Abstract 037: Internal carotid artey agenesis and posterior cerebral artery aneurysm, casd illustration

Introduction Agenesis of the internal carotid artery (ICA) constitutes a rare congenital anomaly, with a reported incidence of less than 0.01% in the general population. The condition was first described during a postmortem examination by Tode in 1787; it is characterized by the complete absence of the ICA, a feature that distinguishes it from aplasia, which involves a lack of development with vestiges of non‐patent vessels, and hypoplasia, defined as incomplete development with a reduced vascular lumen.Individuals with ICA agenesis demonstrate a significantly elevated incidence of cerebral aneurysms, ranging from 25% to 67%, in comparison to the 2%–4% observed in the general population. Subarachnoid hemorrhage (SAH) represents the most common clinical presentation, typically occurring as a consequence of the rupture of an associated intracranial aneurysm. The formation of these aneurysms is postulated to arise from either congenital factors or hemodynamic alterations resulting from abnormal blood flow patterns within collateral pathways. Material and Method A 63‐year‐old male presented to the emergency department with a sudden onset of severe headache and a Glasgow Coma Scale (GCS) score of 14/15. Computed tomography (CT) imaging of the brain revealed subarachnoid hemorrhage (SAH), predominantly within the left sylvian fissure, graded as Fisher grade 3 Figure 1A . Subsequent CT angiography (CTA) confirmed agenesis of the left internal carotid artery (ICA) and revealed a triple A2 configuration and a posterior communicating artery (PCA) aneurysm originating from thalamoperforating branches. Of note, imaging revealed a connecting artery between the anterior communicating artery (AComA) complex and the left posterior cerebral artery (PCA), indicative of a left anterior cerebral artery (ACA) to left PCA connection Figure 1B, 1C . The absence of the left carotid foramen further differentiated the agenesis from ICA hypoplasia or thrombosis.Given the patient's presentation with a ruptured PCA aneurysm, endovascular coil embolization was performed. The patient's post‐operative hospital course was uneventful, with complete resolution of symptoms. Follow‐up brain CTA at 6 months post‐intervention demonstrated no aneurysm recurrence or residual abnormalities. Result The term agenesis is used when the ICA is absent, complete failure of arterial development without the carotid canal at the skull base, whereas aplasia is a lack of development with presence of vestiges of non‐patent vessels and also of the carotid canal; and hypoplasia is an incomplete development of the ICA artery with a reduced but patent vascular lumen accompanied by a smaller but normally structured carotid canal. Conclusion Unilateral ICA agenesis is a rare condition with a high association with intracranial aneurysms. An understanding of collateral pathways, aneurysm location, and appropriate treatment strategies is essential for managing these complex cases. Figure 1. (A) Computed tomography (CT) imaging of the brain revealed subarachnoid hemorrhage (SAH), predominantly within the left sylvian fissure. (B & C) CT angiography (CTA) with 3D reconstruction confirmed agenesis of the left internal carotid artery (ICA) and revealed a triple A2 configuration a connecting artery between the anterior communicating artery (AComA) complex and the left posterior cerebral artery (PCA).