CN III is the primary ocular motor nerve, supplying four of the six extraocular muscles plus the levator palpebrae and pupil constrictor. Complete CN III palsy produces the classic ptosis, exotropia ('down and out' eye from unopposed lateral rectus and superior oblique), and a dilated unreactive pupil. A posterior communicating artery aneurysm compressing CN III produces a surgical third nerve palsy with early pupil involvement.
| Origin | Oculomotor nucleus in the midbrain (superior colliculus level) |
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Pupil-involving CN III palsy (aneurysmal) requires emergency CT angiography or MR angiography to exclude a posterior communicating artery aneurysm — the dilated pupil is from compression of the superficial parasympathetic fibres. Medical CN III palsy from diabetes or hypertension spares the pupil (ischaemic core involvement with peripheral fibre sparing). The distinction is critical: aneurysmal palsy requires urgent neurosurgical or endovascular intervention.
Posterior communicating artery aneurysm compressing CN III producing ptosis, exotropia, and dilated pupil requiring emergency vascular imaging and intervention.
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