Abstract: Background: The dissection of the internal carotid artery (ICA) is commonly associated with miosis in Bernard-Horner syndrome (BHS). The presence of mydriasis is exceptional but can occur in the context of Pourfour du Petit syndrome (PDPS), a rare entity opposite of BHS accompanied by eyelid retraction and hyperhidrosis and caused by hyperactivity of the sympathetic cervical chain. Aim: To report on a case of PDPS as the first manifestation of an ICA dissection. Method: A 54-year-old man presented with isolated left mydriasis with no other abnormalities in the examination. Six months later, he suffered an ischemic stroke in the left middle cerebral artery territory secondary to a left ICA dissection. Results: The initial study with Intracranial computed tomographic angiography and brain magnetic resonance imaging ruled out compressive cause of the third cranial nerve or structural lesion in the midbrain. The absence of hypersensitivity to Pilocarpine discarded postganglionic parasympathetic involvement. Conclusions: In the presence of unilateral mydriasis and once common causes are ruled out an imaging examination of the supra-aortic trunks should be completed, since it could represent the first sign of carotid pathology in the context of PDPS.

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