Cerebellopontine Angle Tumours and Management

ANATOMY OF THE CPA

• Anteriorly - CN VI and the lateral clivus
• Laterally - medial surface of the petrous bone
• Medially - pons, middle cerebellar peduncle, and ventral cerebellum
• Superiorly - CN V (trigeminal)
• Inferiorly - CN IX, X, XI
• Posteriorly - cerebellar flocculus

CLASSIFICATION OF CPA TUMOURS

Common (>90% collectively)

Uncommon CPA Lesions

Intra-axial (not strictly CPA but encroach)

1. VESTIBULAR SCHWANNOMA (ACOUSTIC NEUROMA)

Definition and Epidemiology

• Benign Schwannoma of the 8th cranial nerve, most commonly the vestibular division (superior = inferior divisions equally affected)
• Accounts for 8-10% of all intracranial tumours; incidence ~1 in 100,000
• Most common tumour of the IAC and CPA
• Peak age: 40-60 years (sporadic, unilateral)
• Official term since 1992 NIH Consensus Conference: vestibular schwannoma

Genetics

• Sporadic (95%): unilateral, non-hereditary
• NF1 (von Recklinghausen): chromosome 17; acoustic neuromas in <5%; bilateral acoustic neuroma is NOT a feature
• NF2: chromosome 22 (merlin/schwannomin protein mutation); bilateral acoustic neuromas in up to 96% of patients; onset before age 21; also associated with other CN schwannomas, meningiomas, ependymomas

Pathology

• Arises from Schwann cells of nerve sheath; consists of Schwannoma cells in a collagenous matrix
• Circumscribed; displaces neural structures without direct invasion
• Consistency: firm/dense to soft/cystic
• Histology: Antoni A fibres (fibrillary, polar, elongated tissue) and Antoni B fibres (loosely reticulated tissue)
• Average growth rate: 1-2 mm/year; some tumours do not grow at all

Clinical Features (in order of frequency)

1. Unilateral SNHL - progressive; speech discrimination loss disproportionate to pure-tone loss (retrocochlear pattern); up to 20% present with sudden SNHL
2. Tinnitus - unilateral, high-pitched
3. Dysequilibrium/imbalance - mild; often under-reported; Fukuda stepping test reveals deviation toward affected side; head thrust test shows catch-up saccades
4. Facial hypesthesia/numbness - CN V compression; more common with medium/large tumours
5. Facial weakness - late; due to CN VII compression
6. Brainstem and cerebellar signs (large tumours) - ataxia, long tract signs
7. Raised ICP (very large) - headache, nausea, vomiting, hydrocephalus

Investigations

• Pure tone audiometry: asymmetric SNHL, high-frequency loss
• Speech audiometry: reduced speech discrimination score, rollover phenomenon (score worsens at higher intensities)
• Impedance audiometry: absent ipsilateral acoustic reflex at low sensation levels (acoustic reflex decay)
• ABR (Auditory Brainstem Response): interaural latency difference of wave V >0.2 ms is abnormal; historically 95-100% sensitivity, now supplementary to MRI

• Caloric testing: unilateral canal paresis (Fitzgerald-Hallpike)
• VEMP (Vestibular Evoked Myogenic Potentials): assesses saccule and inferior vestibular nerve

• Gadolinium-enhanced MRI (T1 + T2): gold standard - homogeneous or heterogeneous enhancement; ice-cream cone shape within IAC; CISS/FIESTA sequences detect tumours as small as 1-2 mm
• CT temporal bone: bony erosion of IAC (> 2 mm asymmetry), erosion of porus acusticus (funnel sign); used when MRI contraindicated

Grading (Koos Grading)

• Grade I: purely intracanalicular
• Grade II: extends into CPA, <2 cm, no brainstem contact
• Grade III: fills CPA cistern, touches brainstem
• Grade IV: compresses brainstem

Management

1. Observation (Conservative/Wait and Scan)

• Indications: small tumours (<1.5 cm), elderly patients, poor surgical candidates, serviceable hearing, tumours not showing growth, patient preference
• Serial gadolinium MRI at 6 months, then annually
• Rationale: many tumours grow <1 mm/year or not at all

2. Stereotactic Radiosurgery (SRS)

• Gamma Knife (most common), CyberKnife, LINAC-based
• Dose: 12-13 Gy to tumour margin
• Indications: tumours <2.5 cm in greatest diameter, enlarging tumours, elderly/high-risk surgical patients, residual after surgery
• Tumour control rate: 90-95%
• Does NOT typically preserve hearing; hearing can deteriorate post-SRS
• Risks: facial nerve dysfunction ~1%, trigeminal paresthesia/pain, malignant transformation (1 in 1000 over 30 years)
• Fractionated stereotactic radiotherapy (FSRT) used for larger or near-optic tumours

3. Microsurgical Resection

• Goals: complete tumour removal + preservation of facial nerve + hearing where possible
• Total removal is curative, but not always possible without sacrificing facial nerve
• Residual tumour often does not grow; can be observed or treated with SRS
• Complications: CSF leak (~10%), meningitis, facial paralysis, SNHL, headache, balance disturbance

• Serviceable hearing: speech reception threshold <50 dB, speech discrimination >50% (Class A/B)
• Attempt only with retrosigmoid or middle fossa approach
• More realistic for small (<1.5 cm) lateral tumours

• Intraoperative EMG monitoring of CN VII is mandatory
• House-Brackmann (HB) grading used to assess post-operative facial function

2. MENINGIOMA

• Second most common CPA tumour
• Arises from arachnoid cap cells of the arachnoid covering
• WHO classification: Grade 1 (benign - meningiothelial, fibrous, transitional, psammomatous, angiomatous), Grade 2 (atypical - chordoid, clear cell, atypical), Grade 3 (anaplastic/malignant - papillary, rhabdoid)
• Histology: psammoma bodies (calcified whorls)

• Dural tail sign on MRI
• Eccentric location to IAC (does not funnel into IAC)
• Calcifications on CT
• Hyperostosis of adjacent bone
• More homogeneous enhancement with gadolinium
• Hearing preservation or improvement more common than with schwannomas

3. EPIDERMOID (PRIMARY CHOLESTEATOMA)

• Arises from ectodermal inclusions trapped during neural tube closure
• Lined by stratified squamous epithelium; filled with keratin debris and cholesterol
• "Cauliflower-like" irregular margins on imaging
• Characteristically envelops rather than displaces cranial nerves (unlike acoustic neuroma)

• T1: isointense to CSF (hypointense)
• T2: isointense to CSF (hyperintense)
• FLAIR: heterogeneous hyperintense foci (differentiates from arachnoid cyst which is CSF-isointense on ALL sequences)
• DWI: restricted diffusion (bright) - key differentiating feature from arachnoid cyst
• No gadolinium enhancement

4. ARACHNOID CYST

• CSF-containing cyst lined by arachnoid membrane
• MRI: isointense to CSF on ALL sequences (T1, T2, FLAIR, DWI - does NOT restrict)
• Smooth, homogeneous walls (vs irregular epidermoid)
• Often asymptomatic; managed conservatively; surgical drainage if symptomatic

5. FACIAL NERVE SCHWANNOMA

• Most common tumour of facial nerve (schwannoma > hemangioma)
• Most commonly affects geniculate ganglion
• MRI: gadolinium enhancement
• CT: expansion/erosion of facial nerve canal
• Unlike schwannomas elsewhere, may cause facial nerve tic - helps distinguish from acoustic neuroma
• Management based on facial function:
  • Normal/near-normal function (HB Grade 1-2): observation
  • HB Grade 3 or worse: surgical removal + nerve grafting
  • Decompression for early/mild paresis
  • Approach: middle fossa/transmastoid (serviceable hearing); translabyrinthine (non-serviceable hearing)

DIFFERENTIAL DIAGNOSIS OF CPA LESION (Summary Table)

MANAGEMENT ALGORITHM SUMMARY

CPA Tumour Diagnosed on MRI
         |
    ┌────┴────┐
  Small     Large (>2.5 cm) / Symptomatic
  (<1.5 cm) |                    |
    |      SRS (12-13 Gy)   Microsurgery
  Observe   (Gamma Knife)   ┌────────────────┐
  Serial MRI               Non-serviceable  Serviceable
                            hearing          hearing
                               |                |
                        Translabyrinthine  Retrosigmoid OR
                           (preferred)     Middle Fossa

SURGICAL COMPLICATIONS

RECENT EVIDENCE

KEY EXAM POINTS TO REMEMBER

1. CPA tumour = acoustic neuroma (vestibular schwannoma) in >85% of cases
2. Antoni A and B fibres = histology of schwannoma
3. Psammoma bodies = meningioma
4. DWI restriction = epidermoid (not arachnoid cyst)
5. Dural tail + eccentric to IAC + calcification = meningioma
6. NF2 = bilateral acoustic neuromas, chromosome 22
7. Gold standard investigation = gadolinium-enhanced MRI
8. Rollover phenomenon on speech audiometry = retrocochlear lesion
9. ABR: interaural wave V latency difference >0.2 ms = abnormal
10. Three surgical approaches: translabyrinthine (no hearing), middle fossa (hearing, IAC tumour <1.5 cm), retrosigmoid (potential hearing, any size)
11. SRS dose = 12-13 Gy; control rate 90-95%