In Horner syndrome, which test specifically helps determine if the lesion is preganglionic?

• A. Apraclonidine Test
• B. Hydroxyamphetamine Test
• C. CBC w/ Diff
• D. Chest CT

Answer: (B)

In Horner syndrome, telling apart a preganglionic from a postganglionic lesion hinges on whether the postganglionic sympathetic nerve ending still works. Hydroxyamphetamine directly tests that: it stimulates the release of endogenous norepinephrine from intact postganglionic fibers. If the lesion is preganglionic, the postganglionic neuron remains functional, so hydroxyamphetamine will cause the affected pupil to dilate. If the lesion is postganglionic, the postganglionic neuron is damaged, and there’s little or no NE release in response, so the pupil on the affected side won’t dilate. This makes hydroxyamphetamine the test that specifically localizes to a preganglionic problem.

Apraclonidine can help confirm Horner’s by revealing denervation supersensitivity, but it doesn’t localize the lesion. CBC with differential and chest CT aren’t the functional tests used to distinguish preganglionic from postganglionic Horner’s.