Which topical medication is known to be useful in the diagnosis of Horner's syndrome?

Apraclonidine is the correct choice for diagnosing Horner's syndrome because it is an alpha-2 adrenergic agonist that can help identify the presence of a sympathetic dysfunction. In a healthy eye, apraclonidine causes a slight mydriatic effect due to its ability to stimulate the dilator muscle. However, in the case of Horner's syndrome, the affected pupil, which is typically miosis (constricted), will show a paradoxical dilation when apraclonidine is applied. This occurs because the denervated pupil is overly sensitive to the action of the medication.

In contrast, other options do not serve this diagnostic purpose effectively. Atropine is a non-selective anticholinergic that causes mydriasis, but it does not specifically differentiate between the types of pupil reactions associated with Horner's syndrome. Brimonidine, while useful in glaucoma management, does not provide clear diagnostic information regarding sympathetic activity; it can cause miosis instead. Tropicamide is another anticholinergic used primarily for pupil dilation during examinations but does not have specific diagnostic capabilities for identifying Horner’s syndrome. Thus, apraclonidine stands out as the most effective medication for this purpose.