Variations of the double superior mesenteric vein are not rare: An observational study using computed tomography, three-dimensional image reconstruction, and surgery.

INTRODUCTION: Few studies have evaluated variations of the main trunk of the superior mesenteric vessels. Particularly, the double superior mesenteric vein (DSMV) has not been described in detail. This study aimed to establish the definition, anatomical characteristics, and underlying clinical significance of the DSMV. MATERIALS AND METHODS: A total of 115 patients with colorectal cancer were included in this retrospective study between March 2020 and March 2022. The anatomical characteristics were analyzed using computed tomography, three-dimensional image reconstructions, and surgical videos. RESULTS: Among the patients enrolled, 22 (19.1%) had DSMVs. The median diameters of the right and left superior mesenteric veins were similar. The superior mesenteric artery was sandwiched between the right and the left superior mesenteric veins. The left superior mesenteric vein mainly crossed the ventral side of the superior mesenteric artery (63.6%). In 1 case, the right superior mesenteric vein was mistakenly resected intraoperatively. The DSMV was classified into types I and II based on whether the right and left trunks formed a common trunk; it was further classified into subtypes a and b based on the colonic vein confluence. The proportions of type I-a, I-b, II-a, and II-b were 4.5%, 27.3%, 9.1%, and 59.1%, respectively. The middle colic veins drained into the left superior mesenteric vein in 19 cases (86.4%). CONCLUSIONS: The DSMV is more common than previously thought. For the first time, the definition and four types of the DSMV were proposed. The presence of a DSMV should be considered during right hemicolectomies.