Which drug can dilate a Horner's pupil due to denervation supersensitivity?

• A. Brimonidine
• B. Apraclonidine
• C. Phenylephrine
• D. Tropicamide

Answer: (B)

The key idea is denervation supersensitivity of the iris dilator muscle in Horner’s syndrome. When sympathetic supply to the eye is disrupted, the postganglionic dilator fibers become hypersensitive to adrenergic stimulation. Apraclonidine has enough alpha-adrenergic activity (especially after this supersensitivity) to stimulate the dilator muscle and cause dilation of the affected pupil, while a normal pupil remains largely unchanged. This makes apraclonidine particularly useful as a diagnostic sign of Horner’s, because the abnormal pupil responds to the drug due to the supersensitivity.

Tropicamide works by blocking muscarinic receptors to paralyze the sphincter, causing dilation in both pupils and is not specific for Horner’s. A pure alpha-1 agonist like phenylephrine can dilate the affected pupil as well, but apraclonidine is preferred in this test context because it exploits the supersensitive state to produce a clearer, more specific diagnostic response. Brimonidine is mainly an alpha-2 agonist and is less reliably diagnostic for Horner’s.