What is Small Bowel Obstruction (SBO)?

What is Small Bowel Obstruction (SBO)?

Small bowel obstruction is a surgical emergency characterized by mechanical blockage of the small intestine that prevents the normal passage of intestinal contents, typically presenting with abdominal pain, vomiting, distension, and constipation. 1

Definition and Pathophysiology

SBO occurs when a mechanical obstruction hinders the passage of intestinal contents through the small bowel. 1 The intestine responds to this blockage with increased motor activity upstream from the obstruction, creating characteristic hyperactive bowel sounds and intermittent crampy abdominal pain. 2 This process can progress to mucosal ischemia, necrosis, and perforation if left untreated. 3

Classification Systems

The most clinically important distinction is between complete versus partial obstruction and complicated versus simple obstruction, as complete complicated SBO more commonly requires surgical intervention. 3

• Partial (incomplete) obstruction: Sufficient luminal patency exists to allow some contrast material to flow beyond the point of obstruction 1
• Complete obstruction: No passage of intestinal contents beyond the blockage 1
• Complicated obstruction: Involves bowel ischemia, strangulation, or perforation requiring immediate surgery 3
• Simple obstruction: Mechanical blockage without vascular compromise 3

Common Etiologies

In adults, postoperative adhesions are the most common cause, accounting for 55-75% of all SBO cases. 2, 3 Other important causes include:

• Hernias (internal or external) 1
• Malignancy 1, 4
• Inflammatory bowel disease 4
• Bezoars 1
• Congenital adhesions (in virgin abdomen patients) 1

Clinical Presentation

Cardinal Symptoms

• Colicky abdominal pain that worsens as the bowel attempts to overcome the obstruction 2
• Nausea and vomiting (more prominent and earlier in proximal SBO) 1, 2
• Abdominal distension (occurs in 65.3% of cases with a positive likelihood ratio of 16.8) 2
• Absence of flatus (90% of cases) 2
• Absence of bowel movements (80.6% of cases) 2

Physical Examination Findings

• Hyperactive bowel sounds with audible rushes in early stages 2
• Abdominal tenderness 2
• Visible peristalsis in thin patients 2
• Abdominal distension 2

Critical Warning Signs of Complications

When hyperactive bowel sounds transition to absent sounds, this indicates progression to bowel ischemia or strangulation with mortality rates up to 25% if not immediately treated surgically. 2 Additional red flags requiring urgent intervention include:

• Fever, tachycardia, tachypnea, and confusion 2
• Intense pain unresponsive to analgesics 2
• Diffuse abdominal tenderness, guarding, or rebound tenderness 2
• Hypotension, cool extremities, mottled skin, and oliguria 2
• Elevated lactate levels 2
• Leukocytosis and neutrophilia 2
• Metabolic acidosis (low serum bicarbonate and arterial pH) 2

Diagnostic Approach

Imaging Modalities

CT abdomen and pelvis with IV contrast is the diagnostic standard with >90% accuracy for diagnosing SBO and should be the initial imaging study in most cases. 1, 2 Key advantages include:

• Identifies the site and cause of obstruction 1
• Detects complications including signs of ischemia (abnormal bowel wall enhancement, mesenteric edema, pneumatosis) 2
• No oral contrast is needed in suspected high-grade obstruction 2
• Provides 3-D anatomical information preferred by surgeons 1

Plain abdominal radiographs have limited diagnostic value with only 50-60% sensitivity and are non-diagnostic in 36% of cases. 2 While dilated bowel loops with air-fluid levels and paucity of colonic gas favor SBO, radiographs are often not definitive and can prolong evaluation. 1

Ultrasound is a valid alternative with 91% sensitivity and 84% specificity, particularly useful in pregnancy and for bedside evaluation. 1, 2 However, CT generally provides more comprehensive information for surgical planning. 1

Special Considerations for Low-Grade or Intermittent SBO

Standard CT has reduced sensitivity (48-50%) for low-grade obstruction. 1 In these cases:

• CT enterography or CT enteroclysis with optimized bowel distention may be needed 1
• Re-imaging within 24 hours after oral contrast administration can demonstrate passage beyond a transition point, indicating partial obstruction 1

Laboratory Evaluation

• Complete blood count to assess for leukocytosis 2
• Electrolyte panel to identify imbalances 2
• Renal function tests to evaluate dehydration 2
• Lactate levels to assess for intestinal ischemia 2

Common Diagnostic Pitfalls

Mistaking incomplete obstruction with watery diarrhea for gastroenteritis can lead to dangerous delays in diagnosis. 2, 5 Other critical errors include:

• Overlooking SBO in elderly patients where pain may be less prominent 2
• Relying solely on plain radiographs when clinical suspicion is high 2, 5
• Delaying CT when warning signs of complications are present 2

Management Principles

Surgery is immediately required for strangulation or signs of bowel ischemia. 3 For simple obstruction:

• Intravenous fluid resuscitation 3
• Analgesia 3
• Nasogastric tube decompression for patients with significant distension and vomiting 3
• Surgical service evaluation and admission are recommended 3
• Nonoperative management may be appropriate for partial obstruction 1, 3