Which drug can determine whether a Horner's syndrome lesion is pre or post-ganglionic?

Phenylephrine 1% is used to differentiate between pre-ganglionic and post-ganglionic lesions in Horner's syndrome. In cases of Horner's syndrome, there is a disruption in sympathetic innervation to the eye, which can either occur at the level of the central nervous system (pre-ganglionic) or at the level of the sympathetic ganglion (post-ganglionic).

When phenylephrine, an alpha-1 adrenergic agonist, is administered, it acts by causing the pupil to dilate (mydriasis) through vasoconstriction of the dilator pupillae muscle. In cases of post-ganglionic lesions, the absence of sympathetic innervation means that phenylephrine will not effectively cause pupil dilation. Conversely, if the lesion is pre-ganglionic, the post-ganglionic neurons remain intact, so the presence of pupil dilation from phenylephrine indicates that the lesion is indeed located before the sympathetic ganglion.

This pharmacological test allows clinicians to pinpoint the location of the interruption in sympathetic innervation, which is critical for diagnosis and subsequent management of Horner's syndrome.