Which of the following agents would distinguish between a preganglionic versus a postganglionic lesion?

The location of the lesion in Horner syndrome (either preganglionic or postganglionic) can be determined by the use of certain sympathomimetics. Which of the following agents would distinguish between a preganglionic versus a postganglionic lesion?

(A) amphetamine
(B) atenolol
© epinephrine
(D) isoproterenol
(E) phenylephrine

(A) Indirectly acting sympathomimetics (e.g., amphetamines, cocaine) are useful in this
situation because their action requires the presence of intact postganglionic noradrenergic neurons. Thus administration of hydroxyamphetamine into the eye will cause mydriasis in a Horner patient if the lesion is preganglionic (postganglionic neuron intact), but not if the lesion is postganglionic. In contrast, directacting sympathomimetics (choices C, D, E) will have the same effect regardless of the location of the lesion. Beta antagonists (choice B) will not produce any effect on the pupil.