Hydroxyamphetamine testing is used to determine the location of Horner's syndrome.

• A. Determines the location of Horner's syndrome
• B. Increases intraocular pressure
• C. Causes cycloplegia
• D. Is a carbonic anhydrase inhibitor

Answer: (A)

Hydroxyamphetamine testing looks at whether the postganglionic sympathetic nerve endings can release norepinephrine. In Horner's syndrome, the problem can be central/pre-ganglionic or postganglionic. Hydroxyamphetamine triggers release of NE from intact postganglionic terminals. If the postganglionic neuron is intact (central or preganglionic lesion), NE is released and the pupil dilates after instillation, indicating the lesion is not postganglionic. If there is a postganglionic lesion, the nerve ending cannot release NE, so there is little or no pupil dilation, localizing the problem to the postganglionic segment. Thus, this test helps determine the location along the oculosympathetic pathway. It does not affect intraocular pressure, cycloplegia, or act as a carbonic anhydrase inhibitor.