Third Cranial Nerve Lesions

Anatomy

Course of the Oculomotor Nerve

• The somatic motor nucleus of the oculomotor nerve is located in the mid region of the midbrain.
• The parasympathetic autonomic nucleus is the Edinger-Westphal nucleus, also in the midbrain.

The nerve:

• Emerges from the ventral brainstem, superior to the pons and at the medial margin of the cerebral peduncle of the midbrain.
• Runs forward within the lateral wall of the cavernous sinus, just above the trochlear nerve.
• Divides into:
  • Superior ramus
  • Inferior ramus

Both rami enter the orbit via the superior orbital fissure.

Mnemonic for structures passing superior to inferior:
“Lazy French Tarts Sitting Naked In Anticipation”: Lacrimal nerve, Frontal nerve, Trochlear nerve, Superior ramus of the oculomotor nerve, Nasociliary nerve, Inferior ramus of the oculomotor nerve, Abducens nerve.

Oculomotor Nerve Innervations

Ramus	Structures Supplied
Superior ramus	– Superior rectus (elevates eye maximally when abducted) – Levator palpebrae superioris (raises upper eyelid; also receives sympathetic fibres)
Inferior ramus	– Inferior rectus (depresses eye, maximally when abducted) – Medial rectus (pulls eye medially) – Inferior oblique (pulls eye up and out) – Sends branch to ciliary ganglion → parasympathetic fibres to eye via short ciliary nerves

Pathology

Causes of a third cranial nerve lesion include:

1. Demyelinating disease
   • Multiple sclerosis
2. Vascular disease
   • Brainstem microvascular strokes
3. Space-occupying lesions
   • Tumours
   • Aneurysm of the posterior communicating artery (must be excluded)
   • Abscesses
4. Raised intracranial pressure
5. Venoms
   • Snakebite
6. Thiamine deficiency
   • Seen in Wernicke’s encephalopathy (as one manifestation of ophthalmoplegia)
7. Trauma
   • Skull base trauma
   • Unilaterally dilated pupil ± ipsilateral third nerve palsy may indicate impending cerebral herniation
8. Mononeuritis
   • Diabetes
   • Toxins
   • Microvascular disease
   • Paraneoplastic disease
   • Connective tissue disease
   • Infections (HIV, Lyme disease, syphilis)
9. Idiopathic
   • No clear cause found in some cases
10. Rarely
    • Cavernous sinus thrombosis

Clinical Assessment

Important Points of History

• Presenting problem usually one of diplopia.
• If headache present, raises suspicion for:
  • Cerebral aneurysm (acute bleed or expansion)
  • Intracranial tumours
  • Raised intracranial pressure

Important Points of Examination

1. Strabismus
   • Obvious squint of the affected eye
   • Eye typically lies “down and out” (inferior and lateral)
2. Pupil dilation
   • Pupil may or may not be dilated
3. Ptosis
   • Complete or partial ptosis
4. Eye movement testing
   • Eye is “down and out” with third nerve lesion
   • To test for associated fourth nerve (trochlear) lesion:
     • Ask patient to look down then across to opposite side
     • Eye will intort if fourth nerve is intact

Investigations

Blood Tests

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

CT Angiogram

• Urgent imaging required to rule out posterior communicating artery aneurysm.
• CT angiography reasonable option, but not as sensitive as MRI/MRA.
• Will also detect other space-occupying lesions causing third nerve palsy.

MRI

• Preferred imaging for detecting cerebral aneurysm.
• Most sensitive and specific for other intracranial or orbital space-occupying lesions.

Angiography

• Formal angiography is gold standard but invasive; rarely required.

Management

• Management directed at the underlying cause.
• If posterior communicating artery aneurysm is found:
  • Patient should be kept in hospital pending neurosurgical consultation.
  • Clear follow-up plan must be arranged with neurosurgery before discharge from Emergency Department.

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Appendix 1

Appendix 2

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References

Publications

• Golshani K, Ferrell A, Zomorodi A, Smith TP, Britz GW. A review of the management of posterior communicating artery aneurysms in the modern era. Surg Neurol Int. 2010 Dec 22;1:88.
• Zelman S, Goebel MC, Manthey DE, Hawkins S. Large Posterior Communicating Artery Aneurysm: Initial Presentation with Reproducible Facial Pain Without Cranial Nerve Deficit. West J Emerg Med. 2016 Nov;17(6):808-810.
• 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. More Befuddling Pupillary Asymmetry. LITFL
• Nickson C. Third Cranial Nerve Lesions. LITFL
• Nickson C. Cranial nerve lesions DDx. LITFL