Incomplete Third Nerve Palsy Approach

Summary: • A complete third nerve palsy manifests as a complete or near complete ptosis and involvement of all the extraocular muscles (EOM) innervated by CN III o Reminder: this does NOT include the superior oblique muscle (innervated by CN IV) or the lateral rectus muscle (innervated by CN VI) • Pupil involvement helps dictate likelihood of aneurysm o Pupil involved -> assume posterior communicating artery aneurysm until proven otherwise o Pupil not involved but complete palsy in vasculopathic patient -> likely to be small vessel ischemia • Third nerve palsy may be considered "partial" when: o There are any signs of an abnormal pupil including slightly larger pupil, lack of pupil reaction and recent surgery (i.e. for cataracts) o Divisional muscle involvement such as incomplete ptosis or incomplete involvement of all of the EOM innervated by CN III > ALL partial palsies must be worked up for aneurysm • Standard work-up for aneurysm in a partial third nerve palsy o Non-contrast CT scan > followed by contrast CTA (to look for aneurysm) o Also obtain a contrast MRI (for non-aneurysmal causes)