BACKGROUND: Agenesis of one or both internal carotid arteries (ICA) is usually a benign congenital anomaly. Many patients are diagnosed incidentally and remain asymptomatic, however associated cerebral aneurysms can be life-threatening and result in high morbidity and mortality rates in this population.
MATERIAL & METHODS: Based on the timing of ICA agenesis, during the fetal life, we classified the collateral blood flow pattern into three major types; type I: Collateral blood flow via primitive vessels, type II: Collateral blood flow via ICA branches, and type III: collateral blood flow via branches of the external carotid artery (ECA). The type of collateral blood flow pattern in the reported patients with ICA agenesis and cerebral aneurysm was reviewed in order to determine the relation between the type of collateral blood flow and the development of CAs.
RESULTS: Twenty nine patients with ICA agenesis and CA were reported, of these 27 patients (93%) were found to have type II collateral blood flow, 2 patients (7%) had type I collateral blood flow pattern and none had type III collateral flow pattern. The majority of patients (79%) with CA secondary to ICA agenesis presented with subarachinoid hemorrhage (SAH).
CONCLUSION: Although ICA agenesis is a symptomatic in the majority of cases, it must be recognized because it promotes the development of CA and SAH. Our new classification system for collateral blood flow patterns is easy to use and can predict those at high risk to develop cerebral aneurysms and SAH, and therefore, need annual screening MRA.