An unusual variant of the anterior cerebral and anterior communicating arterial system: anatomical description with clinical and radiological implications.

Anatomical variations of the circle of Willis are frequently encountered; however, certain configurations may have significant clinical and hemodynamic implications. Variations involving the anterior cerebral artery and anterior communicating artery are particularly relevant due to their role in collateral circulation and their importance during surgical and endovascular interventions. A previously undescribed configuration of the anterior cerebral circulation was identified in a 69-year-old woman who presented with a transient ischemic attack. Magnetic resonance angiography demonstrated complete absence of the left A1 segment and the anterior communicating artery, with a single right A2 segment supplying the medial surfaces of both cerebral hemispheres. This configuration was accompanied by a left-sided fetal-type posterior cerebral artery. No significant stenosis was identified in the carotid or vertebrobasilar systems. This unique arterial arrangement does not correspond to currently available classification systems of anterior cerebral artery and anterior communicating artery variations. From a clinical perspective, reliance on a single A2 segment may limit collateral capacity and increase vulnerability to ischemia affecting bilateral medial frontal regions. Awareness of such variants is essential for accurate radiological interpretation and for informed procedural planning in neurosurgical or endovascular interventions involving the anterior circulation.