Internal hernia is an uncommon cause of small bowel obstruction that may be increasing in frequency. Because the clinical diagnosis of internal hernia is difficult, imaging studies such as computed tomography (CT) and small bowel follow through play an important role. Transmesenteric hernia is the most common type and is usually related to prior abdominal surgery, especially with creation of a Roux-en-Y anastomosis (eg, liver transplantation, gastric bypass). CT may allow confident diagnosis in most cases.

When the small bowel herniates through a defect in the mesentery or omentum, the herniated bowel is compressed against the abdominal wall, with no overlying omental fat in most cases and at most levels of anatomic section through the herniated bowel. The herniated bowel tends to appear clustered and lies outside the colon, a reversal of the normal anatomic arrangement. As a result, the adjacent colon is displaced centrally (eg, transverse colon displaced dorsally, ascending colon displaced medially). There will be some degree of compression, crowding, displacement, and obstruction of both the bowel and blood vessels.

The herniated bowel may also twist within the hernia sac, which results in volvulus and a predisposition to bowel ischemia. Twisting of the mesenteric vessels, or the whirl sign, or twisting of the bowel itself is diagnostic of volvulus, and engorged blood vessels, mesenteric ascites, and bowel wall thickening suggest bowel ischemia.

CT signs most suggestive of transmesenteric hernia are those that depict the abnormal cluster of bowel loops along the periphery of the peritoneal cavity and lack of omental fat covering the clustered small bowel.