Small bowel obstruction (SBO) refers to mechanical blockage of the transit of intestinal contents through the small bowel. CT in particular plays a key role in the diagnosis and can help identify the cause of obstruction and assess for potential complications.

Terminology

The degree of obstruction can be described as complete or incomplete (alternatively high-grade or partial) depending on whether enteric contents can pass beyond the site of obstruction. This is often inferred based on the degree of upstream small bowel dilation, the severity of narrowing at the transition point, and the passage of oral contrast (if administered).

Closed-loop small bowel obstruction occurs when the bowel is obstructed at two points, typically close to one another, entrapping enteric contents in a closed-loop segment and leading to progressive vascular compromise with a high risk of developing ischemia and infarction. The importance of this distinction is discussed in detail below. Non-closed-loop obstruction is also known as open-loop or simple obstruction ¹¹.

Epidemiology

The incidence of small bowel obstruction is variable depending on the etiology of the obstruction. The most common cause of SBO is adhesive disease, and the greatest risk factor for developing adhesions is prior surgery ¹.

Diagnosis

The diagnosis is usually made through CT and can be confirmed during laparotomy if surgery is required ^ref. Abdominal radiographs can serve as a first step toward diagnosing an obstruction but do not provide information about the cause, grade, or complications of the obstruction, and thus a CT is appropriate in most cases to guide management ^ref.

Clinical presentation

The clinical presentation of small bowel obstruction varies according to its etiology. Gradual partial obstruction typically manifests with intermittent abdominal discomfort, decreased bowel movements, and vomiting. In contrast, closed-loop obstruction presents more acutely, often with sudden onset of abdominal pain and signs of shock due to rapid vascular compromise. Vomiting may be absent in such cases, as the proximal bowel may not have had sufficient time to dilate before vascular compromise leads to symptoms of severe pain.

Pathology

Simple obstruction (non-closed loop)

A simple (a.k.a. open loop or non-closed loop) obstruction is defined by the presence of a single transition point causing obstruction. This type of obstruction can be further classified into two categories: (1) complete or high-grade obstruction, where no fluid or gas passes beyond the obstruction site, and (2) incomplete or partial obstruction, where some fluid or gas can still traverse the obstructed area ².

Extrinsic compression

• adhesions (most common) ^ref

• abdominopelvic hernia

• less common causes of extrinsic compression

  • serosal/peritoneal metastases: may extrinsically compress small bowel or cause abrupt kinking by limiting its mobility

  • inflammation secondary to peritoneal endometriosis, fistulas (e.g. fistulizing diverticulitis), abscess, or aneurysm may pull in and tether small bowel, causing kinking and obstruction by a similar mechanism to adhesive disease.

Primary bowel pathology

• inflammatory conditions

  • Crohn disease

  • tuberculosis

  • vasculitis

  • eosinophilic gastroenteritis

  • radiation enteritis

• malignancy

  • primary small bowel tumor, e.g. adenocarcinoma, neuroendocrine tumor (NET), gastrointestinal stromal tumor (GIST), lymphoma

  • small bowel metastases

    • hematogenous metastases (melanoma, lung, etc.)

    • serosal/peritoneal metastases (ovarian, colorectal, peritoneal mesothelioma, pseudomyxoma, etc.)

• intraluminal conditions

  • intussusception

  • gallstone ileus

  • bezoar

  • gastric balloon migration

  • ingested foreign body (e.g. fish bone, tooth pick, grill brush bristle)

Closed-loop obstruction

Closed-loop small bowel obstruction occurs when the bowel is obstructed at two points, typically close to one another, entrapping enteric contents in a closed-loop segment and leading to progressive vascular compromise with a high risk of developing ischemia and infarction. The most common causes are adhesions, hernia (internal and external), and volvulus. The etiology is discussed in detail in the dedicated closed-loop article.

Radiographic features

Plain radiograph

Abdominal radiographs have a sensitivity of ~55% (range 50-60%) for small bowel obstruction ³. In most cases, the abdominal radiograph will have the following features ^ref:

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

• predominantly central dilated loops

• multiple dilated loops of bowel

• valvulae conniventes are visible

• gas-fluid levels if the radiographic 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 ^ref:

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

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

CT

CT is more sensitive than radiographs and will demonstrate the cause in most cases. 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

• fat notch sign (typically indicates adhesive SBO) ¹¹

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

• asymmetric mesenteric edema involving the closed-loop segment

• stretching or sharp angulation/narrowing of the mesenteric vessels as they are distorted by the entrance/exit of the closed loop

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

• U-shaped or C-shaped configuration of the closed-loop, which often is best seen on the coronal reformats

• beak sign at the site of fusiform tapering, involving both ends of the closed loop

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

• intramural hemorrhage: high-density fluid within the wall of the affected small bowel is most prominent on non-contrast imaging

Strangulation refers to impaired blood flow (decreased perfusion) to the small bowel caused by obstruction, which can lead to bowel ischemia. Closed-loop obstructions are at particular risk for ischemia and infarction. This is mainly seen when the diagnosis is delayed (up to 10% of small bowel obstructions) and associated with high mortality ¹. Features are non-specific and include ^ref:

• thickened and increased attenuation of the bowel wall

• Increased attenuation of bowel contents due to hemorrhage 

• halo or target sign

• pneumatosis intestinalis

• portal venous gas

• localized fluid or hemorrhage in the mesentery

Positive oral contrast is not usually necessary for the diagnosis of small bowel obstructions as it tends to dilute in the setting of SBO, does not reach the transition point, and may obscure the evaluation of the bowel wall (thereby limiting the evaluation of bowel ischemia)  ⁴.

In uncomplicated cases of adhesional small bowel obstruction, where complications such as ischemia or perforation are absent, a water-soluble contrast challenge may be employed. This approach serves both diagnostic and therapeutic purposes ⁵.

MRI

MRI is not commonly used for the diagnosis of small bowel obstruction, but has a role for the evaluation of pregnant patients for whom ionizing radiation is contraindicated.

Ultrasound

Ultrasound is a bedside test that can help to diagnose small bowel obstruction. Findings suggestive of small bowel obstruction ⁶:

• 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 ^ref:

• 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

Radiology reporting

Key features to include in the radiology report:

• whether the obstruction is a simple, single-point obstruction or closed-loop

• location of obstruction and the appearance of the transition point

• number of points of obstruction

  • especially in obstruction secondary to serosal disease or small bowel tethering by infective or malignant disease

• favored etiology

• presence of complications (e.g. perforation or ischemia)

Treatment and prognosis

Non-operative treatment of small bowel obstruction usually consists of gut rest and decompression via a nasogastric tube. Parenteral fluids or nutrition are typically given in conjunction with this ^ref.

Water-soluble contrast challenges are used as a diagnostic and therapeutic tool for non-closed loop small bowel obstruction. Various protocols exist, but typically involve the administration of around 100 mL of water-soluble contrast orally or via a nasogastric tube, followed by abdominal radiographs 6-12 hours later to assess for passage of contrast into the colon ⁷. If contrast has not passed into the colon at 24 hours after administration, this suggests the obstruction is unlikely to resolve spontaneously and is a relative indication for surgery ^8-9. Traditionally Gastrografin was used, but other water-soluble agents are similarly effective ¹⁰.

Differential diagnosis

• ileus

  • clinically, it may present similarly with nausea, vomiting, and abdominal pain

  • air in the sigmoid colon or rectum is more likely to represent ileus

  • diffusely dilated loops of bowel without a transition point (including colon) are more likely to represent ileus

  • recent or concurrent trauma, surgery, or infectious/inflammatory process is more likely to represent ileus

  • fecal bowel sign is more likely to represent small bowel obstruction

• causes of small bowel dilation, but not necessarily obstruction

  • endometriosis

  • ischemia

  • enteritis

    • this often causes bowel wall thickening and mild dilation with mesenteric edema, but without a transition point. Involvement of the stomach and large bowel can also be seen.

  • medication-related

    • angioedema

    • chemotherapy

• large bowel obstruction

  • colon cancer

  • ileocecal tumor

• pediatric small bowel atresia

See also

• bowel obstruction

  • closed-loop obstruction

  • large bowel obstruction

• bowel ischemia

• 3-6-9 rule