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Abducens Neurovascular Conflict

Summary

  • Vascular compression of CN VI causing horizontal diplopia from lateral rectus palsy
  • The offending vessel contacts the nerve at its root exit zone or cisternal (prepontine) segment
  • High-resolution 3D T2 (CISS/FIESTA/DRIVE) demonstrates the neurovascular contact and any nerve distortion1

Pathophysiology

  • Pulsatile arterial compression causes focal demyelination and axonal injury of CN VI
  • Common offending vessels: AICA, basilar artery, vertebral artery (often dolichoectatic) and SCA
  • Contact is usually at the root exit zone (pontomedullary junction) or within the prepontine cistern

Demographics

  • Rare; most often adults 40–70 years, with no clear sex predilection
  • Associations: hypertension, atherosclerosis and vertebrobasilar dolichoectasia

Diagnosis

  • Horizontal diplopia worse on lateral gaze, esotropia and failure of abduction, without other cranial neuropathies
  • Requires a clinical CN VI palsy plus MRI neurovascular contact, having excluded other causes

Imaging

  • High-resolution 3D heavily T2-weighted sequences (CISS/FIESTA/DRIVE), ≤1 mm, are the mainstay
    • Show the vessel contacting, displacing or indenting the cisternal nerve
  • TOF-MRA identifies the offending vessel and any dolichoectasia for surgical planning
  • Chronic compression may cause nerve atrophy; secondary lateral rectus denervation/atrophy may be seen

panels-1

  • A 40-year-old patient presented with a progressive left abducens palsy.
  • MRI showed the cisternal segment of the abducens nerve (red) distorted by the AICA before entering Dorello's canal.
  • The left lateral rectus muscle was subtly T2-hyperintense and atrophic.

Treatment

  • Conservative first-line: observation (spontaneous resolution possible), prisms, botulinum toxin
  • Microvascular decompression is reserved for persistent, disabling symptoms

Differential diagnosis

Imaging differential Differentiating feature
Abducens schwannoma Enhancing fusiform mass along the cisternal nerve or Dorello's canal, rather than a vascular loop
Petroclival / Dorello's canal meningioma Enhancing dural-based mass with a dural tail
Vertebrobasilar dolichoectasia Ectatic tortuous vessel contacting the nerve; asymptomatic contact is common, limiting specificity
Petrous apex lesion (cholesterol granuloma, apical petrositis) Expansile petrous apex lesion abutting Dorello's canal
Cavernous sinus lesion Mass or abnormal enhancement in the cavernous sinus affecting the intracavernous CN VI

  1. Elder et al. Isolated Abducens Nerve Palsy: Update on Evaluation and Diagnosis. 2016. Current neurology and neuroscience reports - Open in new tab. ↩