Internal carotid artery agenesis: A case report and review of literature.

Congenital agenesis of the internal carotid artery is a very rare anomaly. Intracranial circulation in the involved internal carotid artery territory is maintained by collateral circulation from the contralateral internal carotid artery through the anterior communicating artery and from the vertebrobasilar system through the posterior communicating artery. Usually, patients with internal carotid artery agenesis are asymptomatic due to collateral circulation, but they may present with headache, seizures, or transient ischemic attack. Aneurysms have also been reported to be associated with this entity. Computed tomography angiography, magnetic resonance angiography, and digital subtraction angiography are commonly used modalities to make the diagnosis. Unenhanced skull base computed tomography will show the absence of carotid canal, thus differentiating from carotid hypoplasia. We report on a case of right internal carotid artery agenesis with discussion of embryogenesis, clinical presentation, and imaging findings.