A 65-year-old man with a history of previous abdominal surgery presents with acute onset colicky abdominal pain, abdominal distension, and vomiting. Plain radiograph shows dilated small bowel loops with air-fluid levels. What is the most common cause of small bowel obstruction in this clinical scenario?

Explanation

## Most Common Cause of Small Bowel Obstruction **Key Point:** Adhesions account for 60–75% of all small bowel obstructions in developed countries, particularly in patients with prior abdominal surgery. ### Epidemiology | Cause | Frequency | Risk Factors | | --- | --- | --- | | Adhesions | 60–75% | Prior abdominal/pelvic surgery, peritonitis | | Hernia | 10–15% | Congenital, incisional, inguinal | | Malignancy | 10–15% | Primary bowel cancer, metastatic disease | | Crohn's disease | 5–10% | Chronic inflammation, stricture formation | | Volvulus | 3–5% | Sigmoid > cecal | **High-Yield:** In post-operative patients (>90% of adhesion cases), adhesions are the default diagnosis until proven otherwise on imaging. ### Imaging Features of Adhesive Obstruction 1. **Plain radiograph findings:** - Dilated small bowel loops (>3 cm diameter) - Air-fluid levels on upright view - Transition zone (abrupt change from dilated to collapsed bowel) - No free air (unless perforation) 2. **CT imaging (gold standard):** - Clustered small bowel loops in the pelvis or abdomen - Transition point with no visible mass or stricture - Absence of other obstructive causes - Mesenteric tethering or folding pattern **Clinical Pearl:** The clinical history of prior surgery is the strongest predictor of adhesive obstruction. Most adhesions form within the first 2 years post-operatively but can occur decades later. **Warning:** Do not assume adhesions without imaging confirmation; always exclude malignancy, hernia, and inflammatory strictures, especially in first-time obstruction. ### Why Adhesions Are Most Common - Virtually all abdominal/pelvic surgery causes some degree of peritoneal trauma - Adhesion formation is a natural healing response - Incidence increases with number of prior operations - No specific preventive measure is universally effective **Mnemonic:** **CHASM** — Causes of small bowel obstruction: Crohn's, Hernia, Adhesions, Stricture/malignancy, Malrotation/volvulus. Adhesions top the list in post-op patients.