Destruction and degeneration of preganglionic or postganglionic autonomic axons is followed by greatly increased sensitivity of the denervated autonomic effectors to their specific neurotransmitters and to pharmacological agonists.1 Because of this principle of denervation sensitivity, Horner syndrome produced by a lesion interrupting the postganglionic fibres should dilate the pupil when 1% phenylephrine is placed in the conjunctival sac.2 We report two cases that show that denervation supersensitivity to 1% phenylephrine testing in postganglionic Horner syndrome is not detectable at 3 days, but can be detected 10 days after the onset of symptoms.

A 34-year-old man noted a right upper lid ptosis associated with ipsilateral headache and jaw pain after swimming. He was seen in our department 9 days after the onset of symptoms. …