What are the differences between small‑bowel obstruction and ileus?

Distinguishing Small Bowel Obstruction from Ileus

Small bowel obstruction (SBO) is a mechanical blockage requiring identification of surgical indications, while ileus is a functional motility disorder managed conservatively—the critical distinction lies in the presence of a physical obstruction point versus diffuse dysmotility. 1

Fundamental Pathophysiologic Differences

Small Bowel Obstruction (Mechanical)

• A physical barrier blocks intestinal passage, causing proximal bowel dilation with increased motor activity attempting to overcome the obstruction 1, 2
• The intestine responds with hyperactive peristalsis upstream from the blockage, creating characteristic hyperactive bowel sounds with audible "rushes" 2
• Colicky abdominal pain directly correlates with hyperperistaltic waves as the bowel attempts to push contents through the obstruction 2
• A transition point is identifiable on imaging where dilated proximal bowel meets collapsed distal bowel 3

Ileus (Functional/Paralytic)

• No mechanical obstruction exists—instead, there is diffuse intestinal dysmotility without a physical blockage 4
• Bowel sounds are typically absent or hypoactive rather than hyperactive, reflecting the lack of coordinated peristalsis 2
• Ileus commonly occurs after surgery, trauma, or with metabolic disturbances (electrolyte abnormalities, medications, sepsis) 2
• No discrete transition point is present—the entire bowel shows uniform dilation without a focal obstruction site 4

Clinical Presentation Differences

Small Bowel Obstruction Features

• Colicky, intermittent abdominal pain that waxes and wanes with peristaltic waves 2
• Hyperactive bowel sounds with high-pitched rushes on auscultation 2
• Nausea and vomiting occur earlier and more prominently, especially in proximal obstructions 2
• Absence of flatus occurs in 90% of cases and absence of bowel movements in 80.6% 2
• Abdominal distension is present in 65.3% of cases with a positive likelihood ratio of 16.8 2
• Pain is NOT relieved by defecation, distinguishing it from functional disorders 2

Ileus Features

• Constant, diffuse abdominal discomfort rather than colicky pain 5
• Absent or hypoactive bowel sounds throughout the abdomen 2, 4
• More gradual symptom onset without the acute cramping characteristic of mechanical obstruction 5
• Often occurs in postoperative patients (typically days 3-5 after surgery) or those with metabolic derangements 4
• Uniform bowel dilation without a focal point of obstruction 4

Diagnostic Imaging Distinctions

CT Findings for Small Bowel Obstruction

• CT with IV contrast is the gold standard with >90% accuracy for diagnosing mechanical SBO 3
• Identifiable transition point where dilated proximal bowel (>2.5-3 cm) meets collapsed distal bowel 3
• Oral contrast is not needed—fluid-filled dilated bowel provides natural contrast 3
• Signs of ischemia may be present: abnormal bowel wall enhancement, mesenteric edema, pneumatosis 3
• CT is 100% sensitive and specific in distinguishing complete mechanical obstruction from ileus 4

CT Findings for Ileus

• Uniform bowel dilation throughout small and large intestine without a discrete transition point 4
• No focal obstruction site identifiable 4
• Gas present throughout the entire bowel including the colon and rectum 4
• Bowel wall appears normal without signs of ischemia 4

Ultrasound Capabilities

• Ultrasound has 91% sensitivity and 84% specificity for diagnosing SBO and can differentiate it from ileus 3, 6
• Demonstration of intestinal peristalsis on real-time ultrasound helps distinguish functional from obstructive ileus 6
• Large amounts of free fluid between dilated loops suggests high-grade mechanical obstruction requiring immediate surgery 6
• Ultrasound shows altered peristaltic activity in mechanical obstruction versus absent peristalsis in ileus 6

Critical Management Implications

When SBO Requires Emergency Surgery

• Peritoneal signs (guarding, rebound tenderness) indicating perforation or peritonitis 1, 3
• Signs of strangulation/ischemia: fever, tachycardia, tachypnea, confusion, intense unremitting pain 2, 3
• Transition from hyperactive to absent bowel sounds indicates progression to ischemia with mortality up to 25% 2
• Elevated lactate, leukocytosis, metabolic acidosis suggesting bowel compromise 2, 3
• Complete obstruction failing conservative management within 24-48 hours 7

Ileus Management (Conservative)

• Bowel rest (NPO), IV fluid resuscitation, correction of electrolyte abnormalities 7
• Nasogastric decompression if significant distension or vomiting present 7
• Address underlying causes: discontinue offending medications, treat metabolic disturbances, manage sepsis 5
• Serial clinical examinations to ensure no progression to mechanical obstruction 4
• Most cases resolve spontaneously with supportive care 5

Common Diagnostic Pitfalls

• Mistaking incomplete SBO with watery diarrhea for gastroenteritis—the passage of small amounts of liquid stool does not exclude mechanical obstruction 2
• Relying solely on plain radiographs which have only 60-70% sensitivity and are non-diagnostic in 36% of cases 3
• Overlooking SBO in elderly patients where pain may be less prominent but mortality risk is higher 2
• Assuming ileus in all postoperative patients with distension—mechanical obstruction can occur postoperatively and requires CT differentiation 4
• Delaying CT when clinical suspicion is high—proceed directly to CT with IV contrast rather than starting with plain films 3

Practical Clinical Algorithm

For suspected bowel obstruction:

1. Obtain focused history: prior abdominal surgeries (85% sensitivity for adhesive SBO), recent trauma, medications, metabolic conditions 2

2. Physical examination: assess for hyperactive versus absent bowel sounds, peritoneal signs, hernias at all orifices 1, 2

3. Laboratory tests: CBC, electrolytes, renal function, lactate to assess for complications 1, 3

4. Imaging decision:

   • High clinical suspicion or any alarm signs → proceed directly to CT abdomen/pelvis with IV contrast 3
   • Stable patient, low suspicion → bedside ultrasound if available and operator experienced 3
   • Plain radiographs have limited value and should not delay definitive imaging 3

5. Disposition based on CT findings:

   • Mechanical SBO with transition point → surgical consultation, initiate resuscitation, NPO, NG tube 3, 7
   • Ileus (no transition point, uniform dilation) → conservative management, address underlying causes 4, 7
   • Any signs of ischemia or strangulation → emergency surgery 3, 8