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ACOUSTIC NEUROMA (VESTIBULAR SCHWANNOMA)

A Comprehensive 50-Mark Answer

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📌 DEFINITION

Misnomer Note (Dhingra, p. 434): The term "acoustic neuroma" is a misnomer — it is not a neuroma, and it does not arise from the acoustic (cochlear) nerve. The preferred term is Vestibular Schwannoma.

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📌 EPIDEMIOLOGY

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📌 ETIOPATHOGENESIS

A. Sporadic (Unilateral) — 95%

• Loss of function mutation in NF2 tumor suppressor gene on chromosome 22q12 (merlin/schwannomin protein)
• Radiation exposure (prior head & neck radiotherapy) is a known risk factor
• Mobile phone use — controversial (INTERPHONE study — no definitive link)

B. Hereditary (Bilateral) — 5%

• Neurofibromatosis Type 2 (NF2)
• Autosomal dominant — bilateral vestibular schwannomas are pathognomonic
• Associated with meningiomas, ependymomas, and other cranial nerve schwannomas

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📌 PATHOLOGY

Gross Appearance (Scott-Brown, Vol 3):

• Well-encapsulated, firm, whitish-yellow tumor
• Lobulated surface
• Cut section: solid with areas of cystic degeneration and yellowish discoloration (due to lipid accumulation)
• Attached to the vestibular nerve; cochlear nerve is displaced

Microscopic Appearance:

┌─────────────────────────────────────────────────────────┐
│              HISTOLOGICAL PATTERNS                       │
├─────────────────────┬───────────────────────────────────┤
│  ANTONI TYPE A       │  ANTONI TYPE B                   │
├─────────────────────┼───────────────────────────────────┤
│ Compact, cellular    │ Loose, hypocellular              │
│ Palisading of nuclei │ Myxoid/microcystic change        │
│ Verocay bodies       │ No Verocay bodies                │
│ (nuclear palisades   │ Lipid-laden macrophages          │
│  around pink matrix) │ Hemorrhage, edema               │
│ Orderly arrangement  │ Disorganized                     │
└─────────────────────┴───────────────────────────────────┘

Immunohistochemistry:

• S-100 positive (marker of Schwann cells) — key diagnostic marker
• SOX10 positive
• GFAP negative (distinguishes from glioma)

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📌 ANATOMICAL RELATIONSHIPS

                    INTERNAL AUDITORY CANAL (IAC)
                    ┌─────────────────────────┐
                    │  Facial nerve (VII)      │  ← Anterior Superior
                    │  Superior vestibular     │  ← Posterior Superior
                    │  Cochlear nerve          │  ← Anterior Inferior
                    │  Inferior vestibular     │  ← Posterior Inferior
                    └─────────────────────────┘
                              ↓
                    TUMOR ORIGIN: Superior or Inferior
                    Vestibular nerve at Obersteiner-
                    Redlich zone
                              ↓
                    GROWTH → into CPA cistern

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📌 GROWTH PATTERN & STAGES

Koos Grading System (most widely used):

Growth Rate:

• Average: 1–2 mm/year (highly variable)
• Up to 30% show no growth over observation period
• Some tumors may enlarge rapidly (>3 mm/year)

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📌 CLINICAL FEATURES

Symptoms (in order of frequency):

┌─────────────────────────────────────────────────────────────────┐
│                    CLINICAL TRIAD                                │
│   1. Unilateral SNHL       2. Tinnitus      3. Vertigo          │
└─────────────────────────────────────────────────────────────────┘

1. Hearing Loss (~95% cases)

• Unilateral sensorineural hearing loss (SNHL) — most common presentation
• Insidious, slowly progressive
• High-frequency loss initially
• Sudden SNHL in ~10% — due to vascular compromise (labyrinthine artery occlusion) or rapid tumor expansion
• Rollover phenomenon: Speech discrimination worsens at high intensity (retrocochlear pattern)

2. Tinnitus (~70%)

• Unilateral, high-pitched, continuous
• Often the first symptom noticed by the patient

3. Vertigo (~20–30%)

• Typically mild dysequilibrium rather than true episodic vertigo
• Central compensation occurs due to slow growth
• Frank vertigo is less common (Dhingra)

4. Facial Nerve Features (late, large tumors):

• Facial weakness is surprisingly rare despite facial nerve being intimately related — due to its resistance to slow compression
• Facial numbness/tingling more common (involvement of V nerve)
• Hemifacial spasm (very rare)

5. Features of Large/Giant Tumors (Grade III–IV):

• Trigeminal nerve involvement: facial numbness, loss of corneal reflex
• Cerebellar signs: Ataxia, dysdiadochokinesia, intention tremor
• Raised ICP: Headache, nausea, vomiting, papilledema
• Abducens (VI nerve) palsy: Diplopia
• Hydrocephalus: Due to obstruction of CSF pathways

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📌 SIGNS ON EXAMINATION

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📌 INVESTIGATIONS

FLOWCHART: Diagnostic Algorithm

Unilateral SNHL / Tinnitus / Vertigo
              ↓
      Pure Tone Audiogram (PTA)
              ↓
    ┌─────────────────────────────┐
    │ Asymmetric SNHL ≥10 dB     │
    │ at 2 or more frequencies   │
    └─────────────────────────────┘
              ↓
      Speech Discrimination Score
      (SDS) — Rollover phenomenon
              ↓
      Auditory Brainstem Response (ABR)
      ┌──────────────────────────────┐
      │ Increased I–V IPL (>0.3 ms) │
      │ Absent Wave V                │
      │ Interaural latency diff >0.2│
      └──────────────────────────────┘
              ↓
      GOLD STANDARD:
      Gadolinium-Enhanced MRI
      (T1 with Gd — CISS/FIESTA sequence)
              ↓
    ┌─────────────────────────────────────┐
    │   CONFIRMED VESTIBULAR SCHWANNOMA   │
    └─────────────────────────────────────┘

1. Pure Tone Audiometry (PTA):

• Asymmetric SNHL
• High-frequency loss
• 50/50 rule (Cummings): If SDS <50% or PTA gap >50 dB at high frequencies — suspect retrocochlear pathology

2. Speech Audiometry:

• Rollover index >0.45 — suggestive of retrocochlear pathology
• Poor speech discrimination out of proportion to hearing loss

3. Auditory Brainstem Response (ABR) / BERA:

• Sensitivity: ~90%, Specificity: ~80% for tumors >1 cm
• Increased wave I–V interpeak latency (IPL)
• Interaural latency difference >0.2 ms
• Absent or prolonged Wave V

4. MRI (Gold Standard):

• Gadolinium-enhanced MRI — sensitivity ~99%
• T1 with Gadolinium: Intensely enhancing lesion in IAC/CPA
• CISS/FIESTA sequence (heavily T2-weighted): Tumor appears as filling defect in fluid-bright IAC
• "Ice-cream on a cone" appearance — tumor in CPA with extension into IAC
• Brainstem compression, hydrocephalus assessment

5. CT Scan:

• Less sensitive than MRI
• Shows widening/erosion of IAC (>8 mm or asymmetry >2 mm)
• Useful when MRI is contraindicated
• CT Cisternogram (air or contrast) — historical, largely replaced by MRI

6. Caloric Testing (ENG/VNG):

• Reduced or absent caloric response on the affected side (canal paresis)
• Useful baseline before surgery

7. Electronystagmography (ENG):

• Directional preponderance to normal side
• Spontaneous nystagmus absent or mild (central compensation)

8. Electrocochleography (ECochG):

• SP/AP ratio elevated — may suggest Meniere's disease (differential)

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📌 DIFFERENTIAL DIAGNOSIS

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📌 MANAGEMENT

FLOWCHART: Management Algorithm

CONFIRMED VESTIBULAR SCHWANNOMA
              ↓
    ┌─────────────────────────────────────────┐
    │ Assess: Tumor Size, Age, Hearing,       │
    │ Symptoms, Comorbidities                 │
    └─────────────────────────────────────────┘
              ↓
    ┌─────────┬──────────────┬────────────────┐
    │         │              │                │
    ▼         ▼              ▼                ▼
WATCHFUL  STEREOTACTIC   MICROSURGERY    COMBINED
WAITING   RADIOSURGERY                  APPROACH
(Grade I)  (≤3 cm)       (All grades)

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OPTION 1: WATCHFUL WAITING (Wait and Scan)

• Elderly patients (>65–70 years) with small intracanalicular tumors
• Significant comorbidities (unfit for surgery)
• Serviceable hearing in only-hearing ear
• Patient preference
• Incidentally discovered small tumors

• MRI at 6 months, then annually × 5 years, then every 2–3 years
• Growth >2–3 mm/year → consider intervention

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OPTION 2: MICROSURGICAL EXCISION

Three Main Approaches:

┌─────────────────────────────────────────────────────────────────┐
│           SURGICAL APPROACHES — COMPARISON                       │
├──────────────┬──────────────────┬──────────────┬───────────────┤
│ Feature      │ Translabyrinthine│ Middle Fossa │ Retrosigmoid  │
│              │     (TL)         │    (MF)      │    (RS)       │
├──────────────┼──────────────────┼──────────────┼───────────────┤
│ Hearing      │ Sacrificed       │ May preserve │ May preserve  │
│ preservation │ (No)             │ (Yes)        │ (Yes)         │
├──────────────┼──────────────────┼──────────────┼───────────────┤
│ Tumor size   │ Any size         │ <1.5 cm, IAC │ Any size      │
│              │                  │ confined     │               │
├──────────────┼──────────────────┼──────────────┼───────────────┤
│ Facial nerve │ Excellent        │ Good         │ Good          │
│ exposure     │ visualization    │              │               │
├──────────────┼──────────────────┼──────────────┼───────────────┤
│ Cerebellar   │ No retraction    │ No           │ Required      │
│ retraction   │                  │ retraction   │               │
├──────────────┼──────────────────┼──────────────┼───────────────┤
│ CSF leak     │ Less common      │ Uncommon     │ More common   │
├──────────────┼──────────────────┼──────────────┼───────────────┤
│ Best for     │ Non-serviceable  │ Small tumors │ Large tumors, │
│              │ hearing, any     │ with useful  │ useful hearing│
│              │ size             │ hearing      │               │
└──────────────┴──────────────────┴──────────────┴───────────────┘

a) Translabyrinthine Approach (TL) (Scott-Brown; Hazarika):

• Most direct access to IAC
• Mastoidectomy → labyrinthectomy → IAC drilling
• Facial nerve identified at stylomastoid foramen and traced
• Complete tumor removal with excellent facial nerve preservation
• Sacrifices all residual hearing — used when hearing is already non-serviceable
• Dura closed with abdominal fat graft
• Advantage: No cerebellar retraction; excellent visualization of facial nerve throughout its course

b) Middle Cranial Fossa (MF) / Middle Fossa Approach:

• Temporal craniotomy, elevation of temporal lobe dura
• Access to IAC from above
• Best for small intracanalicular tumors with serviceable hearing
• Risk: Temporal lobe retraction, facial nerve injury at geniculate ganglion

c) Retrosigmoid / Posterior Fossa Approach (Suboccipital):

• Most versatile — access to entire CPA and posterior fossa
• Hearing preservation possible in tumors with small/no IAC component
• Risk: Cerebellar retraction, headache (10–20%), CSF leak

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Intraoperative Monitoring (Critical for Facial Nerve Preservation):

• Facial Nerve Monitoring (EMG) — mandatory in all approaches
• BERA (Intraoperative) — for hearing preservation approaches
• Monopolar/bipolar stimulation for nerve identification

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OPTION 3: STEREOTACTIC RADIOSURGERY (SRS)

Types:

1. Gamma Knife Radiosurgery (GKRS) — Leksell system (most studied)
2. CyberKnife — robotic, frameless
3. Linear Accelerator (LINAC) — photon-based
4. Fractionated Stereotactic Radiotherapy (FSRT) — used for larger tumors

Principle:

• High-dose focused radiation to the tumor in a single fraction (radiosurgery) or multiple fractions (radiotherapy)
• Causes radiation-induced fibrosis and vascular thrombosis → tumor growth arrest
• Does NOT eliminate the tumor — achieves growth control

Indications:

• Tumor ≤3 cm (Koos Grade I–III) without significant brainstem compression
• Elderly/medically unfit patients
• Patient refusal of surgery
• Residual/recurrent tumor post-surgery

Dose (GKRS):

• 12–13 Gy to the tumor margin (50% isodose line)
• Older protocols used 18–20 Gy → higher complication rates

Results:

• Tumor control rate: 93–97% at 5–10 years (multiple series)
• Hearing preservation: 50–60% at 5 years
• Facial nerve preservation: >95%
• Trigeminal neuropathy: 2–4%

Disadvantages:

• Risk of malignant transformation (very rare, <0.1%)
• Radiation-induced complications (facial numbness, tinnitus worsening)
• Delayed tumor control (months to years)
• Post-radiation swelling → transient neurological worsening

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Facial Nerve Management — The Critical Issue

INTRAOPERATIVE FACIAL NERVE STATUS
            ↓
    ┌───────────────────────────┐
    │ Anatomically Intact?      │
    └───────────────────────────┘
       Yes ↓              No ↓
  Monitor & EMG      Immediate Repair
  Stimulate distal    ┌──────────────┐
  segment             │ End-to-end   │
                      │ anastomosis  │
                      │ Cable graft  │
                      │ (sural nerve)│
                      └──────────────┘
                            ↓
                    DELAYED (if no recovery >12 months)
                      ┌──────────────────┐
                      │ Hypoglossal-Facial│
                      │ anastomosis      │
                      │ (XII-VII reinnv.)│
                      └──────────────────┘

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📌 COMPLICATIONS

Surgical Complications:

Radiosurgery Complications:

• Hearing deterioration: ~40% at 5–10 years
• Tinnitus worsening
• Trigeminal neuralgia
• Hydrocephalus (rare)
• Tumor progression requiring surgery (~5%)

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📌 HEARING REHABILITATION POST-TREATMENT

POST-ACOUSTIC NEUROMA TREATMENT — HEARING OPTIONS
                    ↓
        ┌───────────────────────────┐
        │ Contralateral ear normal? │
        └───────────────────────────┘
           Yes ↓              No ↓
    CROS hearing aid    Bone-anchored hearing
    (Contralateral      aid (BAHA)
    Routing of Signals) or Cochlear Implant
                        or Auditory Brainstem
                        Implant (ABI)

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📌 BILATERAL ACOUSTIC NEUROMA (NF2)

• Pathognomonic of Neurofibromatosis Type 2 (NF2)
• Gene: NF2 on chromosome 22q12
• Manchester Criteria for NF2 diagnosis includes bilateral vestibular schwannoma
• Management: Surgery + Bevacizumab (anti-VEGF) has shown tumor control and hearing improvement in NF2-related schwannomas (recent advance)
• Auditory Brainstem Implant (ABI) — placed at cochlear nucleus when cochlear nerve is sacrificed in NF2

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📌 RECENT ADVANCES (2015–2024)

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📌 PROGNOSIS

• Surgery: 95%+ tumor control; best outcomes in experienced centers (>25 cases/year)
• GKRS: 93–97% control at 10 years for tumors ≤3 cm
• Watchful waiting: Safe for small tumors; ~60% remain stable
• Facial nerve recovery: Best if anatomically intact intraoperatively; improves over 12–24 months
• Quality of life: Majority return to normal activities; tinnitus and imbalance may persist
• Recurrence: After complete resection — <1%; after subtotal resection — 10–15%
• Malignant transformation: Extremely rare (<0.05%); reported in irradiated cases

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📌 SUMMARY TABLE

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📚 REFERENCES

1. Scott-Brown's Otorhinolaryngology, Head and Neck Surgery (8th Ed.) — Vol. 3, Chapters on CPA tumors and Vestibular Schwannoma
2. Cummings Otolaryngology: Head and Neck Surgery (7th Ed.) — Brackmann et al., Chapter on Acoustic Neuroma
3. Stell & Maran's Head and Neck Surgery — Schwannoma of VIII nerve
4. Dhingra PL — Diseases of Ear, Nose and Throat (8th Ed.) — p. 434–441
5. Hazarika P — Textbook of Ear, Nose & Throat — Chapter on CPA Tumors
6. Zakir Hussain — Textbook of ENT — Acoustic Neuroma
7. Harrison's Principles of Internal Medicine (21st Ed.) — p. 726 (Vestibular Schwannoma)
8. Jackler RK — Neurotology (2nd Ed.)
9. Samii M & Matthies C — Neurosurgery 1997 — Management of 1000 vestibular schwannomas
10. Kondziolka D et al. — Gamma Knife for acoustic neuroma — long-term results
11. Plotkin SR et al. — Bevacizumab for NF2 schwannomas — NEJM 2009
12. NANOS (North American Neuro-Ophthalmology Society) — Current guidelines 2022
13. European Consensus on Vestibular Schwannoma Management — EAONO 2020

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Examiner's Tip (RGUHS): For a 50-mark answer, ensure you cover — Definition + Epidemiology (5m) + Pathology with Antoni types and diagram (10m) + Clinical features (10m) + Investigations with ABR and MRI (10m) + Management with surgical approaches table and flowchart (12m) + Complications + Recent advances (8m) = 55m framework; trim to 50m based on time.