What is the diagnostic agent of choice for suspected CN III palsy?

• A. 0.125% Pilocarpine
• B. 1% Pilocarpine
• C. Cocaine
• D. 1% Apraclonidine

Answer: (B)

Pupillary function tells you whether the parasympathetic fibers traveling with the oculomotor nerve are intact. In suspected CN III palsy, testing the iris sphincter with pilocarpine directly probes this parasympathetic pathway.

Using a 1% pilocarpine drop, if the pupil constricts, the parasympathetic innervation is preserved, which points toward a pupil-sparing (often ischemic) CN III palsy. If there is little or no constriction, the parasympathetic fibers are involved—this raises concern for a compressive lesion (such as an aneurysm) and requires urgent evaluation. Thus pilocarpine at this concentration is the most informative pharmacologic test for distinguishing these scenarios.

Other agents test different conditions: very dilute pilocarpine (0.125%) is used to identify Adie's tonic pupil due to denervation supersensitivity; cocaine and apraclonidine tests are used to differentiate Horner syndrome from true anisocoria or CN III involvement.