Sixth Cranial Nerve Palsy

Abducens nerve palsy causes loss of abduction of the affected eye, with horizontal diplopia worse at distance and in gaze toward the paretic side.

• The nerve has the longest subarachnoid course and a sharp bend over the petrous apex, making it vulnerable to raised intracranial pressure (false localising sign).
• Causes: microvascular (diabetes, hypertension), raised intracranial pressure (idiopathic intracranial hypertension, tumour), trauma, skull base tumours (clivus chordoma, nasopharyngeal carcinoma), petrous apicitis (otitis media, pain and discharge), cavernous sinus lesions, multiple sclerosis, giant cell arteritis, meningitis, Wernicke encephalopathy, post-viral in children, post lumbar puncture.

• Esotropia in primary position, worse at distance; limited or absent abduction; compensatory face turn toward the paretic side.
• Slowed abducting saccades; mild narrowing of the palpebral fissure with globe retraction on attempted adduction in Duane type, which is a differential.
• Associated signs localise the lesion: papilloedema (raised pressure), ipsilateral facial weakness and deafness (pontine, cerebellopontine angle), Horner syndrome (cavernous sinus), facial pain or numbness (fifth nerve), ear pain and discharge (Gradenigo).
• Bilateral palsy suggests raised pressure, Wernicke, trauma or a clival lesion.

• Exclude mimics: thyroid eye disease (forced duction test), medial rectus restriction, orbital wall fracture entrapment, myasthenia (ice pack test, antibodies), divergence insufficiency, convergence spasm.
• Fundus examination for papilloedema; visual fields.
• Adults over 50 with vascular risk and isolated palsy: blood pressure, glucose, HbA1c, lipids, ESR, CRP; observe 6 to 12 weeks. Image if no recovery by 3 months, or at once for any additional sign, pain or age under 50.
• MRI brain and orbits with contrast and MR venography; lumbar puncture with opening pressure (normal below 25 cmH2O in adults) after imaging.
• Children: MRI always, as brain tumour must be excluded; post-viral palsy resolves in weeks.
• Skull base CT for petrous apex, trauma.

• Treat the cause: antibiotics for petrous apicitis (ceftriaxone 2 g IV daily plus surgical drainage if needed), steroids for giant cell arteritis (methylprednisolone 1 g IV daily for 3 days then prednisolone 1 mg/kg), thiamine 500 mg IV 3 times daily for 3 days then 250 mg daily in Wernicke, acetazolamide 500 mg twice daily (increase to 1 g twice daily) for idiopathic intracranial hypertension, tumour referral.
• Symptom control: monocular occlusion; Fresnel or ground in base out prism for small angles.
• Botulinum toxin (2.5 to 5 units) into the ipsilateral medial rectus in the first months prevents contracture and may restore alignment in partial palsy.
• Surgery after 6 months of non-recovery:
  • Residual lateral rectus function: ipsilateral medial rectus recession (4 to 6 mm) with lateral rectus resection (6 to 8 mm).
  • Complete palsy with no function: vertical rectus transposition (full tendon or Hummelsheim) to the lateral rectus, with botulinum toxin or recession of the medial rectus.
• Amblyopia treatment in children with patching and spectacle correction.