Missing A1 Unmasked During Surgery for Ruptured Anterior Communicating Artery (ACoA) Aneurysm: Navigating the Unexpected.

The infraoptic anterior cerebral artery (ACA) is a rare vascular anomaly that is often regarded as merely an anatomical curiosity. We report the case of a 54-year-old hypertensive man who presented with subarachnoid hemorrhage (World Federation of Neurological Surgeons (WFNS) Grade 4) due to rupture of a bilobed anterior communicating artery (ACoA) aneurysm. CT angiography confirmed the aneurysm, and the patient underwent right pterional craniotomy and clipping. Intraoperatively, the right A1 segment was absent in its usual supraoptic location and was instead found coursing beneath the optic nerve, consistent with a Wong type II infraoptic ACA. The aneurysm was successfully clipped with preservation of perforators, and postoperative angiography confirmed complete occlusion. This case highlights the importance of recognizing an infraoptic A1 variant in patients with a "missing" A1 segment on imaging or during ACoA aneurysm surgery, as failure to identify this anomaly may compromise surgical safety and completeness.