Phenylephrine test for Horner syndrome, used to help localize the sympathetic lesion.
Typically 1% phenylephrine ophthalmic is placed in both eyes and you watch for resolution of the miosis, ptosis, and third-eyelid elevation.
The principle is denervation hypersensitivity: with a postganglionic (third-order) Horner lesion, the affected eye becomes supersensitive to adrenergic stimulation, so dilute phenylephrine produces relatively rapid mydriasis and improvement of the other signs—often within roughly 20 minutes.
Historically, more dilute phenylephrine (e.g. 0.1%) has been used specifically to demonstrate denervation hypersensitivity, while higher concentrations are less useful for precise localization because they can eventually dilate a normal eye too.
So if you’re looking at that dog and thinking elevated nictitans + possible Horner, that’s the test you’re remembering.
