What is the diagnostic approach for a patient with suspected bowel obstruction?

Diagnostic Approach for Bowel Obstruction

CT abdomen and pelvis with intravenous contrast is the definitive imaging study of choice and should be performed immediately in patients with suspected bowel obstruction, achieving diagnostic accuracy exceeding 90% for detecting presence, location, cause, and life-threatening complications. 1, 2

Initial Clinical Assessment

History and Physical Examination

• Identify the classic tetrad: abdominal pain (colicky initially, becoming constant with ischemia), nausea/vomiting (earlier and more prominent in small bowel obstruction), abdominal distension, and absence of flatus/feces passage 1, 3
• Examine all hernia orifices (umbilical, inguinal, femoral) and previous surgical scars—hernias cause 15-25% of small bowel obstructions and carry the highest strangulation risk 1, 4
• Assess for peritoneal signs: rebound tenderness, involuntary guarding, and rigidity indicate ischemia, perforation, or strangulation requiring immediate surgery 1, 3
• Evaluate vital signs for shock: tachycardia, tachypnea, hypotension, cool extremities, mottled skin, and oliguria suggest advanced disease with potential ischemia 1, 2
• Perform digital rectal examination to detect blood or rectal mass suggestive of colorectal malignancy 1

Initial Laboratory Testing

• Order complete blood count, serum lactate, electrolytes, renal function, and liver function tests immediately 1, 2
• Elevated lactate, low bicarbonate, low arterial pH, marked leukocytosis, and hyperamylasemia are critical markers suggesting intestinal ischemia requiring urgent surgical evaluation 1
• Obtain coagulation profile given potential need for emergency surgery 1

Imaging Strategy

First-Line Imaging: CT with IV Contrast

• CT abdomen/pelvis with IV contrast (without oral contrast) is superior to all other modalities with >90% diagnostic accuracy for obstruction presence, transition point location, underlying cause, and complications 1, 2
• Do not administer oral contrast in suspected high-grade obstruction—the intraluminal fluid and gas provide excellent intrinsic contrast, and oral contrast delays diagnosis, increases aspiration risk, and obscures bowel wall enhancement patterns critical for detecting ischemia 1
• CT identifies critical high-risk features requiring immediate surgery: reduced or absent bowel wall enhancement, increased abnormal enhancement, mesenteric edema, ascites, pneumatosis, mesenteric venous gas, closed-loop obstruction, and free intraperitoneal air 1, 5

CT Interpretation Priorities

The radiologist must address four critical questions 6:

1. Confirm obstruction presence and distinguish from adynamic ileus
2. Identify transition point (single vs. closed-loop obstruction)
3. Determine underlying cause (adhesions 55-75%, hernias 15-25%, malignancy 5-10% for small bowel; cancer 60%, volvulus 15-20%, diverticular disease 10% for large bowel) 1
4. Detect complications particularly ischemia/strangulation

Critical caveat: CT sensitivity for ischemia is limited (15-52% prospectively), though specificity is high (61-93%) when signs are present—clinical correlation significantly improves diagnostic accuracy 1, 7

Alternative Imaging Modalities

Abdominal Plain Radiographs

• Plain X-rays have limited diagnostic value with only 50-60% sensitivity, 20-30% inconclusive results, and 10-20% misleading findings 1, 8
• Do not order plain films after CT has been performed—they add no diagnostic value and can delay appropriate management 7, 8
• Plain films may be considered as initial screening only when CT is unavailable or contraindicated 1

Ultrasound

• Ultrasound demonstrates 90% sensitivity and 96% specificity when dilated bowel loops >2.5 cm are visualized with decreased peristalsis 1, 8
• CT remains superior for etiologic diagnosis and complication detection in both small and large bowel obstruction 1

MRI

• MRI is the preferred alternative to CT in pregnant women and children to minimize radiation exposure, with 95% sensitivity and 100% specificity 1, 7, 8

Colonoscopy

• Colonoscopy is indicated for large bowel obstruction to exclude alternative causes, obtain tissue diagnosis in suspected malignancy, and potentially place endoscopic stents 1
• Use carbon dioxide insufflation rather than air to reduce ischemia risk, bloating, and patient discomfort 1

Water-Soluble Contrast Studies

Diagnostic and Therapeutic Role

• Administer 50-150 mL of water-soluble contrast (Gastrografin) orally or via nasogastric tube after adequate gastric decompression in adhesive small bowel obstruction without peritoneal signs 1, 2, 8
• Obtain abdominal X-ray at 24 hours: contrast reaching the colon predicts successful non-operative management with 96% sensitivity and 98% specificity; absence of colonic contrast indicates need for surgery 1, 2, 8
• Water-soluble contrast reduces need for surgery, shortens time to resolution, and decreases hospital length of stay without increasing complications or mortality 1, 2

Safety Precautions

• Administer only after nasogastric decompression to prevent aspiration pneumonia and pulmonary edema 1, 8
• Monitor for dehydration especially in elderly and pediatric patients—hyperosmolar contrast shifts fluid into bowel lumen and can cause shock-like states 1, 8
• Consider delaying administration to 48 hours after initial resuscitation to reduce aspiration and dehydration risks 1, 8
• Use caution in patients with gastropathy risk and be aware of rare anaphylactoid reactions 1, 8

Initial Supportive Management

Begin immediate supportive treatment concurrently with diagnostic workup 1, 2:

• Intravenous isotonic crystalloid resuscitation with potassium supplementation to match losses
• Nasogastric tube decompression to prevent aspiration and analyze gastric contents (feculent aspirate indicates distal obstruction)
• Nil per os (NPO) status
• Foley catheter placement to monitor urine output
• Anti-emetics for symptom control

Critical Decision Points

Immediate Surgical Consultation Required

Obtain urgent surgical evaluation for 1, 2:

• Signs of peritonitis (rebound, guarding, rigidity)
• CT evidence of ischemia (abnormal bowel wall enhancement, pneumatosis, mesenteric venous gas)
• Closed-loop obstruction or volvulus
• Free intraperitoneal air indicating perforation
• Clinical deterioration despite resuscitation
• Hemodynamic instability or septic shock

Trial of Non-Operative Management

Conservative management is appropriate for adhesive small bowel obstruction without high-risk features with 70-90% success rate, but surgical consultation is needed if no improvement by 48-72 hours 1, 2, 7

Common pitfall: Do not delay repeat imaging or surgical consultation if clinical deterioration occurs—ischemia develops rapidly and mortality reaches 25% when present, increasing to 30% with bowel necrosis/perforation 1, 9, 3