Oculomotor nerve (CN III)
Concise, exam‑focused guide to recognition, initial investigation and emergency management of CN III palsy with emphasis on red flags and disposition decisions relevant to the ED.
Function and clinical consequence CN III supplies:
- Motor innervation to most extraocular muscles: medial, superior and inferior recti, and inferior oblique.
- Motor innervation to levator palpebrae superioris (raises the upper eyelid).
- Parasympathetic fibres (Edinger-Westphal → ciliary ganglion) to the sphincter pupillae and ciliary muscle for pupillary constriction and accommodation.
A lesion of CN III therefore produces:
- External ophthalmoplegia with the eye typically deviated “down and out” due to unopposed lateral rectus (CN VI) and superior oblique (CN IV).
- Ptosis from levator palpebrae superioris weakness.
- Pupillary dilatation and loss of light reflex when parasympathetic fibres are involved.
Key anatomy and clinical implications
- Origin: anterior midbrain (oculomotor nucleus and Edinger-Westphal nucleus).
- Intracranial course: passes between the superior cerebellar and posterior cerebral arteries, runs in the lateral wall of the cavernous sinus, and enters the orbit via the superior orbital fissure.
- Fibre arrangement: parasympathetic fibres run on the periphery of the nerve while motor fibres are more central.
Clinical implications: