The oculomotor nerve controls four of the six extra-ocular muscles, the levator palpebrae superioris, and the parasympathetic supply to the pupil constrictor and accommodation. Its parasympathetic fibres travel on the outer surface of the nerve, explaining the classic finding in posterior communicating artery aneurysm — a painful third nerve palsy with a dilated (blown) pupil from external compression affecting the parasympathetic fibres first. Ischaemic third nerve palsy from diabetes spares the pupil because the ischaemia affects the central fibres while the peripheral parasympathetics are preserved.
| Origin | Oculomotor nucleus in the midbrain periaqueductal grey at the level of the superior colliculus |
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A complete CN III palsy produces the classic down and out eye (adduction, elevation, and intorsion lost), ptosis from levator paralysis, and a fixed dilated pupil. The pupil-involving versus pupil-sparing distinction is critical: a pupil-involving third nerve palsy requires urgent MRI/MRA to exclude a posterior communicating artery aneurysm or other mass compressing the nerve externally. A pupil-sparing third nerve palsy in a diabetic patient is presumed ischaemic and managed expectantly.
CN III compression by a PCOM aneurysm producing painful pupil-involving third nerve palsy requiring urgent neurosurgical clipping or endovascular coiling to prevent aneurysm rupture.
