Transient ischemic attack in a patient with absent carotid circulation and dilated vertebrobasilar vessels.

Internal carotid artery (ICA) agenesis is a rare vascular anomaly that, in combination with occlusion of the contralateral ICA, produces significant strain on the posterior circulation for collateral flow. We report a patient who presented with transient ischemic attack and was found to have congenital ICA agenesis and contralateral ICA occlusion with cerebral perfusion maintained through dilated vertebrobasilar circulation.