A 35-year-old woman with known Crohn's disease on mesalamine presents with acute onset of severe abdominal pain, fever (38.5°C), and signs of peritonitis. Plain abdominal X-ray shows free air under the diaphragm. Which investigation should be performed immediately after clinical stabilization to determine the site and extent of perforation?

## Investigation of Perforation in Inflammatory Bowel Disease **Key Point:** High-resolution CT abdomen and pelvis with IV contrast is the imaging investigation of choice to localize perforation, assess disease extent, and identify complications (abscess, fistula) before surgical intervention. ### Why CT with IV Contrast? 1. **Localizes perforation site** — identifies the segment of bowel involved (small bowel vs. colon) 2. **Assesses disease extent** — shows distribution of inflammation, strictures, and skip lesions 3. **Detects complications** — identifies abscesses, fistulas, and free fluid 4. **Guides surgical planning** — determines whether primary repair, resection, or diversion is needed 5. **Non-invasive** — safe even in acute peritonitis, unlike endoscopy ### CT Findings in IBD Perforation | Finding | Significance | |---------|---------------| | **Free air** | Confirms perforation; already evident on plain film | | **Focal bowel wall thickening** | Identifies site of perforation | | **Loculated fluid/abscess** | May allow percutaneous drainage vs. surgery | | **Fistulous tracts** | Suggests complex CD | | **Strictures** | Indicates chronic disease; guides resection extent | **Clinical Pearl:** IV contrast is essential to assess bowel wall enhancement (inflammation) and differentiate perforation from sealed microperforation. **High-Yield:** In acute peritonitis with confirmed free air, **CT is faster and safer than diagnostic laparoscopy** and provides more anatomical detail for surgical planning. ### Why Other Options Are Inappropriate ```mermaid flowchart TD A[IBD patient with peritonitis + free air]:::outcome --> B{Immediate investigation?}:::decision B -->|CT with IV contrast| C[Localizes perforation<br/>Assesses extent<br/>Guides surgery]:::action B -->|Diagnostic laparoscopy| D[Invasive; delays definitive care<br/>Limited therapeutic value]:::urgent B -->|Barium enema| E[Contraindicated in acute<br/>perforation; risk of<br/>extraperitoneal spillage]:::urgent B -->|Repeat plain X-ray| F[No additional diagnostic<br/>information; delays imaging]:::urgent ``` **Warning:** Barium studies are absolutely contraindicated in suspected perforation — barium leakage into peritoneal cavity causes severe peritonitis and granulomatous reactions. **Mnemonic — IBD Perforation Imaging (SAFE):** - **S**ite localization → CT - **A**bscess detection → CT - **F**istula assessment → CT - **E**xtent of disease → CT [cite:Harrison 21e Ch 297]