Hydroxyamphetamine can be used in diagnosing Horner's syndrome. If postganglionic sympathetics are affected, will there be dilation or no dilation?

In the context of diagnosing Horner's syndrome, hydroxyamphetamine acts as a sympathomimetic agent used to evaluate the function of the sympathetic nervous system in the eye. In a healthy response, hydroxyamphetamine would typically lead to dilation of the pupil due to increased sympathetic stimulation.

When postganglionic sympathetic fibers are affected, as occurs in Horner's syndrome, there is a disruption in the normal signaling that results in diminished or absent dilation. The affected fibers fail to respond to stimulation from hydroxyamphetamine, leading to a lack of dilation. This indicates that the postganglionic pathway is impaired, which is a characteristic finding in Horner's syndrome.

Thus, in cases where the postganglionic sympathetics are involved, there will indeed be no dilation of the pupil following administration of hydroxyamphetamine. This lack of response is crucial in helping clinicians diagnose and confirm the presence of Horner's syndrome.