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, accounting for 55-75% of all cases, followed by hernias (15-25%) and malignancies (5-10%) 1.

Major Etiologies of Small Bowel Obstruction

Adhesions (55-75% of cases)

• Adhesions are fibrous tissue that connects surfaces or organs within the peritoneal cavity that are normally separated 1
• Adhesions typically form after peritoneal injury from abdominal surgery, but can also occur in patients with no prior abdominal surgery (virgin abdomen) 1
• In patients with virgin abdomen (SBO-VA), adhesions still account for 26-100% of small bowel obstructions 1
• Types of adhesions include:
  • Solitary band adhesions (65% of adhesive SBO in virgin abdomen) 1
  • Matted adhesions (35% of adhesive SBO in virgin abdomen) 1

Hernias (15-25% of cases)

• External hernias (inguinal, femoral, umbilical, incisional) 1
• Internal hernias (paraduodenal, foramen of Winslow) 1
• Abdominal wall hernias account for approximately 10% of SBO cases 1

Malignancies (5-10% of cases)

• Primary small bowel tumors (NETs, lymphoma, carcinomas) 1
• Metastatic tumors (commonly from colon, ovary, or prostate) 1
• Malignancy as a cause of SBO-VA is encountered in 4-41% of cases 1

Less Common Causes (10-15% of cases)

• Intussusception: telescoping of one segment of bowel into another 1, 2
• Volvulus: twisting of bowel around its mesenteric axis 1
• Gallstone ileus: obstruction by a gallstone that has eroded through the gallbladder into the small bowel 1
• Bezoars or foreign bodies 1
• Meckel's diverticulum: most common congenital anomaly of the GI tract, present in 2% of population 3
  • Can cause obstruction due to volvulus around a fibrous band connecting the diverticulum to the umbilicus 3
  • Can also cause obstruction due to entrapment of small bowel beneath a mesodiverticular band 3
• Inflammatory bowel disease causing strictures 1
• Radiation-induced strictures 1
• Post-anastomotic strictures 1
• Sclerosing encapsulating peritonitis 1
• Uterine fibroids (rare cause) 4

Etiology Based on Patient History

Patients with Previous Abdominal Surgery

• Adhesions are the predominant cause (55-75%) 1
• Having a history of previous abdominal surgery has 85% sensitivity and 78% specificity to predict adhesive small bowel obstruction 1
• Matted adhesions are more common (67%) than band adhesions in patients with previous surgery 1

Patients with Virgin Abdomen (No Previous Surgery)

• Adhesions still account for 26-100% of cases 1
• Malignancy is more common (4-41%) compared to patients with previous surgery 1
• Other causes like congenital bands, internal hernias, and Meckel's diverticulum should be considered 1

Diagnostic Approach for Identifying Cause

• CT scan is the most accurate imaging modality for determining etiology with 76% accuracy compared to operative findings 1
• Water-soluble contrast studies can help in diagnosis and have prognostic value 5
• Negative laparotomies (no identifiable cause found during surgery) occur in 6-40% of SBO-VA cases 1

Clinical Pearls

• Always consider malignancy in elderly patients with new-onset SBO, even with history of previous surgery 6, 7
• Meckel's diverticulum should be considered in young patients with SBO, especially when accompanied by GI bleeding 3
• Rare causes like uterine fibroids should be considered in women of reproductive age with SBO and no other obvious etiology 4
• The risk of SBO is highest following colorectal, oncologic gynecological, or pediatric surgery 1