Sixth Cranial Nerve Lesions

Anatomy

Course of the Abducens Nerve

• Originates in the abducens nucleus within the pons.
• Exits the ventral brainstem in the posterior fossa at the junction of the pons and medulla, medial to the facial nerve, which itself lies medial to the vestibulocochlear nerve.
• Passes forward over the petrous temporal bone, into the middle cranial fossa, lateral to the sella turcica, within the cavernous sinus:
  • Lies lateral to the internal carotid artery
  • Lies medial to the ophthalmic nerve
• Leaves the cavernous sinus and enters the orbit through the superior orbital fissure and common tendinous ring.
• Runs along the medial aspect of the lateral rectus muscle, which it supplies.

Abducens Nerve Innervations

Function	Structure Innervated
Motor	Lateral rectus muscle

Pathology

Causes of a sixth cranial nerve lesion include:

1. Demyelinating disease
   • Multiple sclerosis
2. Vascular disease
   • Brainstem microvascular strokes
3. Space-occupying lesions
   • Tumours
   • Aneurysms
   • Abscesses
4. Raised intracranial pressure
   • Cerebral oedema
   • Intracerebral haemorrhage (ICH)
   • Subarachnoid haemorrhage (SAH)
5. Venoms
   • Snake bite
6. Thiamine deficiency
   • Wernicke’s encephalopathy (manifestation of ophthalmoplegia)
7. Trauma
   • Especially involving the petrous temporal bone, where the abducens nerve crosses
8. Mononeuritis
   • Diabetes
   • Toxins
   • Microvascular disease
   • Paraneoplastic disease
   • Connective tissue disease
   • Infectious disease (HIV, Lyme disease [US], syphilis)
9. Idiopathic
   • No clear cause found in some cases
10. Rare causes
    • Cavernous sinus thrombosis
      • Usually in combination with lesions of other cranial nerves within the cavernous sinus
11. Congenital causes
    • Congenital absence of the sixth nerve (e.g. Duane syndrome)

Clinical Assessment

Important Points of History

1. Presenting problem usually diplopia.
2. Patients may also present with a head-turned attitude in an attempt to maintain binocular vision.

Important Points of Examination

1. Strabismus
   • May be an obvious medially directed squint of the affected eye.
2. Eye movement testing
   • Failure of lateral movement of the affected eye.
   • Test both eyes together, and if abnormality found, each eye separately.
3. Double vision
   • Signs are maximal when looking to the affected side.
   • Images are horizontal and parallel.
   • Outermost image (from affected eye) disappears on covering that eye.
   • Outermost image usually more blurred.

Investigations

When Clinical Diagnosis Is Clear

• None may be necessary (e.g. in snake envenomation).

Otherwise Consider:

Blood Tests

1. FBC
2. CRP
3. ESR
4. U&Es / glucose

CT Scan / CT Angiogram

• Good screening test for intracranial mass lesions.
• CT angiogram for suspected aneurysmal disease.

MRI

• Best imaging investigation for the sixth cranial nerve.
• Especially useful for:
  • Intracranial / intraorbital space-occupying lesions (tumours, abscesses, aneurysms)
  • Multiple sclerosis

Management

• Management depends on the underlying cause.

Diplopia

• Patients should be warned not to perform high-risk activities (e.g. driving).
• Use of an eye patch may relieve debilitating diplopia.

Disposition

• Disposition depends largely on cause:
  • Mass lesions or bleeds → Urgent referral to Neurosurgery.
  • Isolated lesions in otherwise well patients → Referral to Neurology and/or Ophthalmology.

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Appendix 1

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Appendix 2

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References

Publications

• Brazis PW, Masdeu JC, Biller J. Localization in Clinical Neurology. 8e 2021
• Fuller G. Neurological Examination Made Easy. 6e 2019
• O’Brien M. Aids to the Examination of the Peripheral Nervous System. 6e 2023

FOAMed

• Coni R. Neuro 101: Cranial Nerves. LITFL
• Nickson C. The Brainstem Rules of Four. LITFL
• Ercleve T. The rule of 4 of the brainstem. LITFL
• Nickson C. Third Cranial Nerve Lesions. LITFL
• Nickson C. Cranial nerve lesions DDx. LITFL