OCULOMOTOR PALSY Remember: "DOWN and OUT" with a dilated pupil.
OVERVIEW OF CAUSES: MEDICAL vs SURGICAL CAUSES CN 3 lesions are generally divided into medical or surgical (compressive) causes: classically, medical third nerve lesions spare the pupil.
DIFFERENTIAL DIAGNOSIS
MEDICAL CAUSES
Isolated third nerve lesions may occur with:
Branch occlusion of the posterior cerebral artery. Most often these occur with contralateral weakness (fascicular third nerve) or with ataxia (Claude’s syndrome) or choreoathetosis (Benedict’s syndrome) from thalamoperforate branch occlusion of the P1 segment of the posterior cerebral artery.
Rarely, an ascending interpeduncular branch occlusion from the top of the basilar artery.
Myasthenia gravis frequently affects the third nerve with pupillary sparing and increased ptosis with fatigue.
Rarely, demyelinating disease affects the third nerve.
Symmetrical ptosis and oculomotor weakness is characteristic of:
Mitochondrial disease (Kearns–Sayre syndrome, progressive external ophthalmoplegia)
Congenital myopathy (centronuclear or myotubular myopathy)
Oculopharyngeal dystrophy.
Surgical (compressive) third nerve is seen with:
Aneurysms as follows:
Posterior communicating artery
Internal carotid artery
Superior cerebellar artery
Top of the basilar (often in conjunction with the fourth nerve)
Tumors:
Parasellar neoplasms
Medial sphenoid wing meningiomas
Skull-based tumors.