Apraclonidine test for Horner syndrome. A, A patient with suspected oculosympathetic defect (ptosis and miosis) on the right side. B, Following instillation of topical apraclonidine in both eyes, the right pupil has dilated and the anisocoria is now reversed, confirming Horner syndrome on the right side. Note also the resolution of lid ptosis, even to the point of retraction on the right side.