Intersigmoid hernia (ISH) is the herniation of the small bowel into the intersigmoid fossa . CT plays an important role in the diagnosis of internal hernias, including ISH. The diagnosis of internal hernia with CT images consists of identifying the sac-like appearance of the strangulated small intestine and the causative congenital structures on the surrounding vasculature and organs [8–11]. A sac-like appearance is a direct sign of a closed-loop obstruction. This means a fluid-filled, distended intestinal loop or a radial array of distended loops with stretched and thickened mesenteric vessels converging to a central point [8, 11]. The CT findings of ISH have already been reported as an entrapped small bowel that shows a sac-like appearance between the stretched sigmoid colon and the left psoas muscle [5, 6]. However, to the best of our knowledge, no studies have investigated the CT findings in multiple ISH cases.
In this study, we evaluated the CT findings of seven intersigmoid hernia cases and found the following findings in all cases: The small bowel (a) ran under the point that the inferior mesenteric vessels bifurcated to the superior rectal vessels and the sigmoid vessels (Fig. 3, 5a), (b) was strangulated at the anterior of the left ureter (Fig. 4, 5b), (c) formed a sac-like appearance (d) medial to the gonadal vein (Fig. 4, 5b), and (e) lay between the left psoas muscle and the sigmoid colon (Fig. 4, 5b). Among these findings, (c) and (e) are consistent with previous reports. Because the ISH is the only internal hernia that related to the intersigmoid fossa, the remaining findings (a, b, d) will be discussed based on the anatomy of the intersigmoid fossa.
The intersigmoid fossa (ISF) is present in 50–80% of autopsies with no difference between sexes . It is an inverted V-shaped cul-de-sac situated at the top of the two roots of the parietal brim of the sigmoid mesocolon and is formed by a defect of fusion between the mesentery and the parietal peritoneum, so its space is enclosed by both structures . The orifice points downward and slightly to the left [12, 13]. Some structures surround the orifice of ISF; dorsal to the orifice is either the left common iliac artery or two branches of bifurcation (left internal or external iliac artery) and ureter, and above it is the superior rectal artery and the sigmoid arteries . Somé et al. reported that the most frequently encountered structures during the dissection of the ISF fundus were the ureter, the common iliac artery, gonadal pedicle, and the left external iliac artery .
As for the finding in (a) and (b) above, it is consistent with the description that the ureter was situated dorsal to the orifice and the superior rectal artery and the sigmoid arteries above . As for (d), there is a description of the gonadal pedicle running backward to the ISF fundus , but we could not find any report of it running laterally to the ISF. This anatomical positioning is assumed to be related to embryological mechanisms, but we could not find any reports that discussed this.
Despite no sex differentiation in the presence of ISF , all patients were male. In females, the uterus may inhibit the protrusion of the small bowel into the ISF because of its position. The ISF is located near the ovarian fossa, which is surrounded posteriorly by the ureter and the internal iliac artery, superiorly by the external iliac artery . Therefore, the left ovary may prevent small bowel protrusion. These anatomical features may explain the sex differentiation of the presence of ISH in this study.
We evaluated the size of the ISF by measuring the diameter of the small bowel loop. In all cases, the diameter in the craniocaudal direction was the longest, and the diameters in the lateral and anterior-posterior directions were comparable. Chiarini et al.  and Somé et al.  reported the size of ISF in previous studies. Chiarini et al. reviewed 114 studies of ISH and reported that the mean size of ISF in 96 patients with ISH was 2.65 cm (SD 1.15 cm, range 1–10 cm). Somé et al. measured the size of orifice (transverse and longitudinal diameter, TD, and LD) and depth of fossa in 48 cases of cadavers and calculated the volume by applying the formula of the truncated cone volume (volume = TD × LD × depth × 1/3). They reported that 55.5% of cases were smaller than 4 cm3, and 13.9% were larger than 9 cm3. Our results tended to be larger than the findings of Chiarini et al and Somé et al. Differences in the measurement and calculation methods may explain the differences between our results and those of previous reports. It may also be a factor that the herniated bowel stretches the ISF, increasing its size in this study.
Regarding the orifice location, Testut described that the left common iliac artery is situated dorsal to the orifice . Whereas in all our cases, the orifice was located inferior to the bifurcation of the common iliac artery. Our cases may have a lower orifice position, and the lower position of the orifice may correlate to the risk of ISH.
Our study had several limitations. Although reconstructed images in two or more directions were obtained in all cases, the image acquisition protocol was inconsistent. This variety may not be a significant problem in assessing anatomical evaluation but may cause some bias. Although there are some cases that require resection of the small bowel due to ischemia , all our cases were treated by the reduction of the incarcerated hernia. This bias may be due to the small sample size. Further investigations with a larger sample size are required.