Third nerve palsy – surgical or medical?

Ischaemic oculomotor nerve palsies (IONP) usually are pupil-sparing. Diabetes, hypertension, and atherosclerosis are commonly associated with IONP. Vasculopathic third nerve mononeuropathies are caused by extra-axial infarcts of the core of the third nerve, which derives its blood supply from the vasa nervorum. The pupillomotor fibers are spared because of their more peripheral location and blood supply from the pial vessels. The superficial location of the pupillomotor fibers renders them more susceptible to damage from compressive lesions, most commonly posterior communicating artery aneurysms. The rule of the pupil states that an isolated, atraumatic third nerve palsy with complete external ophthalmoplegia and a normal pupil has a greater than 90% probability of being due to microvascular ischaemia. However, there are exceptions and more importantly misinterpretation of the rule can lead to unnecessary investigations and complications. This review is an attempt to clarify what a pupil sparing oculomotor palsy is and when to investigate further for a compressive lesion such as an aneurysm.