What is the role of a topical hydroxyamphetamine test in the diagnosis of Horner syndrome?

Updated: May 01, 2019
  • Author: Christopher M Bardorf, MD, MS; Chief Editor: Edsel Ing, MD, MPH, FRCSC  more...
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The localization of a lesion causing Horner syndrome may be aided by the use of the topical hydroxyamphetamine test. Hydroxyamphetamine stimulates the release of stored endogenous norepinephrine from the postganglionic axon terminals into the neuromuscular junction at the iris dilator muscles. This test may distinguish a postganglionic third-order neuron lesion from a presynaptic second-order or first-order neuron lesion.

To perform the test, 2 drops of 1% hydroxyamphetamine solution are instilled into each eye. A period of 24-48 hours must be allowed to elapse between the cocaine test and the hydroxyamphetamine test because cocaine has the ability to inhibit the uptake of hydroxyamphetamine into the presynaptic vesicles, thereby reducing the accuracy of the latter test.

Hydroxyamphetamine drops instilled into an eye with Horner syndrome with intact postganglionic fibers (ie, first- or second-order neuron lesions) dilate the affected pupil to an equal or greater extent than they do the normal pupil. However, hydroxyamphetamine drops instilled into an eye with Horner syndrome with damaged postganglionic fibers (ie, third-order neuron lesions) do not dilate the affected pupil as well as they do the normal pupil.

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