How does Apraclonidine help in diagnosing Horner's syndrome?

The correct answer highlights that Apraclonidine does not affect pupils in healthy eyes. This property is essential when diagnosing Horner's syndrome because, in this condition, the affected eye has a sympathetic nerve deficit, leading to a characteristic ptosis and miosis.

When Apraclonidine, an alpha-2 adrenergic agonist, is administered, it stimulates the alpha-2 receptors in the eye. In healthy eyes with normal sympathetic tone, there will be no significant pupillary response as there is no underlying sympathetic deficiency to reveal. Conversely, in the eye affected by Horner's syndrome, the administered Apraclonidine leads to a rebound dilation due to the loss of sympathetic tone, as the drug temporarily overcomes the parasympathetic predominance.

This contrast in reactions between the affected and unaffected eye helps to confirm the diagnosis of Horner's syndrome. Therefore, the lack of effect on the pupil in healthy eyes is critical for accurately identifying the syndrome.