Small Bowel Obstruction (SBO)

As you will recall, the upper limits of normal for the diameter of the small bowel lumen is 3 cm. When the luminal diameter is larger than this, it is DILATED. The differential diagnosis for a dilated bowel lumen is mechanical small bowel obstruction vs. adynamic ileus.

As the name implies, SBO occurs as result of a physically obstructing lesion. Ninety percent (90%) of SBO are caused by adhesions or hernias. Less common causes include volvulus, intussusception, and bowel wall lesions such as masses or strictures. 

SBO from adhesions as a result of abdominal surgeries and should be suspected in patients presenting with signs and symptoms of SBO with a history of previus abdominal surgeries. Adhesions may cause partial or complete obstruction and may spontaneously resolve.

Hernias may be external or internal. External hernias are protrusions of bowel through defects in structures external to the peritoneal cavity (e.g. inguinal, femoral, incisional). Internal are protrusions of bowel through intraperitoneal defects (e.g. paraduodenal, paracecal). Incarceration of a hernia confers that the hernia is not reducible. Strangulation of a hernia confers that its blood supply has been cut off. Incarcerated hernias are at risk for strangulation.

Small bowel volvulus involves the torsion of bowel around its mesentery. Small bowel volvulus is much less common in adults than its counterparts, cecal and sigmoid volvulus. Primary volvulus is seen in children with midgut malrotation. Secondary volvulus is seen in adults and associated with adhesions, internal hernias, and tumors. Volvulus can cut off the blood supply, leading to mesenteric ischemia or infarction.

Intussusception is the telescoping of bowel to form inner loop (intussusceptum) and outer loop (intussuscipiens). It is much more common in young children, usually idiopathic in nature. In adults, the lead point is often a mass.

Plain films are often that first studies ordered to evaluate for SBO. The hallmark of obstruction on plain films (obviously) is dilated loops of bowel. To distinguish SBO from large bowel obstruction, the dilated loops are usually centrally located with valvulae conniventes present. On supine images, SBO often demonstrates a “stepladder” pattern of dilated loops of bowel that appear to be stacked on one another. Variable amount of gas is seen in the colon depending on the severity and duration of obstruction. A gasless abdomen can also be seen if distended loops are fluid-filled. Erect or lateral decubitus films demonstrate air-fluid levels. If the loops are predominantly filled with fluid, a “string of pearls” can be seen, representing small collections of air trapped between valvulae.

It can be difficult (if not impossible) to distinguish SBO from ileus on plain films, as both present with dilated loops of bowel and air-fluid levels. However, in contrast to ileus, SBO tends to have differential air-fluid levels (air-fluid levels at different levels); whereas, ileus tends to have air-fluid levels at the same height.

CT is a valuable tool in the evaluation of SBO.  CT detects intramural or extraintestinal causes of SBO, such as hernias, masses, etc. CT assists in determining the site of obstruction.  The site of obstruction is often heralded by a transition zone, in which the caliber of the bowel wall transforms from dilated to decompressed. On CT, hernias be identified as abnormally located loops of small bowel. CT also helps to define other hernia contents (e.g. omentum, colon). In volvulus, CT may demonstrate radially distributed dilated small bowel around twisted, edematous mesentery. Intussusceptions present on CT as  edematous bowel walls with a “bull’s eye” appearance from the layering of bowel walls.

Upright abdominal radiograph demonstrates air-fluid levels and small bowel dilatation. Supine abdominal plain film demonstrates dilated loops of small bowel.