What is the diagnostic agent of choice for a suspected Tonic pupil?

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

Answer: (A)

Tonic pupil arises from parasympathetic denervation of the iris sphincter with postganglionic receptors becoming supersensitive. Because of this supersensitivity, a very small amount of cholinergic stimulus can cause constriction. Using a dilute cholinergic agent, specifically 0.125% pilocarpine, will trigger constriction in the affected pupil while a normal pupil—lacking denervation supersensitivity—remains largely unresponsive at this low concentration. This makes the dilute pilocarpine test the most diagnostic for Adie’s (tonic) pupil.

In contrast, a higher concentration of pilocarpine (like 1%) would constrict both normal and tonically dilated pupils, providing no diagnostic distinction. Cocaine and apraclonidine are used primarily to evaluate Horner syndrome and sympathetic pathways, not to confirm a tonic pupil, so they don’t serve this diagnostic purpose here.