Small bowel obstruction

Last revised by Daniel J Bell on 23 Mar 2024

Small bowel obstruction (SBO) accounts for 80% of all mechanical intestinal obstruction, and the remaining 20% results from a large bowel obstruction. It has a mortality rate of ~5%.

Bowel obstruction may be complete or incomplete 8:

  • complete or high grade obstruction means that no fluid or gas can get beyond the obstruction

  • incomplete, low grade or partial obstruction means that fluid or gas are still able to proceed

The classical presentation is cramping abdominal pain and abdominal distension with nausea and vomiting. Radiographic findings can be evident 6-12 hours before the onset of clinical symptoms 9.

Causes can be divided into congenital and acquired. Acquired causes may be extrinsic causing compression, intrinsic, or luminal.

In developed countries, adhesions are by far the most common cause, accounting for ~75% of obstructions while in developing countries incarcerated hernias are much more common accounting for 80% of obstructions 3.

  • fibrous adhesions

    • the main cause in developed countries (75% of cases)

    • almost all are related to post-operative adhesions with a small percentage secondary to peritonitis

    • diagnosis of exclusion as adhesive bands are not seen on CT

    • an abrupt change in caliber without mass lesion, inflammation or bowel wall thickening at the transition point

  • abdominal hernia

    • 10% of cases in developed countries

    • external hernia related to abdominal or pelvic wall defect (congenital weakness or previous surgery)

    • internal hernia with protrusion of viscera through peritoneum or mesentery into another abdominal compartment

  • endometriosis

    • a rare cause of SBO

    • endometrial implants are typically on the anti-mesenteric edge of the bowel

    • solid enhancing nodule contiguous with or penetrating the thickened bowel wall

    • may infiltrate the submucosa with a hypoattenuating layer between the muscularis and mucosa

  • masses

  • inflammation, e.g. Crohn, tuberculosis, eosinophilic gastroenteritis

    • small bowel obstruction in Crohn disease may relate to:

      • acute flare with luminal narrowing secondary to transmural inflammation

      • cicatricial stenosis in long-standing disease

      • adhesions or incisional hernias from previous surgery

  • tumor (rare)

    • primary small bowel neoplasms are rare (<2% all GI malignancy) and usually advanced at the time of SBO.

    • small bowel involvement by metastatic disease is more common

    • cecal malignancy involving ileocecal valve

  • radiation enteritis

    • produces adhesive and fibrotic changes in the mesentery with luminal narrowing and dysmotility

    • may obstruct the late phase (>1 year after therapy)

  • intestinal ischemia

    • occlusion or stenosis of the mesenteric arterial or vascular supply

    • produces small bowel wall thickening and obstruction

    • pneumatosis and portal venous gas if advanced

  • intramural hematoma

    • trauma, iatrogenic, anticoagulant therapy, Henoch-Schonlein purpura

    • produces luminal narrowing

    • better seen on non-enhanced CT with homogenous, regular and spontaneously hyper-attenuating wall

  • intussusception

    • rare in adults (<5% of SBO)

    • a lead point may relate to neoplasm, adhesion or foreign body

    • bowel-within-bowel with or without mesenteric fat and mesenteric vessels

    • leading mass should be carefully interpreted and differentiated from the soft-tissue pseudotumor that represents the intussusception itself

  • swallowed, e.g. foreign body, bezoar

  • gallstone ileus

    • a rare complication of recurrent cholecystitis

    • biliary-intestinal fistula with impaction of a gallstone in the small bowel

  • meconium ileus (or meconium ileus equivalent, distal intestinal obstruction syndrome)

  • migration of a gastric balloon

Abdominal radiographs are only 50-60% sensitive for small bowel obstruction 3. In most cases, the abdominal radiograph will have the following features:

  • dilated loops of small bowel proximal to the obstruction (see 3-6-9 rule)

  • predominantly central dilated loops

  • three instances of dilatation > 2.5 - 3 cm ref required

  • valvulae conniventes are visible

  • gas-fluid levels if the study is erect, especially suspicious if 8

    • >2.5 cm in width

    • in the same loop of the bowel but at different heights (> 2 cm difference in height)

However, obstruction (which may be high-grade mechanical obstruction) may also present with the following features:

  • gasless abdomen: gas within the small bowel is a function of vomiting, NG tube placement and level of obstruction

  • string-of-beads sign: small pockets of gas within a fluid-filled small bowel

Bedside tests help to diagnose small bowel obstruction, findings suggestive of small bowel obstruction 7:

  • dilated bowel loop (diameter > 3 cm)

  • ineffective peristalsis

    • results in "to-and-fro" or "whirling" appearance of intraluminal contents

  • prominence of the valvulae conniventes

    • present in dilated jejunal loops

The extent of obstruction is typically implied rather than sought directly based on the involvement of the ascending/descending colon, the morphology of the small bowel loops (high mucosal folds pattern present in the jejunum, absent in the ileum), and involvement of the stomach.

​Findings suggestive of bowel ischemia/infarction (will need urgent surgical evaluation): 

  • extraluminal free fluid

    • the "pointy" triangular appearance of interloop free fluid is sometimes referred to as the tanga sign

  • loss of peristalsis

  • bowel wall thickening >3 mm

    • with effacement of mural architecture

  • mural gas

CT is more sensitive than radiographs and will demonstrate the cause in ~80% of cases 3. Features on CT may include:

  • dilated small bowel loops >2.5 cm up from outer wall to outer wall

  • normal caliber or collapsed loops distally

  • small bowel feces sign

Closed-loop obstructions are diagnosed when a bowel loop of variable length is occluded at two adjacent points along its course. May be partial or complete with characteristic features:

  • radial distribution of several dilated, fluid-filled bowel loops

  • stretching of prominent mesenteric vessels converging towards the point of torsion

  • U-shaped or C-shaped configuration

  • beak sign at the site of fusiform tapering

  • whirl sign reflecting rotation of bowel loops around a fixed point

Strangulation is defined as closed-loop obstruction associated with intestinal ischemia. Mainly seen when the diagnosis is delayed (up to 10% of small bowel obstructions) and associated with high mortality. Features are non-specific and include:

Positive oral contrast is not usually necessary for the diagnosis of small bowel obstruction 4:

  • usually, becomes dilute in the setting of SBO and does not usually reach the transition point before the scan occurs

  • may obscure the evaluation of the small bowel wall, limiting evaluation of bowel ischemia

In cases of adhesional small bowel obstruction with no complications such as ischemia or perforation, a water-soluble contrast challenge may be administered. This has both a diagnostic and therapeutic effect 5.

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