Etiology of Third Cranial Nerve Disorders Third cranial (oculomotor) nerve disorders that cause palsies and affect the pupil commonly result from Aneurysms (especially of the posterior communicating artery) Transtentorial brain herniation Less commonly, meningitis affecting the brain stem (eg, tuberculosis meningitis) The most common cause of palsies that spare the pupil, particularly partial palsies, is Ischemia of the 3rd cranial nerve (usually due to diabetes or hypertension) or of the midbrain Occasionally, a posterior communicating artery aneurysm causes oculomotor palsy and spares the pupil.