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.

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