What are the primary causes of small bowel obstruction?

Primary Causes of Small Bowel Obstruction

Adhesions are the most common cause of small bowel obstruction (47.9-75% of cases), followed by hernias (15-25%), malignancies (4-13%), and several less common etiologies including bezoars, volvulus, and intussusception. 1, 2

Major Causes by Frequency

1. Adhesions (55-75% of cases)

• Most common overall cause of small bowel obstruction 2
• Types of adhesions:
  • Post-surgical adhesions: Most common type, forming after peritoneal injury from abdominal operations 1
  • Congenital adhesions: Can occur in patients with no prior surgery (virgin abdomen) 1
  • Other causes of adhesions: Radiotherapy, endometriosis, inflammation, and local response to tumors 1

2. Hernias (15-25% of cases)

• Second most common cause 2
• Types include:
  • External hernias (inguinal, femoral, umbilical, incisional)
  • Internal hernias

3. Malignancies (4-13% of cases)

• Can be primary or metastatic 1, 2
• Sources include:
  • Primary small bowel tumors (NETs, lymphoma, carcinomas)
  • Metastatic tumors from colon, ovary, or prostate 1, 3
  • Peritoneal carcinomatosis 3

4. Less Common Causes

• Intussusception: Telescoping of one segment of bowel into another 1, 4
• Small bowel volvulus: Twisting of bowel around its mesenteric axis 1
• Gallstone ileus: Obstruction by a large gallstone 1
• Bezoars/Foreign bodies: Accumulation of ingested material 1, 5
• Meckel's diverticulum: Congenital abnormality of the small intestine 1
• Inflammatory bowel disease: Causing strictures 2
• Radiation-induced strictures: Following radiation therapy 2
• Enteroliths: Rare cause of obstruction 5
• Uterine fibroids: Extremely rare external compression 4

Causes by Patient Population

In Patients with Prior Abdominal Surgery

• Adhesions are overwhelmingly the predominant cause (55-75%) 2
• Risk is highest following:
  • Colorectal surgery
  • Oncologic gynecological procedures
  • Pediatric surgery 1

In Virgin Abdomen (No Prior Surgery)

• Adhesions still account for 26-100% of cases, but typically solitary band adhesions rather than matted adhesions 1
• Malignancy is more common in this population (4-41%) 1
• Congenital abnormalities are more likely to be the cause 1

Clinical Pearls

• The distinction between complete vs. partial and complicated vs. simple obstruction is critical for management decisions 6
• Complete, complicated SBO more commonly requires surgical intervention 6
• Most reliable clinical findings include prior abdominal surgery, constipation, abdominal distension, and abnormal bowel sounds 6
• Signs of strangulation (requiring immediate intervention) include fever, hypotension, diffuse abdominal pain, and peritonitis 6
• CT imaging has over 90% diagnostic accuracy and can help differentiate between mechanical obstruction and ileus 2
• In cancer patients, bowel obstruction may be caused by peritoneal carcinomatosis or direct tumor invasion, not just adhesions 7, 3

Diagnostic Approach

• Abdominal CT with IV contrast is the most accurate diagnostic tool (>90% accuracy) 2
• Plain radiographs have limited sensitivity (60-70%) and cannot exclude the diagnosis 1, 6
• Ultrasound is a reliable alternative diagnostic method, especially in patients who cannot undergo CT 6

Understanding these causes is essential for proper management, as the etiology directly influences treatment decisions and patient outcomes.