If a central or preganglionic lesion is present, instillation of hydroxyamphetamine should cause mydriasis of the pupil, as the postganglionic neuron is intact and therefore able to release NA.
Damage to the postganglionic neuron will result in less or no NA being released, and mydriasis will be incomplete or absent in response to topical hydroxyamphetamine.
Pholedrine is the N-methyl derivative of hydroxyamphetamine and has been suggested as an alternative drug in localising the causative lesion in OSP.
A study comparing pupil responses to phenylephrine 1% and hydroxyamphetamine 1% in 14 consecutive patients with OSP after previously confirming the diagnosis with cocaine 10%21 found that phenylephrine 1% increased pupil size in postganglionic OSP by a mean of 2.
Given the limited availability of hydroxyamphetamine and the similarities in sensitivity and specificity of the two drugs in localising a lesion to the postganglionic neuron, phenylephrine 1% appears to be the more reliable and readily available alternative.
For more than 30 years, cocaine 2-10% has been the gold standard in the diagnosis of unilateral Horner's syndrome and hydroxyamphetamine 1% recommended to localise the causative lesion.
Hydroxyamphetamine test for diagnosis of postganglionic defects.
False-negative hydroxyamphetamine (Paredrine) test in acute Horner's syndrome.
Pholedrine: A substitute for hydroxyamphetamine as a diagnostic eyedrop test in Horner's syndrome.