A 68-year-old man presents to the emergency department with acute-onset severe abdominal pain and vomiting for 6 hours. He has a history of previous abdominal surgery 15 years ago. On examination, he is in distress, with a distended abdomen and hyperactive bowel sounds. Plain abdominal radiograph (supine and erect views) shows multiple dilated small bowel loops with a "step-ladder" appearance, and air-fluid levels at different heights on the erect film. No free air is visible under the diaphragm. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Small Bowel Obstruction ### Key Radiographic Signs **Key Point:** The "step-ladder" pattern of dilated small bowel loops with air-fluid levels at different heights on the erect radiograph is pathognomonic for small bowel obstruction. ### Radiographic Features of Small Bowel Obstruction | Feature | Appearance | Significance | |---------|-----------|---------------| | **Step-ladder sign** | Dilated small bowel loops arranged in a staircase pattern | Hallmark of SBO; caused by peristaltic waves against the obstruction | | **Air-fluid levels** | Horizontal levels at different heights (erect view) | Indicates fluid accumulation proximal to obstruction | | **Dilated loops** | >3 cm diameter for small bowel | Proximal to mechanical obstruction | | **Valvulae conniventes** | Complete transverse bands crossing the lumen | Distinguishes small bowel from colon | | **Absence of colonic gas** | No gas in colon distal to SBO | Suggests complete obstruction | ### Pathophysiology 1. Mechanical obstruction (adhesions from prior surgery in this case) 2. Proximal bowel dilates with fluid and gas 3. Peristalsis against the obstruction creates the step-ladder appearance 4. Fluid accumulates, creating air-fluid levels on upright radiography **Clinical Pearl:** The history of previous abdominal surgery is the strongest risk factor for adhesive small bowel obstruction, which accounts for ~60% of SBO cases in developed countries. **High-Yield:** On a supine radiograph alone, you may miss SBO — always obtain an erect or left lateral decubitus view to visualize air-fluid levels. This is essential for diagnosis. ### Why Plain Radiography Works Here - Sensitivity ~60–70% for SBO when both supine and erect views are obtained - Specificity is high when classic signs are present - CT is more sensitive but plain films are the initial screening tool in resource-limited settings and are often sufficient for clinical decision-making **Mnemonic: SILO** — Small bowel, Intestinal dilation, Levels (air-fluid), Obstruction