[Agenesis of the internal carotid artery with an unusual collateral pathway].

A 65-year-old male developed severe headache of sudden onset 3 months prior to admission. The right retrograde brachial angiogram revealed that the right internal carotid artery was absent. The extremely dilated external carotid artery on the right side continued to the extracranial portion of the right internal carotid artery. No other collateral pathways were found. From an embryological point of view, it is speculated that the aortic arch III might have been mal-developed and the distal portion of the dorsal aorta might have received a collateral circulation via a persistent aortic arch I or II.