A patient with Horner syndrome from a carotid dissection is given a dilute cocaine eye drop test. Cocaine fails to dilate the affected pupil because it acts by:
- A Directly stimulating alpha-1 receptors of the iris dilator muscle
- B Inhibiting acetylcholinesterase at the iris sphincter neuromuscular junction
- C Blocking norepinephrine reuptake; an absent norepinephrine supply prevents mydriasis ✓
- D Releasing stored catecholamines from iris adrenergic terminals
Correct answer: C. Blocking norepinephrine reuptake; an absent norepinephrine supply prevents mydriasis
Explanation
Cocaine blocks the neuronal norepinephrine transporter (NET), allowing sympathetically released NE to accumulate at the synapse and dilate the normal pupil. In Horner syndrome, the sympathetic supply to the iris dilator is interrupted; with no NE being released, cocaine has no substrate to potentiate, so the pupil fails to dilate. This confirms a pre-ganglionic or first-order Horner lesion when used alongside hydroxyamphetamine differentiation. Options B and D would not require intact sympathetic innervation.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.