In Horner syndrome, which pharmacological test confirms a preganglionic (second-order neuron) lesion as opposed to a postganglionic (third-order) lesion?

• A Cocaine 4% — dilation of the affected eye confirms the diagnosis
• B Hydroxyamphetamine 1% — failure of pupil to dilate indicates a postganglionic lesion ✓
• C Apraclonidine 0.5% — reversal of anisocoria confirms sympathetic denervation supersensitivity
• D Pilocarpine 0.1% — failure to constrict indicates a preganglionic lesion

Explanation

Hydroxyamphetamine 1% (Paredrine) releases norepinephrine from intact postganglionic nerve terminals — if the third-order neuron is intact (preganglionic lesion), the affected pupil dilates normally; if there is a postganglionic lesion with degenerated nerve terminals, it fails to dilate. Cocaine confirms the diagnosis of Horner syndrome itself (both pre- and postganglionic). Apraclonidine demonstrates denervation supersensitivity to confirm Horner's presence but does not localize the lesion.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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