A 62-year-old man presents to the emergency department with acute onset severe abdominal pain, abdominal distension, and vomiting. Plain abdominal radiograph shows dilated small bowel loops with air-fluid levels and a transition zone. What is the most common cause of mechanical small bowel obstruction in adults in India?

## Most Common Cause of Mechanical Small Bowel Obstruction **Key Point:** Adhesions account for 60–75% of all mechanical small bowel obstructions in developed countries and remain the leading cause in India, particularly in patients with prior abdominal surgery. ### Epidemiology & Pathophysiology **High-Yield:** Adhesions form as a result of: - Previous abdominal or pelvic surgery (most common) - Peritonitis or intra-abdominal infection - Inflammatory bowel disease - Abdominal trauma Once formed, adhesions can kink, twist, or compress bowel loops, leading to obstruction without any structural defect. ### Plain Radiograph Findings in Small Bowel Obstruction | Finding | Sensitivity | Specificity | Clinical Significance | |---------|-------------|-------------|----------------------| | Dilated small bowel (>3 cm) | High | Moderate | Indicates obstruction | | Air-fluid levels | Moderate | High | Suggests mechanical obstruction | | Transition zone | High | High | Marks the site of obstruction | | Absence of colonic gas | Variable | Moderate | Complete obstruction | ### Why Adhesions Are Most Common 1. **Frequency of prior surgery:** Abdominal and pelvic procedures are extremely common (appendectomy, hysterectomy, C-section, hernia repair). 2. **Inevitable formation:** Nearly all abdominal surgery results in some degree of adhesion formation; most remain asymptomatic, but a subset cause obstruction. 3. **Recurrent episodes:** Patients with one adhesive obstruction have a 10–25% risk of recurrence. **Clinical Pearl:** Adhesions are a diagnosis of exclusion on imaging — plain radiographs cannot directly visualize adhesions. The diagnosis is suggested by: - History of prior abdominal surgery - Absence of external hernia - Absence of inflammatory or malignant features - Transition zone on imaging ### Comparison with Other Causes | Cause | Frequency | Key Distinguishing Feature | |-------|-----------|---------------------------| | Adhesions | 60–75% | Prior surgery history | | Hernia | 10–15% | Visible/palpable defect; may see herniated loop on imaging | | Crohn's disease | 5–10% | Chronic symptoms; imaging shows thickened bowel wall, skip lesions | | Volvulus | <5% | Rare; characteristic "bird's beak" or "coffee bean" sign | | Malignancy | 10–15% | Older age; weight loss; irregular narrowing on imaging | **Mnemonic:** **CHASM** — Common causes of small bowel obstruction: - **C**rohn's disease - **H**ernia - **A**dhesions ← **Most common** - **S**carring / stricture (from prior surgery or IBD) - **M**alignancy **Warning:** Do not assume volvulus or incarcerated hernia without imaging evidence of external hernia or characteristic radiographic signs (bird's beak, coffee bean). Adhesions remain statistically most likely.