A 65-year-old woman presents with progressive diplopia. When looking straight ahead, she has upper eyelid ptosis, a mid-dilated pupil, exotropia, and hypertropia of her left eye. When you ask her to look to her right, her left eyelid raises, and her pupil constricts.
Which of the following is the most likely cause of this presentation?
A. Ischemic CN III palsy
B. Congenital CN III palsy
C. Multiple sclerosis
D. Distant history of trauma
E. Compressive CN III palsy
The correct answer is E. This presentation is classic for aberrant regeneration (synkinesis) of the oculomotor nerve. The key clue is that when she looks to the right, her left eyelid elevates and the pupil constricts—movements that should not normally occur together. That kind of miswiring happens when regenerating CN III fibers grow into the wrong pathways. Aberrant regeneration does not occur with ischemic CN III palsy, but does occur after compression or trauma. In an older patient with progressive symptoms, a compressive lesion (classically a posterior communicating artery aneurysm or tumor) is most likely.