What is the effect of CN III palsy on the pupil?

• A. Pupil constricts
• B. Pupil dilates due to parasympathetic fiber involvement
• C. Pupil remains fixed
• D. Pupil oscillates

Answer: (B)

CN III carries parasympathetic fibers that drive the sphincter pupillae, which constricts the pupil in response to light. If the oculomotor nerve is damaged in a way that involves these parasympathetic fibers (as often happens with compressive lesions like an aneurysm), the pupil loses its ability to constrict. The result is a dilated pupil that is often less reactive to light. In contrast, if CN III palsy is due to ischemia and spares the superficial parasympathetic fibers, the pupil may stay small and reactive. So the effect described—pupil dilation from parasympathetic fiber involvement—is the expected finding.