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Cranial nerves · Motor

III. Oculomotor nerve

Not medically reviewed · Content last updated · 2 min read ·

This is the date the draft was written, not a medical review date.

The oculomotor nerve (CN III) supplies most extraocular muscles and the levator palpebrae superioris, and carries parasympathetic fibres for pupillary constriction and accommodation. It emerges from the interpeduncular fossa of the midbrain.

Terminologia Anatomica (Latin): Nervus oculomotorius

Function

General somatic efferent to the superior, inferior and medial rectus, inferior oblique and levator palpebrae superioris. General visceral efferent (parasympathetic) to the sphincter pupillae and ciliary muscle via the ciliary ganglion.

Nuclei

Oculomotor nucleus (somatic) at the level of the superior colliculus in the midbrain, and the Edinger-Westphal nucleus (parasympathetic) lying dorsally and rostrally to it. The levator palpebrae is supplied by a single midline caudal subnucleus, so ptosis from a nuclear lesion is bilateral.

Exit from the skull

It passes through the cavernous sinus and enters the orbit through the superior orbital fissure, where it divides into a superior division (superior rectus, levator palpebrae) and an inferior division (medial and inferior rectus, inferior oblique, parasympathetic root to the ciliary ganglion).

Testing

Test eye movements in the H pattern, looking for diplopia, ptosis, and failure of adduction, elevation or depression. Examine pupil size and the direct and consensual light reflexes, and the accommodation reflex (near response).

Signs of a lesion

Complete palsy gives ptosis, and the eye is deviated down and out (unopposed lateral rectus and superior oblique) with a dilated, poorly reactive pupil. Pupil-sparing palsy suggests ischaemia of the nerve trunk (for example diabetes); a dilated pupil suggests external compression.

Clinical correlations

A painful third nerve palsy with a dilated pupil is a surgical emergency until a posterior communicating artery aneurysm is excluded. Uncal herniation compresses the nerve against the tentorium with an ipsilateral dilated pupil. Weber syndrome combines ipsilateral CN III palsy with contralateral hemiparesis (midbrain infarct).

Common exam points

  • Supplies all extraocular muscles except lateral rectus (CN VI) and superior oblique (CN IV): LR6 SO4.
  • Complete palsy: ptosis, eye down and out, dilated pupil.
  • Parasympathetic fibres run on the outside of the nerve and are first affected by compression.
  • Painful palsy with a dilated pupil: exclude a posterior communicating artery aneurysm.
  • Edinger-Westphal nucleus mediates pupillary constriction and accommodation.

References

  1. Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students. Elsevier — Tier 3 (textbook)
  2. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. Wolters Kluwer, 8th edition (2018) — Tier 3 (textbook)
  3. Haines DE, Mihailoff GA. Fundamental Neuroscience for Basic and Clinical Applications. Elsevier, 5th edition (2018) — Tier 3 (textbook)
  4. Bickley LS. Bates' Guide to Physical Examination and History Taking (Bickley). Wolters Kluwer, 13th edition (2020) — Tier 3 (textbook)

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