A 28-year-old woman from Mumbai with a 3-year history of Crohn disease now presents with acute abdominal pain, fever, and a palpable mass in the right lower abdomen. She has had multiple prior episodes of inflammation. Which investigation is most appropriate to confirm the suspected complication and guide management?

Explanation

## Investigation of Crohn Disease Complications **Key Point:** High-resolution CT abdomen and pelvis with IV contrast is the investigation of choice for detecting and characterizing complications of Crohn disease, particularly intra-abdominal abscess and fistulas. ### Clinical Context: Why CT? The patient presents with: - Fever + acute abdominal pain (suggests infection/abscess) - Palpable right lower abdominal mass (suggests localized collection or fistula) - Chronic CD history (high risk for transmural complications) These features suggest an **intra-abdominal abscess** or **enterocutaneous fistula**—both common complications of Crohn disease that require imaging for diagnosis and treatment planning. ### Why High-Resolution CT is Superior ```mermaid flowchart TD A[Suspected CD complication with fever/mass]:::outcome --> B{Type of complication?}:::decision B -->|Abscess/collection| C[CT abdomen/pelvis with IV contrast]:::action B -->|Fistula/sinus tract| C B -->|Obstruction/perforation| C C --> D{CT findings}:::decision D -->|Abscess| E[Percutaneous drainage ± antibiotics]:::action D -->|Fistula| F[Surgical or medical management]:::action D -->|Perforation| G[Emergency surgery]:::urgent ``` **High-Yield:** CT with IV contrast can: 1. Detect and localize intra-abdominal abscess (sensitivity >90%) 2. Characterize fistulas and sinus tracts 3. Assess for perforation and peritonitis 4. Guide percutaneous drainage if needed 5. Evaluate for obstruction and strictures **Clinical Pearl:** In acute CD with fever and a palpable mass, imaging is essential before colonoscopy. Colonoscopy is contraindicated in suspected perforation or acute toxic colitis and may worsen abscess formation if not localized first. ### Comparison: Investigation Modalities for CD Complications | Investigation | Sensitivity for Abscess | Sensitivity for Fistula | Guides Intervention? | Can Assess Perforation? | | --- | --- | --- | --- | --- | | **Plain X-ray** | <20% | No | No | Limited | | **CT (IV contrast)** | >90% | 85–95% | Yes (drainage planning) | Yes | | **MR enterography** | 85–90% | 90–95% | Moderate | Yes | | **Colonoscopy** | No | No | No | Contraindicated if acute | | **Biomarkers** | No | No | No | No | **Mnemonic:** **FISTULA** — Fistula, Infection (abscess), Stricture, Toxic megacolon, Ulceration, Lymphadenopathy, Abscess — all are complications of CD that CT can detect. [cite:Harrison 21e Ch 297]