What is the role of a topical apraclonidine 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 topical apraclonidine test is a practical and reliable alternative to the topical cocaine test; it is readily available and adequately sensitive (87%) and is currently the test of choice. [23, 24, 25]

Apraclonidine is an ocular hypotensive agent that acts as a weak alpha1-agonist and a strong alpha2-agonist. [26, 23, 22, 27] Typically given in a 0.5% or 1% solution, it has little to no effect on a normal pupil but has a mydriatic effect on an abnormal pupil.

In Horner syndrome, upregulation of alpha1-receptors increases apraclonidine sensitivity and causes denervation supersensitivity of the iris dilator muscle. The denervation supersensitivity results in pupillary dilatation and lid elevation on the abnormal side but no response or mild miosis on the normal side from alpha2-activity after apraclonidine administration. Reversal of anisocoria occurs after bilateral instillation of apraclonidine.

In acute cases, false-negative test results may occur because the alpha1-receptor upregulation on which the effect of apraclonidine depends may take 5-8 days. [28, 29] Accordingly, a negative apraclonidine test result, especially in acute settings, does not exclude Horner syndrome. In such cases, a cocaine test should be performed to exclude Horner syndrome.

Apraclonidine 0.5% or 1% may cause lethargy, bradycardia, and respiratory depression in infants younger than 6 months as a consequence of the immaturity of the blood-brain barrier. [30]

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