What is the role of a topical cocaine 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 basis for the topical cocaine test is the ability of cocaine to act as an indirect sympathomimetic agent by inhibiting the reuptake of norepinephrine from the synaptic cleft at the nerve ending.

The test is performed by instilling cocaine solution (2-4% or, according to some, 4-10%) into each eye. Cocaine instilled in an eye with intact sympathetic innervation causes the pupil to dilate. A sympathetically denervated pupil (such as is present in Horner syndrome) dilates poorly to cocaine, regardless of the level of the sympathetic interruption, because of the absence of endogenous norepinephrine in the synapse.

For optimal accuracy, test results should be evaluated 30 minutes or longer after cocaine is administered. The maximal response is seen 40-60 minutes after instillation of the drops. Postcocaine anisocoria greater than 0.8 mm is sufficient to diagnose Horner syndrome.

The disadvantages of the topical cocaine test are as follows:

  • The drops are difficult to obtain because they must be made at a compounding pharmacy

  • The drops are relatively expensive

  • The test can yield equivocal results

  • Cocaine metabolites may be detected in urine

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