Which topical adrenergic agonist ocular medication can reverse anisocoria in a (+) Horner's syndrome?

In (+) Horner's syndrome, the affected pupil is typically smaller (miosis) due to a disruption in sympathetic innervation. Topical adrenergic agonists can help to differentiate between the potential causes of anisocoria and provide valuable diagnostic information.

Apraclonidine is particularly effective in this scenario because it has weak agonistic activity on alpha-adrenergic receptors and can cause a temporary dilation of the affected pupil in Horner's syndrome. This occurs because the normal sympathetic pathway is disrupted, and the administration of apraclonidine stimulates the adrenergic receptors in the smooth muscle of the pupil, leading to a relative increase in pupil size.

Cocaine may also be used in this context, as it prevents the reuptake of norepinephrine, leading to pupil dilation if the sympathetic nerve supply is intact. However, in the case of (+) Horner's syndrome, cocaine will not cause dilation and may even lead to pupil constriction in the affected eye.

Phenylephrine, a potent alpha-agonist, is more commonly used to dilate pupils and assess for sympathetic function. However, its effect would not differentiate Horner's syndrome since it relies on an intact sympathetic supply to elicit dilation in a