Diagnostics

• Imaging is required to:

  • Confirm mechanical bowel obstruction

  • Identify the site and assess the severity of the obstruction

  • Identify complications and the underlying etiology of the obstruction

  • Guide treatment planning

• Laboratory studies provide supportive evidence to help assess the severity of the obstruction.

Do not wait for imaging before initiating definitive management if there is an emergent critical finding (e.g., peritonitis).

Bowel obstruction requires a swift diagnostic workup to establish if emergency surgery is required.

• Initial imaging modality: depends on the type of bowel obstruction and hemodynamic stability of the patient

  • Acute bowel obstruction

    • Stable patients: CT abdomen and pelvis with IV contrast

    • Unstable patients: Consider abdominal series x-ray or abdominal ultrasound first, along with urgent surgical consultation.

  • Subacute bowel obstruction

    • Preferred: CT abdomen and pelvis with IV contrast

    • Alternatives: MRI with and/or without IV contrast, water-soluble contrast challenge, and specialized dynamic contrast studies

• Findings: Radiological signs common to all imaging modalities are detailed in the table.

• Indication: most appropriate initial test in hemodynamically unstable patients or in resource-poor centers

• Findings

  • Proximal bowel dilatation

  • Minimal or no intraluminal air distal to the obstruction

  • Stepladder sign (best seen on an upright view): multiple air-fluid levels and stacked dilated loops of small bowel

  • Chest x-ray : Air under the diaphragm is an indicator of bowel perforation.

  • See also “Radiological signs of mechanical bowel obstruction.”

• Important considerations : X-rays have a number of limitations.

  • Variable sensitivity (50–65%)

  • Cannot reliably identify the site of obstruction, underlying etiology, or extent of complications

  • Do not influence the management of acute bowel obstruction to the same extent as CT abdomen

• Indications

  • With IV contrast: most appropriate initial test in hemodynamically stable patients with acute bowel obstruction

  • With water-soluble oral contrast: Consider in patients with subacute bowel obstruction and no evidence of complications.

  • Without contrast: for patients with a contrast allergy

• Findings

  • Similar to those seen on abdominal x-ray; see “Radiological signs of mechanical bowel obstruction.”

  • Transition point: sudden narrowing of the bowel lumen at the site of obstruction

  • Closed-loop bowel obstruction: a type of mechanical bowel obstruction in which the proximal and distal ends of the obstructed loop are closed

    • Single site of obstruction: e.g., in volvulus or incarcerated hernia

    • Multiple sites of obstruction: e.g., obstructing colorectal cancer with a competent ileocecal valve

    • Progresses rapidly and is associated with an increased risk of strangulation

• Important consideration: In acute bowel obstruction, a CT scan is more accurate than an x-ray in the identification of the site of obstruction, complications, and underlying etiology, and, therefore, influences patient management to a greater extent.

Closed-loop bowel obstructions are associated with an increased risk of bowel strangulation and perforation.

POCUS or formal ultrasound can be performed.

• Indication: Hemodynamically unstable patients (may be preferred over abdominal x-ray)

• Findings

  • Multiple fluid-filled dilated bowel loops > 2.5 cm in diameter adjacent to collapsed bowel loops (most specific finding)

  • Thickened bowel wall

  • Prominent plicae circulares of dilated small bowel loops (sometimes referred to as the keyboard sign)

  • Altered peristalsis

    • Increased (early finding) or decreased/absent (late finding)

    • Pendular peristalsis: dysfunctional so-called “to-and-fro” peristalsis

  • Intraperitoneal fluid accumulation may be present.

• Indication: hemodynamically stable patients with contraindications to radiation exposure

• Findings: similar to those identified with a CT scan;

• Indication: suspected distal LBO if CT is unavailable

• Findings

  • Tapering of the bowel lumen at the site of obstruction

    • Complete bowel obstruction: contrast not visible beyond the obstruction

    • Partial bowel obstruction: small amount of contrast visible beyond the obstruction

  • Bird beak sign: in volvulus

  • Apple core sign: in colonic malignancy

Contrast enema helps differentiate complete bowel obstruction from partial bowel obstruction.

Barium enema is contraindicated if bowel perforation is suspected (water-soluble contrast enema can be used instead).

• Indication: SBO, to differentiate partial SBO from complete SBO [7][14]

• Procedure: A water-soluble contrast medium is administered orally or via an enteric tube, followed by abdominal x-ray 8 and 24 hours after ingestion.

• Findings

  • Normal WSCc: contrast reaches the colon within 24 hours of administration

    • Indicates partial bowel obstruction

    • The patient may be a candidate for nonoperative management.

  • Intraluminal and intramural causes of bowel obstruction may be identified

• Additional considerations

  • WSCc is also used to evaluate response to nonoperative management.

• Routine studies: CBC, BMP, serum lactate, and CRP

• Supportive findings

  • In patients who are dehydrated

    • ↑ BUN and creatinine (prerenal acute kidney injury)

    • ↑ Hematocrit (due to hemoconcentration)

  • In patients with recurrent vomiting

    • Hypochloremic hypokalemic metabolic alkalosis

    • Hyponatremia

  • Suggestive of complicated bowel obstruction

    • Hyperkalemia , elevated serum lactate, and metabolic acidosis: suggestive of bowel ischemia

    • Leukocytosis (> 16,000 cells/mcL)

    • Elevated nonspecific inflammatory markers (↑ CRP and serum creatine kinase)

    • ↑ Amylase

    • Altered coagulation panel (e.g., elevated INR in sepsis)

Leukocytosis, metabolic acidosis, and elevated serum lactate in a patient with suspected bowel obstruction are suggestive of bowel ischemia.

• Differential diagnoses of mechanical bowel obstruction

  • Paralytic ileus

  • Bowel perforation (secondary peritonitis)

  • Mesenteric ischemia

  • Inflammatory bowel disease

  • Ovarian torsion

• Differential diagnoses of SBO

  • Acute appendicitis

  • Acute pancreatitis

  • Pelvic inflammatory disease

• Differential diagnoses of LBO

  • Diverticulitis

  • Toxic megacolon

  • Chronic megacolon

  • Acute colonic pseudo-obstruction (Ogilvie syndrome)