Which diagnostic medication will show NO dilation in a (+) Horner's syndrome regardless if postganglionic or preganglionic sympathetics are affected?

In a case of Horner's syndrome, the sympathetic innervation to the pupil is disrupted, leading to a characteristic triad of ptosis, miosis, and anhidrosis. The question pertains to the use of diagnostic medications to evaluate the presence of Horner's syndrome and their effects on pupil dilation.

Cocaine is a sympathomimetic agent that normally promotes pupil dilation by preventing the reuptake of norepinephrine, thus enhancing sympathetic tone. However, in Horner's syndrome, there is a disruption in sympathetic pathways. Both preganglionic and postganglionic lesions prevent the release of norepinephrine at the target tissues, meaning that the drug cannot cause the expected dilation response. Consequently, when cocaine is applied to a patient with Horner's syndrome, there will be no dilation observed, regardless of whether the lesion is preganglionic or postganglionic.

This key characteristic makes cocaine distinctive in diagnosing Horner's syndrome compared to other agents listed. For example:

• Apraclonidine, an alpha-2 agonist, may cause a measurable dilation in Horner's syndrome due to its action on the remaining sympathetic pathway, which is not available when cocaine is used.

• Hydroxyamphetamine works by releasing