What is the differential diagnosis for dilated bowel loops?

Differential Diagnosis of Dilated Bowel Loops

Dilated bowel loops indicate either mechanical obstruction or functional ileus, and CT scan with IV contrast is the definitive first-line test to distinguish between these and identify the underlying cause. 1, 2

Primary Diagnostic Categories

Mechanical Small Bowel Obstruction

• Adhesive disease (most common cause, accounting for approximately 70% of cases) 1, 3
• Hernias (internal or external through abdominal wall defects, including post-bariatric surgery internal hernias through mesenteric defects) 1, 4
• Malignancy (primary bowel tumors or extrinsic compression from masses) 1, 5
• Strictures from inflammatory bowel disease (particularly Crohn's disease with transmural inflammation) 1
• Closed-loop obstruction (U-shaped conglomerate of fixed, dilated loops with thickened walls and extraluminal fluid) 6
• Intussusception (more common in pediatric populations but can occur in adults) 1
• Volvulus (midgut volvulus showing "whirlpool sign" of twisted mesenteric vessels) 1

Mechanical Large Bowel Obstruction

• Colorectal malignancy (most common cause of colonic obstruction) 4
• Diverticular stricture 4
• Volvulus (sigmoid or cecal) 4
• Fecal impaction 4

Functional/Paralytic Ileus

• Postoperative ileus (most common after abdominal surgery) 7
• Metabolic derangements (hypokalemia, hypomagnesemia, uremia) 7
• Medications (opioids, anticholinergics) 7
• Inflammatory conditions (pancreatitis, peritonitis, sepsis) 7
• Ischemia (mesenteric ischemia causing bowel dysmotility) 7

Post-Surgical/Anatomic Variants

• Short bowel syndrome with compensatory dilatation (dilated segments occur at suboptimal anastomoses or watershed areas, promoting bacterial overgrowth) 1
• Peritoneal encapsulation (rare congenital anomaly with accessory peritoneal layer encapsulating small bowel, causing central clustering of dilated loops) 8

Critical Imaging Findings Requiring Immediate Surgery

The following CT findings mandate urgent surgical intervention without delay: 1, 2

• Closed-loop obstruction (fixed U-shaped dilated loops) 6
• Bowel ischemia (abnormal wall enhancement, intramural hyperdensity, pneumatosis intestinalis) 1, 5
• Portal venous gas 2
• Free intraperitoneal air (perforation) 1
• Large amount of free fluid between loops (suggests high-grade obstruction or strangulation) 3
• "Whirlpool sign" (twisted mesenteric vessels indicating volvulus) 1

Diagnostic Approach Algorithm

Step 1: Clinical Assessment

• Examine for peritonitis (rebound, guarding) - if present, proceed directly to surgery 1, 4
• Check all hernial orifices and previous surgical scars 4
• Assess vital signs - tachycardia and shock suggest complications 4
• Obtain plain abdominal radiograph only if CT unavailable (sensitivity 74-84%, specificity 50-72%) 1, 4

Step 2: CT Scan with IV Contrast (No Oral Contrast Needed)

• CT has >90% accuracy for diagnosing obstruction and identifying the cause 1, 2, 4
• Multiplanar reformations significantly improve detection of transition points 4
• Look for transition point (dilated proximal bowel to decompressed distal bowel) - presence indicates mechanical obstruction 1, 4
• Assess for "small bowel feces sign" and "beak sign" at transition point 4
• Evaluate bowel wall enhancement - abnormal enhancement suggests ischemia 5

Step 3: Differentiate Complete vs. Partial Obstruction

• Water-soluble contrast study can be administered after adequate gastric decompression 2, 4
• If contrast reaches colon within 24 hours - high likelihood of non-operative resolution 1, 2
• If no contrast passage - consider complete obstruction requiring surgery 1

Special Considerations

Inflammatory Bowel Disease

• Crohn's disease causes strictures with upstream dilatation, often with penetrating complications (fistulas, abscesses) at the stricture site 1
• Look for asterisk-shaped fistula complexes tethering multiple bowel loops 1
• Colonoscopy to terminal ileum and small bowel imaging define disease extent 1

Post-Bariatric Surgery

• Internal hernias through mesenteric defects (jejunojejunostomy, Petersen space, transverse mesocolon) show clustering of dilated loops with "whirlpool sign" 1
• Herniated bowel through transverse mesocolon appears behind gastric remnant as mass effect 1
• MRI preferred in pregnant patients with bariatric surgery history to avoid radiation 1

Common Pitfalls to Avoid

• Do not delay CT scan - plain radiographs are often nonspecific and prolong evaluation 1
• Do not give oral contrast before gastric decompression - risk of aspiration pneumonia 2
• Do not wait beyond 72 hours for non-operative management without clear improvement 1, 2
• Do not miss closed-loop obstruction - requires immediate surgery even without peritonitis 6
• CT only 20% sensitive for ischemic bowel - maintain high clinical suspicion despite negative CT 5