Non-pupil-sparing oculomotor palsy

The rule of the pupil in third nerve palsy:

Elucidated in 1958 by Rucker. In effect, the rule states, “Third nerve palsy due to extrinsic compression of the nerve will be associated with impaired pupillary constriction.” However, it is often overlooked that in 3% the pupil is spared ¹⁾.

Most cases are due to extrinsic compression of the 3rd nerve. Etiologies include:

1. tumor: the most common tumors affecting the 3rd nerve:

a) chordomas

b) clival meningiomas

2. vascular: the most common vascular lesions:

a) Posterior communicating artery aneurysm-(pupil sparing with aneurysmal oculomotor palsy occurs in < 1%).

★ Development of a new 3rd nerve palsy ipsilateral to a p-comm aneurysm may be a sign of expansion with the possibility of imminent rupture and is traditionally considered an indication for urgent treatment

b) Basilar bifurcation aneurysm or aneurysms of the distal basilar artery (basilar tip)

c) carotid-cavernous fistula: look for pulsatile proptosis

3. uncal herniation

4. cavernous sinus lesions: usually cause additional cranial nerve findings (V1, V2, IV, VI); see Cavernous sinus syndrome. Classically the third nerve palsy, e.g. from enlarging cavernous aneurysm, will not produce a dilated pupil because the sympathetic which dilate the pupil are also paralyzed