During examination of a patient with suspected Horner syndrome, apraclonidine 0.5% is instilled into both eyes. After 30 minutes, the affected pupil dilates and the upper lid rises. This reversal occurs because apraclonidine acts as:

• A A cholinesterase inhibitor increasing acetylcholine
• B A beta-blocker reducing aqueous production
• C A parasympatholytic causing mydriasis
• D An alpha-1 agonist causing denervation supersensitivity response ✓

Explanation

Apraclonidine is a weak alpha-1 agonist. In Horner syndrome, denervation supersensitivity develops in the iris dilator muscle, causing it to respond to the weak alpha-1 stimulation with dilation. The normal pupil does not respond. This reversal of anisocoria confirms Horner syndrome. It is particularly useful in acute Horner syndrome before supersensitivity to hydroxyamphetamine can develop.

Reference: AAO: Neuro-Ophthalmology, 2022 ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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