If Hydroxyamphetamine is instilled in a Horner's syndrome eye and the eye does dilate, what can be concluded about the lesion?

When Hydroxyamphetamine is instilled in the eye affected by Horner's syndrome and results in dilation, it indicates that the sympathetic pathway's post-ganglionic neurons are intact. Hydroxyamphetamine works by stimulating the release of norepinephrine from sympathetic nerve endings, promoting dilation.

In Horner's syndrome, a pre-ganglionic lesion blocks the sympathetic pathway before the synapse occurs in the superior cervical ganglion, while a post-ganglionic lesion means that the sympathetic fibers have already synapsed and the issue lies somewhere beyond that point. Therefore, if the eye dilates in response to Hydroxyamphetamine, it confirms that the pathway is functioning up to that point and that the lesion must be pre-ganglionic. If it were a post-ganglionic lesion, the nerve endings would be affected, and dilation would not occur. This conclusion is critical for diagnosing the specific location of the sympathetic pathway interruption in Horner's syndrome.