Which pharmacologic agent is commonly used topically to reveal Horner's syndrome due to denervation hypersensitivity?

Which pharmacologic agent is commonly used topically to reveal Horner's syndrome due to denervation hypersensitivity?

• A. Apraclonidine
• B. Cocaine
• C. Hydroxyamphetamine
• D. Pilocarpine

Answer: (A)

Horner's syndrome causes denervation of the sympathetic pathway to the iris, which leads to upregulation or hypersensitivity of the iris dilator muscle to adrenergic stimulation. A topical agent that can activate those upregulated receptors will reveal the defect by altering pupil size. Apraclonidine, when used topically, acts on adrenergic receptors and, because of denervation hypersensitivity, the dilator muscle responds strongly enough to cause noticeable dilation of the affected pupil. In practice, this can reverse or lessen the anisocoria seen in Horner's, making the syndrome easy to reveal in the clinic. Cocaine relies on norepinephrine release and reuptake blockade; with Horner's there is reduced NE supply, so little dilation occurs, making it unreliable for revealing the syndrome. Hydroxyamphetamine helps localize the lesion (preganglionic vs postganglionic) rather than simply revealing Horner's. Dilute pilocarpine can cause miosis in Horner's due to muscarinic receptor supersensitivity, but it’s used to confirm the diagnosis after suspicion is raised, not as the primary reveal test.

Horner's syndrome causes denervation of the sympathetic pathway to the iris, which leads to upregulation or hypersensitivity of the iris dilator muscle to adrenergic stimulation. A topical agent that can activate those upregulated receptors will reveal the defect by altering pupil size.

Apraclonidine, when used topically, acts on adrenergic receptors and, because of denervation hypersensitivity, the dilator muscle responds strongly enough to cause noticeable dilation of the affected pupil. In practice, this can reverse or lessen the anisocoria seen in Horner's, making the syndrome easy to reveal in the clinic.

Cocaine relies on norepinephrine release and reuptake blockade; with Horner's there is reduced NE supply, so little dilation occurs, making it unreliable for revealing the syndrome. Hydroxyamphetamine helps localize the lesion (preganglionic vs postganglionic) rather than simply revealing Horner's. Dilute pilocarpine can cause miosis in Horner's due to muscarinic receptor supersensitivity, but it’s used to confirm the diagnosis after suspicion is raised, not as the primary reveal test.