Variation of the Inferior Mesenteric Vein’s Drainage Pattern and its Clinical Significance

The anatomical joining and drainage of the inferior mesenteric vein (IMV) into the splenic vein are usually reported in the anatomical literature. Nevertheless, the joining and drainage of IMV into the superior mesenteric vein or the junction between the splenic vein and the superior mesenteric vein are also possible. During routine dissections of the abdomen for anatomy education in the Department of Anatomy Laboratory in the Faculty of Medicine at the Gazi University, a variaton of IMV was observed in a 66-year-old male cadaver. It is certainly useful for surgeons, as well as interventional radiologists, to be informed about mesenteric venous variations.


INTRODUCTION
The inferior mesenteric vein (IMV) has a number of functions. For instance, it performs the venous drainage of the rectum, sigmoid and descending colon segments (1)(2)(3)(4). In the embryological period, the portal vein occurs in the second month of pregnancy. The right and left vitellin veins develop as parts of the hepatic veins, inferior vena cava, ductus venosus, sinusoids, superior mesenteric vein, inferior mesenteric vein, and also the splenic vein. Eventually, the inferior mesenteric vein may participate the superior mesenteric vein, splenic vein, or splenomesenteric junction(5). Significant anastomoses from left side to right side, between vitelline veins and caudal to the liver are reconstitute to the distal point of the PV by way of two veins: IMV and SV (6,7).
The origin of the IMV is located at the level of the anal canal in the form the superior rectal vein with the junction to inferior and middle rectal veins surrounding the rectal plexus (1, 2). As the superior rectal vein exits the pelvis, it continues adjacent to the superior rectal artery medial to the left ureter and crosses the left common iliac vessels then extends as IMV cranially.
It crosses superiorly on the anterior aspect of the psoas major muscle which is situated laterally to the neighboring artery. It may cross the testicular or ovarian arteries or continue medial to them. Moreover, its joining the splenic vein back to the body of the pancreas higher than the level of the duodenojejunal junction is possible in almost all of the cases (1,2,8). The pattern of the IMV venous drainage may vary (4,9,10). The joining and drainage of the IMV into the splenic vein were traditionally reported in the anatomy textbooks (4,(11)(12)(13)(14). In this study, we aimed to draw attention to the IMV variation we encounter during routine cadaver dissection and the clinical importance of this variation.

CASE REPORT
A variation of IMV was detected in a 66-year-old male cadaver while performing routine dissections of the abdomen in the laboratory of the Department of Anatomy, Faculty of Medicine at Gazi University for the purpose of anatomical education. As seen in figure 1, IMV drained into the superior mesenteric vein instead of splenic vein.

DISCUSSION
The joining and drainage of the IMV into the splenic vein were traditionally reported in the anatomy textbooks (4,(11)(12)(13)(14). The termination of the IMV may vary depending on autopsy series. For instance, its termination at the junction of the splenic and the superior mesenteric veins or its drainage into the superior mesenteric vein are possible (1,14). In the study conducted by Krumm et al., 916 computed tomography scans of the abdomen were examined in order to record anatomical variations of the IMV. In this study, the drainage of the IMV into the splenic vein was reported in about 40% of the cases, into the portal confluence in about 30% of the cases, and into the superior mesenteric vein in about 20% of the cases(15). Graf et al. Stated that 54 cases who have undergone helical computed tomography venography of the pancreas for mesenteric anatomical variants, and the drainage of the IMV into the splenic vein was detected in 56% of the patients, into the superior mesenteric vein in 26% of the patients, and the splenomesenteric angle in 18% of the patients(1). In a study conducted in order to assess mesenteric venous patterns in 102 cases who have undergone multidetector row computed tomography, the IMV's joining the splenic vein was reported in 68.5% of the patients, the superior mesenteric vein in 18.5% of the patients, and the splenoportal confluence in 7.6% of the patients (16). In the present study, IMV drained into the superior mesenteric vein.
Drainage of the inferior mesenteric vein to superior mesenteric vein near to the formation of portal vein decrease the blood content of splenic vein and therefore possibly a healthier condition for the formation of portal vein from the point of portal blood pressure. Bleeding is periodically detected by surgeons during the Access to the peripancreatic head region. The cut of small portal veins caused by incorrect traction is the reason for this. However, it would be possible to eliminate these problems in case the anatomical structure of the portal venous tributaries was studied better (17,18).
The inferior mesenteric vein (IMV) represents an essential branch of the portal venous system and it has a number of important functions. It is periodically used for the purpose of portal decompression in portosystemic shunt operation or venovenous bypass when orthotropic hepatic transplantation is performed (20)(21)(22). Previous studies have reported that the IMV has been occasionally used for the purpose of portal decompression (22). Furthermore, it was reported that inferior mesenteric vein ligation mitigated intractable bleeding from anorectal varices (23).

CONCLUSION
In open surgery and laparoscopic surgery to the liver, pancreas, intestines and other abdominal organs, surgeons should be aware of such variations for safer surgical manipulations. Therefore, the existence of such variations should always be taken into account. GMJ 2021; 32: 436-438 Coşkun et al.