Internal carotid artery agenesis and basilar artery aneurysm with third nerve palsy.

Agenesis of the internal carotid artery is an extremely rare vascular anomaly, occurring in less than 0.01% of the population. In most patients, this vascular anomaly is asymptomatic because the contralateral internal carotid artery, external carotid artery, or vertebrobasilar artery systems provide a sufficient arterial collateral blood supply. Carotid agenesis is evident in adults more often than in pediatric patients. We report on a 2-month-old baby with unilateral agenesis of the right internal carotid artery, basilar artery aneurysm, and right third cranial nerve palsy. These abnormalities were diagnosed incidentally using computed tomography, magnetic resonance imaging, and magnetic resonance angiogram as part of an evaluation for suspected encephalitis. According to our review of the literature, this report is the first to describe unilateral agenesis of the carotid artery presenting with ipsilateral oculomotor nerve palsy.