What are the potential underlying causes of Small Bowel Obstruction (SBO)?

Potential Underlying Causes of Small Bowel Obstruction

Adhesions are the dominant cause of small bowel obstruction, accounting for 55-75% of all cases, followed by hernias (15-25%) and malignancies (5-10%). 1, 2

Primary Etiologies by Frequency

Adhesions (55-75% of cases)

• Adhesions represent fibrous tissue bands connecting normally separated peritoneal surfaces or organs. 2
• Post-surgical adhesions are the most common subtype, developing after peritoneal injury from any abdominal operation. 2, 3
• A history of previous abdominal surgery has 85% sensitivity and 78% specificity for predicting adhesive SBO. 1
• The highest risk follows colorectal, oncologic gynecological, or pediatric surgery. 2
• Adhesions can occur even in virgin abdomen (no prior surgery), accounting for 26-100% of SBO-VA cases. 1, 2
• In virgin abdomen, solitary band adhesions predominate (65%), while matted adhesions account for 35%. 1, 2
• Conversely, patients with previous surgery develop matted adhesions more commonly (67%). 2
• Congenital peritoneal bands represent anomalous intra-peritoneal adhesions present from birth, remnants of physiological organogenesis. 2

Hernias (15-25% of cases)

• External hernias include inguinal, femoral, umbilical, and incisional hernias. 1, 2
• Abdominal wall hernias account for approximately 10% of SBO cases. 1, 2
• Internal hernias (paraduodenal, foramen of Winslow) can trap bowel loops. 2
• Each hernia orifice and all laparotomic/laparoscopic incision scars require careful examination during physical assessment. 1

Malignancies (5-10% of cases overall; 4-41% in virgin abdomen)

• Primary small bowel tumors include neuroendocrine tumors, lymphoma, and carcinomas. 2
• Metastatic tumors commonly originate from colon, ovary, or prostate cancer. 2
• In virgin abdomen patients, malignancy is encountered more frequently (4-41%) compared to post-surgical patients. 1, 2
• Previous rectal bleeding and unexplained weight loss suggest colorectal cancer. 1

Other Causes (15% of cases)

• Gallstone ileus occurs when a large gallstone erodes through the gallbladder wall into adjacent bowel, causing mechanical obstruction. 1
• Meckel's diverticulum (present in 2% of population) causes obstruction through volvulus around a fibrous band connecting the diverticulum to the umbilicus or entrapment beneath a mesodiverticular band. 1, 2
• Small bowel volvulus presents with intense, untreatable pain due to ischemia. 1
• Intussusception involves telescoping of one bowel segment into another. 1
• Bezoars or foreign bodies can cause mechanical obstruction. 1
• Inflammatory bowel disease stenosis develops from chronic inflammation. 1
• Radiation-induced strictures occur following pelvic or abdominal radiation therapy. 1, 2
• Post-anastomotic strictures develop at surgical anastomosis sites. 1, 2
• Ischemic stenosis results from prior mesenteric ischemia. 1
• Carcinomatosis involves peritoneal seeding from intra-abdominal malignancies. 1
• Endometriosis can cause bowel obstruction through implants on bowel serosa. 1
• Sclerosing encapsulating peritonitis is a rare cause involving fibrous encasement of bowel. 1, 2

Critical Diagnostic Considerations

History-Based Risk Stratification

• Previous abdominal surgery strongly predicts adhesive SBO (85% sensitivity, 78% specificity). 1
• Previous diverticulitis episodes suggest diverticular stenosis. 1
• Chronic constipation history (dolichosigmoid) may indicate volvulus. 1
• Medications affecting peristalsis are associated with pseudo-obstruction and adynamic ileus. 1

Physical Examination Priorities

• Abdominal distension has a positive likelihood ratio of 16.8 and negative likelihood ratio of 0.27 for SBO. 1
• Peritoneal signs indicate ischemia and/or perforation requiring emergency surgery. 1
• Examine all hernia orifices (umbilical, inguinal, femoral) and surgical scars. 1
• Digital rectal examination and rectoscopy can detect rectal masses or blood. 1

Imaging Accuracy

• CT scan is the most accurate imaging modality with 76% accuracy compared to operative findings. 1
• Plain radiographs have limited value with only 60-70% sensitivity and specificity. 1, 3
• Negative laparotomies (no identifiable cause found during surgery) occur in 6-40% of SBO-VA cases. 1, 2

Common Pitfalls to Avoid

• Do not assume absence of prior surgery excludes adhesions—adhesive SBO occurs in 26-100% of virgin abdomen cases. 1, 2
• Normal lactate, white blood cell count, and CRP cannot exclude bowel ischemia—clinical judgment and imaging remain essential. 1
• Young patients with SBO should prompt consideration of Meckel's diverticulum, especially with concurrent GI bleeding. 2
• In elderly or unconscious patients, vital signs and cardiopulmonary examination become fundamental when history is unreliable. 1