A 62-year-old man with a history of abdominal surgery 15 years ago presents with acute onset colicky abdominal pain, abdominal distension, and bilious vomiting for 8 hours. On examination, he is dehydrated with visible peristaltic waves. Plain abdominal X-ray shows dilated small bowel loops (diameter 4 cm) with multiple air-fluid levels in a "staircase" pattern, and minimal colonic gas. CT abdomen with IV contrast reveals transition zone with abrupt change in caliber at the distal ileum, with no evidence of mass, perforation, or ischemia. What is the most likely diagnosis?

Explanation

## Diagnosis: Adhesive Small Bowel Obstruction ### Clinical Presentation **Key Point:** The combination of prior abdominal surgery, acute colicky pain, bilious vomiting, and classic imaging findings (dilated small bowel with air-fluid levels in a staircase pattern) is pathognomonic for adhesive small bowel obstruction. ### Imaging Features | Feature | Adhesive SBO | Strangulation | Crohn's | Malignancy | |---------|--------------|---------------|--------|------------| | **Transition zone** | Abrupt, single | Abrupt with ischemic changes | Gradual, fibrotic | Shouldering, mass | | **Bowel caliber** | 3–5 cm (dilated) | Dilated with wall thickening | Thickened wall throughout | Irregular narrowing | | **Mesenteric vessels** | Normal | Engorged, whirled | Normal | Normal | | **Peritoneal fat** | Normal | Stranded | Stranded | Normal | | **Ischemic signs** | Absent | Present (late) | Absent | Absent | **High-Yield:** The "staircase" pattern of air-fluid levels on plain film and the abrupt transition zone on CT without signs of ischemia or mass are the gold standard for diagnosing uncomplicated adhesive obstruction. ### Why This Patient Has Adhesive SBO 1. **Prior surgery** — adhesions form in >70% of patients after abdominal surgery and are the leading cause of SBO in developed countries. 2. **Acute presentation** — adhesions can cause obstruction years or decades after surgery due to kinking or torsion. 3. **No ischemic signs** — absence of bowel wall enhancement loss, mesenteric stranding, or free fluid argues against strangulation. 4. **Transition zone without mass** — rules out malignancy; gradual transition would suggest Crohn's stricture. ### Management Pathway ```mermaid flowchart TD A[Adhesive SBO suspected]:::outcome --> B{Signs of strangulation?}:::decision B -->|No: Normal enhancement, no stranding| C[Conservative management]:::action B -->|Yes: Wall thickening, mesenteric stranding, free fluid| D[Urgent surgical exploration]:::urgent C --> E[NPO, IV fluids, NG tube]:::action E --> F{Resolution in 48-72 hrs?}:::decision F -->|Yes| G[Discharge, dietary advice]:::outcome F -->|No| H[Surgical intervention]:::action ``` **Clinical Pearl:** Approximately 80% of uncomplicated adhesive obstructions resolve with conservative management (bowel rest, IV hydration, nasogastric decompression). Surgery is reserved for failure to resolve, signs of strangulation, or recurrent episodes. **Tip:** On exam, look for the **transition zone** — the single point where caliber changes abruptly. A smooth, gradual transition suggests inflammatory disease (Crohn's); a shouldering or shouldering with shouldering suggests malignancy.