Sixth Nerve Palsies

Commonest cause of neurogenic strabismus

Consider alternative diagnoses that can mimic

• Mechanical

  • Duane syndrome
  • Medial orbital wall fracture
  • Thyroid eye disease

• Myopathic:

  • Myasthenia
  • Myositis

• Convergence spasm

• Distance esotropia with high myopia

• Congenital esotropia with cross-fixation

Anatomy nuggets

• Nucleus lies in the lower pons anterior to the fourth ventricle at the level of the facial colliculus
• Most fibres innervate ipsilaterally, but 40% travel in the MLF to the contralateral subnucleus
• Fasciculus travels over the petrous apex of the temporal bone under the petroclinoid ligament
• Runs within the cavernous sinus lateral to the ICA and inferomedial to IIIn, IVn and Va
• Enters the orbit within the annulus of Zinn

Aetiology

• Idiopathic
• Microvascular
• Tumour eg. of the cerebellopontine angle or nasopharyngeal
• Basilar artery aneurysm
• Increased ICP (false-localising sign)
• Trauma eg skull base fracture
• Demyelination
• Vasculitis
• Meningitis
• Middle ear infection involving petrous temporal bone: Gradenigo (see below)
• Cavernous sinus thrombosis
• Carotid-cavernous fistula
• Congenital: but 1/3rd of children will have an underlying intracranial lesion so most require neuroimaging

Features

• Ductions are better than versions: helps distinguish paralytic from restrictive squint

  • Monocular movements (ductions) produce larger movements than binocular movements (versions)

• Worse for distance

• Primary deviation (when paretic eye fixing) is less than secondary deviation (when non-paretic eye is fixing)

Tests

• Vascular risk factors including arteritis
• Consider MRI if no recovery after 3 months or cause unclear
• Hess chart

Treatment

• Conservative: prisms, occlusion
• Botox injection
• Surgical:
  • LR resection/MR recession
  • Vertical muscle transposition if no residual function ie. total paralysis (causes exotropia if thereis residual function)
    • Hummelscheim procedure: for total sixth nerve palsies
      • Disinsertion of half of inferior and superior recti and reattaching above and below lateral rectus, plus recessing medial rectus
    • Jensen procedure: split-tendon procedure. Superior, inferior, lateral recti are split
    • Foster procedure: entire superior and inferior recti tendons are cut and re-inserted next to lateral rectus

Gradenigo syndrome

• Rare potentially lethal
• Caused by apical petrositis and extradural inflammation
  • Often secondary to chronic otitis media
• Involves 6th nerve and trigeminal nerve
  • 6th nerve palsy
  • Hemifacial pain
• Can progress to intracranial abscess and meningitis