A Rare Variation of the Inferior Mesenteric Vein with Clinical Implications

The portal venous system has unique blood drainage from the gastrointestinal tract, spleen, pancreas and gallbladder to the liver. It consists of 4 large veins: the hepatic portal, splenic (SV), superior mesenteric (SMV) and inferior mesenteric (IMV). The SMV collects the venous return from the small intestine tributaries, ileocolic, right colic, middle colic and marginal veins (MarV). The IMV receives the blood from the superior rectal, sigmoid and left colic veins. However, anatomic variations of the portal venous system have occasionally been noted. During our routine gross dissection over the past years, we have found several common variations of the IMV drainage. For example, the IMV drains into the SMV or confluence of the SMV and SV, as opposed to directly into the SV in most cases. However, this is the first time that the IMV is found to drain into the MarV, one of the SMV tributaries. Besides, the IMV tapers gradually along the ascending course to accommodate its termination at the MarV. This type of uncommon variation could add complications to the surgical procedures involving the IMV. Knowledge of the variations of IMV anatomy helps surgeons to choose an appropriate approach to minimize the likelihood of colorectal ischemia resulting from surgery.