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Facial Neurovascular Conflict

Summary

  • Neurovascular compression of the facial nerve (CN VII) at the root exit zone causing hemifacial spasm
  • Results from vascular loop compression, typically by anterior inferior cerebellar artery (AICA) or posterior inferior cerebellar artery (PICA)
  • High-resolution MRI with CISS/FIESTA sequences demonstrates vascular contact at the cisternal segment of CN VII

Pathophysiology

  • Mechanism of compression
    • Arterial pulsations cause chronic irritation of facial nerve at root exit zone (REZ)
    • REZ is transition zone between central and peripheral myelin (2-3mm from brainstem)
    • Most vulnerable area due to lack of epineurium
  • Pathologic changes
    • Demyelination at compression site
    • Ephaptic transmission between adjacent nerve fibres
    • Hyperexcitability of facial nerve nucleus
  • Offending vessels
    • AICA (most common - 40-50%)
    • PICA (30-40%)
    • Vertebral artery (10%)
    • Basilar artery dolichoectasia (rare)
    • Venous compression (extremely rare)

Demographics

  • Incidence
    • 11 per 100,000 population
    • Accounts for primary hemifacial spasm in >95% of cases
  • Age
    • Peak incidence: 5th-6th decade
    • Mean age at onset: 45-50 years
    • Rare in patients <30 years
  • Gender
    • Female predominance (2:1 ratio)
  • Laterality
    • Left side more commonly affected (60%)
    • Bilateral involvement rare (<1%)

Diagnosis

  • Clinical presentation
    • Involuntary, intermittent tonic-clonic contractions of facial muscles
    • Typically begins in orbicularis oculi muscle
    • Progresses caudally to involve lower face
    • Exacerbated by stress, fatigue, voluntary facial movements
    • Persists during sleep (distinguishes from blepharospasm)
  • Electrophysiology
    • Abnormal muscle response on EMG
    • Lateral spread response on nerve conduction studies
    • Synkinesis between different facial nerve branches
  • Differential diagnosis
    • Secondary hemifacial spasm (tumour, AVM, aneurysm)
    • Facial myokymia
    • Blepharospasm
    • Facial tics
    • Post-Bell's palsy synkinesis

Imaging

  • MRI Protocol
    • High-resolution 3D heavily T2-weighted sequences essential
    • Thin-slice acquisition (0.5-1mm) through cerebellopontine angle
  • T2 CISS/FIESTA
    • Hyperintense CSF with excellent contrast
    • Hypointense cranial nerves clearly visualised
    • Flow voids of vessels seen as hypointense structures
    • Demonstrates vascular contact/compression at REZ
  • T1
    • Limited utility for neurovascular conflict
    • Useful for excluding mass lesions
  • T1+C
    • Not routinely required
    • Helps exclude enhancing lesions (schwannoma, meningioma)
    • May show enhancement if chronic nerve irritation
  • DWI
    • Usually normal
    • Excludes acute ischaemic changes
  • SWI
    • Helpful for identifying vessels
    • Distinguishes arteries from veins
    • Detects calcifications or haemorrhage
  • MRA (TOF or contrast-enhanced)
    • Confirms vascular anatomy
    • Identifies offending vessel origin and course
    • Excludes aneurysms or vascular malformations
  • Imaging findings
    • Direct contact between vessel and CN VII at REZ
    • Indentation or displacement of nerve
    • Perpendicular vessel orientation to nerve most significant
    • Atrophy or signal changes in chronic cases

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  • A patient presenting with right hemifacial spasm has an MRI showing contact between the attached segment of the facial nerve and a superiorly looping PICA.

Treatment

  • Medical management
    • First-line therapy
    • Carbamazepine (initial drug of choice)
    • Baclofen, gabapentin as alternatives

Differential diagnosis

Differential diagnosis for facial palsy Differentiating feature
Bell's palsy Facial nerve may enhance
Facial nerve schwannoma Progressive facial weakness with hearing loss; enhancing mass along facial nerve course on MRI
Meningioma Space-occupying lesion visible on MRI with dural tail sign; progressive symptoms rather than paroxysmal