Last reviewed 01/2018

The level of a complete lesion in Horner's syndrome can be identified by the use of pharmacological agents:

  • 4% cocaine eyedrops fail to dilate the pupil in Horner's syndrome:
    • cocaine inhibits the reuptake of noradrenaline, if there is no noradrenaline then cocaine is inactive
  • hydroxyamphetamine will not dilate a Horner's pupil if the lesion is post-ganglionic
  • weak adrenaline dilates a Horner's pupil when the lesion is post-ganglionic

In summary:

  • a normal pupil is dilated by cocaine and hydroxyamphetamine
  • when the lesion is pre-ganglionic only hydroxyamphetamine causes dilatation
  • when the lesion is post-ganglionic only adrenaline causes dilatation