A 68-year-old man with a history of abdominal surgery 10 years ago presents with acute onset colicky abdominal pain, abdominal distension, and vomiting. Plain abdominal radiograph shows multiple air-fluid levels with a 'stepladder' pattern and dilated small bowel loops. What is the most common cause of mechanical small bowel obstruction in this clinical scenario?

Explanation

## Most Common Cause of Mechanical Small Bowel Obstruction ### Epidemiology and Frequency **Key Point:** Adhesions account for 60–75% of all mechanical small bowel obstructions in the developed world, with the incidence increasing significantly after abdominal or pelvic surgery. **High-Yield:** In patients with prior abdominal surgery (as in this case), adhesions are responsible for approximately 70–80% of mechanical SBO cases. The risk increases with the number of prior surgeries and time elapsed since the operation. ### Pathophysiology Adhesions form as a result of: 1. Peritoneal trauma during surgery 2. Inflammatory response and fibrin deposition 3. Organization and fibrosis over weeks to months 4. Progressive constriction of bowel loops ### Plain Radiograph Findings in Adhesive SBO | Finding | Description | Significance | |---------|-------------|---------------| | Air-fluid levels | Horizontal levels in upright/decubitus films | Indicates transition zone | | Stepladder pattern | Progressive decrease in caliber of dilated loops | Classic for adhesions | | Small bowel dilatation | >3 cm diameter | Proximal to obstruction | | Transition zone | Abrupt change from dilated to collapsed bowel | Locates obstruction site | | Valvulae conniventes | Preserved plicae circulares | Confirms small bowel origin | **Clinical Pearl:** The stepladder pattern is particularly characteristic of adhesive obstruction because adhesions typically cause partial, intermittent obstruction that allows some bowel gas to pass distally, creating the classic appearance. ### Comparative Frequency of SBO Causes | Cause | Frequency (%) | Key Distinguishing Features | |-------|---------------|----------------------------| | Adhesions | 60–75 | History of surgery; gradual onset; recurrent episodes | | Hernias | 10–15 | Visible/palpable mass; acute onset; often irreducible | | Malignancy | 10–15 | Older age; weight loss; insidious onset; irregular narrowing | | Volvulus | 5–10 | Specific anatomical sites (sigmoid > cecal); coffee-bean sign | | Inflammatory bowel disease | 3–5 | Known IBD history; chronic symptoms | **Mnemonic:** **CHAMP** — Common causes of mechanical SBO in order of frequency: - **C**ontractures/Crohn's (inflammatory) - **H**ernias - **A**dhesions (most common overall) - **M**alignancy - **P**ancreatic/Peritoneal pathology ### Why Adhesions Are Most Common in This Case 1. **Prior surgery:** 10-year history of abdominal surgery is the strongest risk factor 2. **Presentation:** Acute colicky pain suggests intermittent obstruction, typical of adhesions 3. **Imaging pattern:** Stepladder appearance is classic for adhesive obstruction 4. **Age:** 68 years old — sufficient time for adhesion formation and maturation **High-Yield:** Post-operative adhesions can form within days but typically cause obstruction months to years after surgery. The risk remains elevated indefinitely. ### Management Implications - **Conservative management:** 70–80% of adhesive SBO resolve with nasogastric decompression and bowel rest - **Surgical intervention:** Reserved for failed conservative management, signs of strangulation, or complete obstruction - **Recurrence:** 10–25% of patients have recurrent adhesive obstruction [cite:Robbins and Cotran Pathologic Basis of Disease 10e Ch 17]