A 68-year-old man presents to the emergency department with acute onset severe abdominal pain, abdominal distension, and vomiting of 6 hours duration. He has a history of previous abdominal surgery 15 years ago. On examination, his abdomen is markedly distended with visible peristaltic waves. Bowel sounds are high-pitched and tinkling. Plain abdominal radiograph shows multiple dilated small bowel loops (maximum diameter 4 cm) with air-fluid levels in a stepladder pattern. The colon appears collapsed. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Mechanical Small Bowel Obstruction ### Key Radiological Findings **Key Point:** The classic plain film triad of mechanical small bowel obstruction consists of: 1. **Dilated small bowel loops** (>3 cm diameter is abnormal; >4 cm is markedly dilated) 2. **Air-fluid levels** in a **stepladder pattern** (multiple levels at different heights on upright/decubitus films) 3. **Collapsed colon** (decompressed distal bowel indicates obstruction proximal to the ileocecal valve) ### Why This Is Mechanical Obstruction **High-Yield:** The stepladder air-fluid level pattern is **pathognomonic for mechanical obstruction**. Each dilated bowel loop fills with fluid and air, creating successive levels of different heights as you move distally—like a staircase. This pattern reflects the peristaltic attempts of the bowel to push contents past the obstruction. **Clinical Pearl:** The presence of a **collapsed colon** is a crucial discriminator. In mechanical small bowel obstruction, the colon distal to the ileocecal valve remains decompressed because the obstruction prevents gas from reaching it. This is different from paralytic ileus, where the entire colon is also dilated. ### Why Adhesions Are the Likely Etiology **Key Point:** In a patient with **prior abdominal surgery** (15 years ago), adhesions are the most common cause of mechanical small bowel obstruction, accounting for ~60–75% of cases in the developed world. The acute presentation with high-pitched bowel sounds and visible peristaltic waves indicates the bowel is still attempting to overcome the obstruction—a sign of **partial or early complete obstruction**. ### Differential Radiological Features | Finding | Mechanical SBO | Paralytic Ileus | Acute Gastroenteritis | |---------|---|---|---| | **Small bowel dilation** | Yes, >3 cm | Yes, mild to moderate | Minimal | | **Stepladder air-fluid levels** | **Yes, characteristic** | No (uniform levels) | Absent | | **Colonic gas** | Absent (collapsed) | **Present (dilated)** | Variable | | **Bowel wall thickness** | Normal | Normal | May be thickened | | **Clinical signs** | Peristaltic waves, high-pitched sounds | Quiet abdomen, hypoactive sounds | Variable | **Mnemonic:** **SBO = Stepladder + Bowel dilation + Obstruction** (the three S's of mechanical obstruction). ### Why Collapsed Colon Rules Out Ileus **Warning:** A common trap is confusing mechanical obstruction with paralytic ileus. Both show dilated small bowel, but: - **Mechanical obstruction**: Collapsed colon (obstruction blocks gas passage) - **Paralytic ileus**: Dilated colon (entire GI tract is atonic) In this case, the **collapsed colon is the key finding** that excludes ileus and confirms mechanical obstruction. [cite:Robbins & Cotran 10e Ch 17; Harrison 21e Ch 298]