What are the common etiologies of small bowel obstruction in adults?

Etiologies of Small Bowel Obstruction

Adhesions are the most common cause of small bowel obstruction in adults, accounting for 55-75% of all cases, followed by hernias (15-25%) and malignancies (5-10%). 1

Primary Etiologic Categories

Adhesions (Most Common)

• Adhesions represent the single leading cause of SBO, responsible for 55-75% of cases 1, 2
• Post-surgical adhesions are the predominant type, with a history of prior abdominal surgery having 85% sensitivity and 78% specificity for predicting adhesive SBO 1, 2
• The risk is highest following colorectal, oncologic gynecological, or pediatric surgery 1
• Even in patients with virgin abdomen (no prior surgery), adhesions still cause 26-100% of cases, with pooled data showing approximately 48% 1
• Two types exist: solitary band adhesions (65% in virgin abdomen) and matted adhesions (35% in virgin abdomen, but 67% in post-surgical patients) 1

Hernias (Second Most Common)

• External hernias (inguinal, femoral, umbilical, incisional) account for 15-25% of SBO cases 1, 2
• Internal hernias (paraduodenal, foramen of Winslow, broad ligament defects) are rare but important causes 1, 3
• Abdominal wall hernias specifically account for approximately 10% of SBO cases 1

Malignancies (Third Most Common)

• Malignancies cause 5-10% of all SBO cases 1, 2
• Primary small bowel tumors include neuroendocrine tumors, lymphoma, and carcinomas 1
• Metastatic tumors commonly originate from colon, ovary, or prostate 1
• In virgin abdomen patients, malignancy accounts for 4-41% of cases, representing a significantly higher proportion than in post-surgical SBO 1, 2
• Carcinomatosis peritonei can cause extrinsic compression leading to obstruction 2

Less Common but Important Etiologies

Inflammatory and Structural Causes

• Crohn's disease causes fibrostenotic strictures, with up to 54% of patients developing SBO requiring surgery 4
• Radiation-induced strictures occur particularly in the sigmoid colon after pelvic radiation 2
• Post-anastomotic strictures 1
• Sclerosing encapsulating peritonitis 1

Mechanical Obstructions

• Intussusception (rare in adults, typically has a pathologic lead point) 1
• Volvulus 1
• Gallstone ileus 1
• Bezoars or foreign bodies 1
• Meckel's diverticulum (present in 2% of population, causes obstruction via volvulus around fibrous band or entrapment beneath mesodiverticular band) 1

Drug-Related Causes

• Bevacizumab causes ulceration, fistulation, or perforation in 0.9% of patients within 1 year 2
• Tyrosine kinase inhibitors are associated with bowel perforation 2

Critical Clinical Pearls and Pitfalls

Diagnostic Considerations

• CT scan is the most accurate imaging modality for determining etiology with 76% accuracy compared to operative findings 1
• The traditional teaching that malignancy is the primary cause of SBO in virgin abdomen is outdated; adhesions remain most common even without prior surgery 1
• However, malignancy still accounts for approximately 1 in 10 virgin abdomen cases, mandating thorough CT evaluation 1
• Negative laparotomies (no identifiable cause found) occur in 6-40% of virgin abdomen SBO cases 1

Age and Population-Specific Considerations

• Meckel's diverticulum should be considered in young patients with SBO, especially when accompanied by GI bleeding 1
• Endometriosis can cause SBO particularly in women of reproductive age 2
• Median age at diagnosis ranges from 58 to 65 years, with male predominance (65-83% of cases) 1

Management Implications

• Modern guidelines suggest treating virgin abdomen SBO similarly to adhesive SBO in post-surgical patients, with initial conservative trial unless signs of ischemia, strangulation, or peritonitis are present 1
• Between 39-83% of virgin abdomen SBO patients undergo operative treatment, notably higher than contemporary rates for post-surgical SBO 1