Third Nerve Palsy

The pupil constricting parasympathetic fibres are located superficially in the oculomotor (3rd) nerve as it exits the brain stem and consequently they are typically affected by a compressive lesion eg tumour, aneurysm but spared by a vascular event eg microvascular infarction secondary to diabetes. Sudden onset painful diplopia from a third nerve palsy with pupil involvement therefore requires urgent evaluation for a posterior communicating artery aneurysm in all age groups.