A 58-year-old man from rural Maharashtra presents with a 3-day history of colicky abdominal pain, abdominal distension, and constipation. He has had two previous laparotomies for adhesiolysis 8 and 15 years ago. On examination, the abdomen is distended with visible peristaltic waves and high-pitched bowel sounds. Plain abdominal X-ray shows dilated small bowel loops with valvulae conniventes crossing the entire width of the bowel, and a transition zone is identified. CT abdomen with IV contrast shows a transition point at the proximal ileum with collapsed distal small bowel and no obvious mass or volvulus. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Adhesive Small Bowel Obstruction ### Key Imaging Features **High-Yield:** Adhesive obstruction is the most common cause of mechanical small bowel obstruction in the developed world and accounts for ~65–75% of cases in patients with prior abdominal surgery [cite:Harrison 21e Ch 297]. ### Radiological Findings Supporting Adhesion | Finding | Significance | |---------|-------------| | **Transition zone** | Single, sharp demarcation between dilated proximal and collapsed distal bowel | | **Valvulae conniventes** | Intact mucosal folds crossing entire bowel width (rules out volvulus, intussusception) | | **No mass, no volvulus** | CT excludes mechanical causes other than adhesion | | **History of prior surgery** | Strongest risk factor for adhesive obstruction | ### Pathophysiology of Adhesions 1. Prior laparotomy causes peritoneal trauma and inflammation 2. Fibrous bands form during healing (can occur years later) 3. Bands kink or compress bowel loops → obstruction 4. Most common site: proximal ileum (as in this case) **Clinical Pearl:** Adhesions are the **only diagnosis of exclusion** in small bowel obstruction — imaging must rule out volvulus, intussusception, hernia, and malignancy before adhesion is confirmed. ### Why Imaging Alone Cannot Definitively Diagnose Adhesion **Key Point:** Adhesions are **radiolucent** and invisible on imaging. The diagnosis is made by: - Excluding other mechanical causes - Identifying a transition zone - Confirming prior abdominal surgery - Clinical response to conservative management (NG decompression, IV fluids) ### Management Algorithm ```mermaid flowchart TD A[Small bowel obstruction]:::outcome --> B{Prior abdominal surgery?}:::decision B -->|Yes| C[Suspect adhesion]:::action B -->|No| D[Look for other causes]:::action C --> E[CT to exclude volvulus, mass, hernia]:::action E --> F{Transition zone present?}:::decision F -->|Yes, no other cause| G[Adhesive obstruction]:::outcome F -->|No, or other findings| H[Alternative diagnosis]:::outcome G --> I[Conservative management: NG tube, IV fluids, rest]:::action I --> J{Resolves in 48–72 hrs?}:::decision J -->|Yes| K[Discharge, avoid repeat surgery]:::action J -->|No| L[Surgical exploration]:::urgent ``` **High-Yield:** ~70% of partial adhesive obstructions resolve with conservative management alone; surgery is reserved for complete obstruction or failure of conservative care.