Congenital Absence of the Right A1 Segment and the Anterior Communicating Artery: A Previously Unreported Variant of the Circle of Willis

Background Variations of the Circle of Willis are common, but the combination of complete agenesis of a unilateral anterior cerebral artery A1 segment and absence of the anterior communicating artery (AComA) is exceptionally rare. Recognition of such patterns is essential for accurate radiologic interpretation and neurosurgical planning. Methods We describe a congenital CoW variant detected by brain MRI and TOF-MRA in an 18-year-old woman. Results Imaging demonstrated complete agenesis of the right A1 segment and total absence of the AComA. The left A1 was dominant and supplied both anterior hemispheric territories. No aneurysms, arteriovenous malformations, or stenoses were present. The patient remains neurologically asymptomatic at long-term follow-up. Conclusion This configuration complete unilateral A1 agenesis together with total absence of the AComA has not been previously described in contemporary angiographic literature. Awareness of this variant is important for preventing misinterpretation as arterial occlusion and for planning endovascular or surgical procedures.