A 62-year-old man presents with acute abdominal pain, abdominal distension, and vomiting. Plain radiography shows dilated small bowel loops with air-fluid levels. On contrast-enhanced CT abdomen, there is a transition point with collapsed distal bowel. What is the most common cause of mechanical small bowel obstruction in developed countries?

## Most Common Cause of Small Bowel Obstruction **Key Point:** Adhesions account for 60–75% of mechanical small bowel obstructions in developed countries, particularly in patients with prior abdominal or pelvic surgery. ### Epidemiology & Risk Factors Adhesions form as a consequence of: - Previous abdominal or pelvic surgery (most common) - Peritonitis (spontaneous or secondary) - Inflammatory bowel disease - Abdominal trauma ### Imaging Features of Adhesive Obstruction | Feature | Finding | |---------|----------| | **Small bowel dilatation** | Proximal loops dilated (>3 cm) | | **Transition point** | Abrupt change from dilated to collapsed bowel | | **Air-fluid levels** | Characteristic on upright plain films | | **CT appearance** | No focal mass, no stricture, no hernia defect | | **Mesenteric vessels** | Normal caliber, no evidence of ischemia | ### Clinical Pearl **High-Yield:** Adhesions are the leading cause in developed countries; in developing countries, hernias (especially external) remain the most common cause. The history of prior surgery is a crucial clinical clue — approximately 75% of adhesive obstructions occur in patients with previous abdominal operations. ### Comparison with Other Common Causes | Cause | Frequency (Developed) | Imaging Clue | |-------|----------------------|---------------| | **Adhesions** | 60–75% | Transition point, no mass, prior surgery history | | **Hernia** | 10–15% | Defect in abdominal wall or mesentery | | **Crohn's disease** | 5–10% | Stricture, mesenteric fat stranding, skip lesions | | **Malignancy** | 5–10% | Focal mass, shouldering, irregular narrowing | ### Mnemonic: CHAMP **C** — Crohn's disease **H** — Hernia **A** — **Adhesions** (most common in developed countries) **M** — Malignancy **P** — Post-surgical stricture **Tip:** When you see a patient with small bowel obstruction and a history of prior abdominal surgery, adhesions should be your first thought. Imaging alone cannot definitively diagnose adhesions (diagnosis is often surgical), but the absence of a focal mass, stricture, or hernia defect on CT makes adhesions the most likely diagnosis. [cite:Harrison 21e Ch 298]