Case : 

Sac-like appearance of herniated small bowel between sigmoid colon and psoas muscle, showing closed-loop formation

Intersigmoid hernia 

Topic : Sigmoid mesocolon-related hernia

- The sigmoid mesocolon ; suspending the sigmoid colon from the posterior parietal peritoneum

- Sigmoid mesocolon-related hernias ; Three subtypes

1. Transmesosigmoid ; A transmesosigmoid hernia is defined as a herniation through a complete defect in both of the peritoneal layers of the sigmoid mesocolon, whereby no hernia sac is present.

2. Intramesosigmoid ; An intramesosigmoid hernia is a herniation into an abnormal defect in only one peritoneal layer, whereby the herniated viscera are trapped in the sigmoid mesocolon within a sac. 

3. Intersigmoid ; An intersigmoid hernia involves the intersigmoid fossa, a congenital retroperitoneal fossa that is located just above and behind the apex of the root of the sigmoid mesocolon. This fossa is formed during the fusion of the lower part of the left mesocolon and the posterior parietal peritoneum. Up to 70% of all bodies have this fossa, and when it is large enough, small bowel loops can herniate into it.

** Tips for differential diagnosis

- At CT, the hernia orifice can be observed between the sigmoid colon and the left psoas major muscle in all three subtypes

- Among them, the key to diagnosis of a transmesosigmoid hernia is absence of a saclike appearance. In contrast, intramesosigmoid and intersigmoid hernias show a saclike appearance, and CT findings of these two subtypes show substantial similarity, although the hernia orifices theoretically lie in different locations. 

- The sigmoid mesocolon contains the sigmoid arteries and veins and the superior rectal artery and vein. At multidetector CT, splaying of the sigmoid vessels, as if they are wrapping the hernia sac, may suggest an intramesosigmoid hernia.