Abstract
Pourfour du Petit syndrome is an uncommon cause of eyelid retraction, associated with unilateral mydriasis and hemifacial hyperhidrosis. This syndrome is caused by hyperactivity of the ipsilateral oculosympathetic pathway and needs to be recognized because it has an opposite clinical presentation but the same topographic and diagnostic value as Horner syndrome. The authors report a rare case of Pourfour du Petit syndrome associated with cluster headache and discuss pathophysiological hypotheses, clinical presentation, complementary exams, pharmacologic testing, treatment options, and prognosis. Early detection of these symptoms may lead to swift diagnosis and treatment.