A 42-year-old woman from Mumbai presents with acute severe left lower quadrant pain, fever (38.5°C), and guarding. She has a history of diverticulitis. CT abdomen with IV contrast shows a localized collection with air-fluid level adjacent to the sigmoid colon, with a small amount of free air confined to the pelvis. Which imaging finding best indicates contained perforation rather than generalized peritonitis?

Explanation

## Imaging Findings in Contained vs. Generalized Perforation ### Contained Perforation: Localized Collection with Confined Free Air **Key Point:** Contained perforation occurs when the perforation is walled off by adjacent viscera, omentum, or peritoneum before widespread spillage occurs. On CT, this manifests as a localized abscess or collection with air-fluid level, with free air confined to the immediate vicinity (e.g., pelvis in sigmoid perforation) rather than distributed throughout the abdomen. **High-Yield:** The distinction between contained and generalized perforation has critical management implications: - **Contained perforation:** May be managed conservatively with antibiotics, percutaneous drainage, and bowel rest in selected cases - **Generalized perforation:** Requires urgent surgical intervention ### CT Findings: Contained vs. Generalized Perforation | Feature | Contained Perforation | Generalized Perforation | |---------|----------------------|------------------------| | Free air distribution | Localized to one region (e.g., pelvis, paracolic gutter) | Extensive, throughout all quadrants | | Collection/abscess | Present, with air-fluid level | Absent or minimal | | Bowel wall | Focal thickening at perforation site | Diffuse thickening | | Peritoneal fluid | Localized | Widespread ascites | | Clinical urgency | May allow conservative management | Requires urgent surgery | | Etiology | Often diverticulitis, appendicitis | Peptic ulcer, trauma, malignancy | **Clinical Pearl:** In diverticulitis-related perforation, the sigmoid colon is often adherent to adjacent structures (bladder, small bowel, pelvic sidewall), which naturally contains the perforation. This is why diverticular perforation may be managed conservatively with percutaneous drainage if the patient is stable, whereas peptic ulcer perforation (which occurs in the anterior duodenum with wide peritoneal exposure) typically requires emergency surgery. ### CT Technique for Perforation Detection **Mnemonic: CAPP** — **C**ontrast-enhanced CT, **A**ir-fluid level, **P**eritoneal findings, **P**erforation site. 1. **Contrast-enhanced CT** is superior to plain films for detecting and localizing perforation 2. **Air-fluid level** within a collection indicates communication with bowel lumen 3. **Peritoneal findings** (free air, fluid, inflammation) guide management 4. **Perforation site** identification helps determine etiology and surgical approach ### Why Confined Free Air Indicates Containment When a perforation is immediately sealed by adjacent structures (omentum, peritoneal fold, visceral adhesions), free air cannot disseminate widely. It remains localized to the region of perforation. A small amount of free air in the pelvis with a localized collection is a classic sign of contained diverticular perforation, allowing for percutaneous drainage rather than emergency laparotomy. [cite:Robbins & Cotran 10e Ch 17; Harrison 21e Ch 297]