Abstract
<title>Abstract</title> <p> <bold>Purpose.</bold> The azygos system, vital for thoracic venous drainage, significantly varies due to complex embryologic development. Predominantly, the azygos, hemiazygos, accessory hemiazygos veins, and their tributaries comprise this system. This case presents a previously unreported azygos system variation with clinical implications for thoracic imaging and surgery. <bold>Methods.</bold> Routine anatomical dissection was performed on a 61-year-old male cadaver donated through the Saint Louis University School of Medicine’s Gift Body Program. <bold>Results.</bold> A previously unreported horseshoe venous confluence, formed by a dual-column azygos venous system and associated with a retroaortic left brachiocephalic vein (LBCV), was identified between the fifth and sixth thoracic vertebral levels (TV5-TV6). Each azygos column received the fifth to eleventh posterior intercostal veins (PICVs) from its respective sides. The right azygos column received the first four PICVs via the right superior intercostal vein (SICV), which drained into the LBCV’s posterior side. The left SICV was absent; the first four left PICVs drained directly into the retroaortic LBCV. <bold>Conclusion.</bold> This horseshoe venous confluence, formed within a unique dual-column azygos system and associated with a retroaortic LBCV, presents important clinical implications for imaging, interpretation, surgical planning, and procedural safety. The retroaortic LBCV and horseshoe venous confluence may complicate central venous catheter placements and increase venous injury risk. Aortic aneurysms may compress this atypical venous pathway, potentially compromising cardiac hemodynamics and venous return. Awareness of this unprecedented venous configuration supports improved medical education and clinical practice by emphasizing the role of anatomical variations in enhancing patient safety and outcomes. </p>