Variations or anomalies in the IMA anatomy are rare. In the report by Kakihara et al., abdominal angiography in 3182 patients revealed occasional IMA bifurcation anomalies, consisting of a defect in one patient (0.03%), running anomaly due to visceral retroversion in two patients (0.06%), and bifurcation from the root of the SMA in two patients (0.06%) [5]. Similarly, studies by Kondo et al. and Minamizawa et al. reported presence of abnormality, in which IMA branched from SMA, in 0.07% (one among 1516 cases) and 0.05% of the primary resection cases of colorectal cancer, respectively [7, 8].
Using “variation” and “inferior mesenteric artery” as search keywords, we identified six papers describing anatomical abnormal branching of the root of IMA in autopsy cases reported on PubMed from 1968 to 2021 [9,10,11,12,13,14]. However, we found only case report in the English literature for colorectal cancer cases wherein IMA arose from the SMA. Okada et al. focused his report on the arterial anatomy, therefore, the details of the surgery for rectal cancer were unknown [14]. Using a Japanese literature search in “Igaku Chuo Zasshi” (Central Journal of Medicine), we found five cases, including our case (Fig. 6) [5, 7, 8, 14, 15]. In most cases, the superior margin of lymphadenectomy was set at the inferior border of the duodenum at the position where the IMA normally originates from the abdominal aorta or at the plexus of the colic branch of the lumbar splanchnic nerve. In this case, as in the case reported by Kakihara et al., Japanese D3 lymphadenectomy was performed by ligating the root of the IMA.
It has also been reported that preoperative 3D-CT is informative and helpful for surgeons to assess the vascular anatomy, especially the vascular variability around the SMA [14, 16, 17]. Bifurcation abnormalities are uncommon with IMA, however, preoperative confirmation of anatomy of IMA using 3D-CT is useful information for surgeons in left-sided colorectal cancer surgery. In our department, 3D-CT is performed in all cases of colorectal cancer; therefore, bifurcation abnormality of the IMA could be identified preoperatively in this case, and the surgery could be performed safely without intraoperative complications such as vascular injury.
ICG fluorescence imaging has been widely used to evaluate the blood and lymphatic flow for colorectal cancer surgery [18,19,20,21]. The mechanism of sometimes selective illumination of lymph nodes is unclear [22]; however, in the present case, the ICG-fluorescence-illuminated lymphatic flow was observed to end at the position where the IMA normally originates from the abdominal aorta. In this case, the application of the latest equipment of a laparoscopic near-infrared camera system helped us perform complete CME as well as Japanese D3 lymphadenectomy.
Tandler [23, 24] reported that during arterial development of abdominal organs in the fetus measuring 5 mm, several primitive mesenteric arteries arise segmentally from the dorsal aorta and supply their respective primordia. In the early stages of development, longitudinal anastomoses connect the arteries vertically in the dorsal mesentery [23, 24]. However, if abnormal partial disappearance of the primitive mesenteric artery and longitudinal anastomosis occurs, branching abnormalities such as that seen in the present case may occur. The arc of Riolan, a vessel assumed to connect the proximal segments of the MCA and left colic artery has an incidence of 8–10% [25]. It is not clear if this longitudinal anastomosis connecting the SMA and IMA is the same as the arc of Riolan; however, we believe that the bifurcation abnormality of the IMA in this case is unrelated to the arc of Riolan based on the vascular morphology. In this case, only the colic branch from the lumbar splanchnic nerve was found in the position where the IMA normally branches from the abdominal aorta, suggesting that the disappearance of the primitive mesenteric artery of the hindgut during development may have caused the abnormal IMA bifurcation, as discussed by Tandler.
Based on the above, we suspected that the IMA originated from the abdominal aorta and was accompanied by lymphatics during fetal life; however, the root of the IMA disappeared during the developmental process, and lymphatics ascending along the IMA persisted even after the disappearance of the root of the IMA and did not get redirected towards the SMA. Although the extent of lymphadenectomy is not established in such cases, ligation at the bifurcation of IMA was considered to be oncologically appropriate in this case. Since surgical cases of colorectal cancer with abnormal branching of the IMA are rare, further studies are required to validate these findings.