If Hydroxyamphetamine is instilled in a Horner's syndrome eye and the eye does not dilate, what does it indicate about the lesion?

When Hydroxyamphetamine is administered in an eye affected by Horner's syndrome and there is no dilation response, it suggests that the lesion is located at the post-ganglionic level. Hydroxyamphetamine works by stimulating the release of norepinephrine from the nerve endings, which would normally cause dilation of the pupil in a healthy response.

In cases of Horner's syndrome characterized by a post-ganglionic lesion—such as a problem affecting the sympathetic nerve after it has synapsed—there is a failure to release sufficient norepinephrine, leading to a lack of pupil dilation upon administration of Hydroxyamphetamine. If the lesion were pre-ganglionic, it would still have intact post-ganglionic fibers that can release norepinephrine, resulting in pupil dilation if Hydroxyamphetamine is applied.

This understanding is crucial for correctly interpreting the underlying cause of Horner's syndrome and determining its location, which is vital in diagnosing and managing the condition appropriately.