Internal carotid artery agenesis: A narrative review.

Background: Congenital anomalies of the internal carotid artery (ICA), including hypoplasia, aplasia, and agenesis, are rare vascular conditions with a prevalence below 0.01%. Most patients remain asymptomatic because of collateral cerebral circulation, yet these anomalies may cause cerebrovascular complications, particularly ischemic events and intracranial aneurysm formation. Methods: A narrative literature review was performed to synthesize available data on demographics, embryology, pathophysiology, clinical presentation, imaging evaluation, classification systems, management, and outcomes of congenital ICA anomalies. Results: ICA agenesis is commonly detected incidentally, especially in unilateral cases with adequate collateral pathways. Symptomatic cases, more frequent in bilateral involvement, may present with headache, transient ischemic attack, stroke, or impaired consciousness. Risk factors include bilateral absence, incomplete collateral circulation, and classification patterns such as Lie types C and D and Jamous type 1b, associated with increased hemodynamic stress and aneurysm formation. Computed tomography (CT) angiography, magnetic resonance angiography, and digital subtraction angiography are central to diagnosis, whereas skull base CT distinguishes agenesis from hypoplasia. Management is usually conservative in asymptomatic patients, whereas surgical or endovascular treatment is reserved for aneurysms or hemodynamic compromise. Conclusion: ICA agenesis is a rare but clinically important anomaly. Recognition of variants, imaging features, and complications is essential for diagnosis and procedural planning. Surveillance imaging and individualized management help reduce cerebrovascular morbidity.