid (cholesteatoma)	Restricts on DWI (bright), insinuates around structures, does not enhance, "cauliflower"
Arachnoid cyst	Follows CSF on all sequences, no DWI restriction, no enhancement
Facial nerve schwannoma	Early facial weakness/twitching, follows the geniculate/labyrinthine VII course
Vascular loop / aneurysm	AICA loop compressing CN VIII; flow void on MRI
Glomus jugulare / metastasis	Lower cranial nerve signs, jugular foramen erosion

The single most important everyday discriminator: acoustic neuroma is centred on the IAC with an ice-cream-cone shape and is S-100 positive; meningioma sits on the dura with a dural tail and is EMA positive.

Neurofibromatosis type 2 (the must-know association)

• Bilateral vestibular schwannomas are pathognomonic of NF2.
• Gene: NF2 (chromosome 22q12) → loss of merlin; autosomal dominant.
• NF2 also features: meningiomas, ependymomas, schwannomas of other nerves, juvenile cataracts → mnemonic "MISME" (Multiple Inherited Schwannomas, Meningiomas, Ependymomas).
• Presents younger than sporadic disease. Differentiate from NF1 (chromosome 17, neurofibromin, café-au-lait spots, Lisch nodules, optic glioma, axillary freckling).

High-yield: Bilateral acoustic neuromas = NF2 (chromosome 22). A classic single-best-answer NEET PG stem. NF1 = chromosome 17; do not confuse the two.

Mnemonics and eponyms

• "VESTibular schwannoma" — comes from the VESTibular nerve, not cochlear.
• CPA cranial nerve order with growth: VIII → V → VII (hearing first, corneal next, face last).
• Hitselberger sign — hypoaesthesia of the posterosuperior external auditory canal wall (sensory CN VII).
• Obersteiner–Redlich zone — the glial–Schwann junction where the tumour originates.
• MISME — for the NF2 tumour spectrum.

Recently asked / exam angle

NEET PG and INI-CET stems on this topic typically test:

• "Commonest site of origin" → internal acoustic meatus; "cell of origin" → Schwann cell of the vestibular (superior vestibular) nerve.
• "Investigation of choice / gold standard" → Gd-enhanced MRI of IAM; "most sensitive audiological test" → BERA with prolonged I–V interpeak latency.
• Bilateral tumours → name the syndrome (NF2) and chromosome (22).
• Audiometry interpretation: unilateral SNHL with disproportionately poor speech discrimination and positive tone decay → asks you to localise to retrocochlear.
• Histology image: Verocay bodies / Antoni A and B, S-100 positivity.
• "Earliest symptom" → unilateral SNHL/tinnitus; "earliest sign of CPA extension" → diminished corneal reflex.
• Surgical approach matching: translabyrinthine (sacrifices hearing) vs middle fossa (small intracanalicular, preserve hearing) vs retrosigmoid (large, hearing attempt).
• Image-based: "ice-cream cone" appearance and funnel-shaped widened porus acusticus.
• Differential trap: dural tail + EMA positive = meningioma, not acoustic neuroma; DWI bright = epidermoid.

Rapid revision

1. Vestibular schwannoma = benign Schwann-cell tumour of the (superior) vestibular nerve at the internal acoustic meatus; commonest CPA tumour (~80%).
2. Earliest & commonest symptom = unilateral progressive SNHL; tinnitus is next.
3. Hallmark audiology clue = speech discrimination disproportionately poor (retrocochlear); positive tone decay, low SISI, no recruitment.
4. BERA: increased wave I–V interpeak latency, interaural wave V difference >0.2 ms — most sensitive audiological test.
5. Investigation of choice / gold standard = Gd-enhanced MRI of the IAM; classic "ice-cream cone" with widened funnel-shaped porus acusticus.
6. Histology: Antoni A (Verocay bodies) + Antoni B, strongly S-100 positive.
7. CN involvement order with growth: VIII → V (corneal reflex loss) → VII (late); facial palsy is a late, uncommon feature.
8. Hitselberger sign = hypoaesthesia of posterosuperior EAC wall; Obersteiner–Redlich zone = site of origin.
9. Bilateral tumours = NF2 (chromosome 22, merlin); spectrum = MISME.
10. Management: small/elderly → observe (wait & scan); small–medium → stereotactic radiosurgery; large/growing → microsurgery.
11. Translabyrinthine sacrifices hearing (best facial-nerve safety); middle fossa for small intracanalicular with hearing preservation; retrosigmoid for large tumours.
12. Top differential = meningioma (dural tail, EMA+, S-100−); epidermoid restricts on DWI; combined V+VII deficit → exposure keratitis — protect the cornea.