A 72-year-old woman from Mumbai presents with acute left lower abdominal pain, fever (38.5°C), and leukocytosis (WBC 14,000/μL). She reports a history of chronic constipation and diverticulosis noted on colonoscopy 2 years ago. CT abdomen with IV contrast shows air in the paracolic gutter on the left side, with focal wall thickening and a small outpouching at the sigmoid colon. There is adjacent fat stranding and a small fluid collection (2 cm). Which of the following is the most likely diagnosis and the key imaging sign that distinguishes this from other causes of pneumoperitoneum?

Explanation

## Imaging Findings in Perforated Diverticulitis ### Clinical and Imaging Correlation **Key Point:** Perforated diverticulitis presents with **air in the paracolic gutter** (ipsilateral to the affected colon), **focal diverticular outpouching**, **colonic wall thickening**, and **adjacent fat stranding**. This constellation of findings is pathognomonic for perforated diverticulitis. ### Characteristic CT Findings in Perforated Diverticulitis | Feature | Perforated Diverticulitis | Significance | |---------|---------------------------|---------------| | **Air location** | Paracolic gutter (same side as affected colon) | Localized to colonic segment | | **Diverticular outpouching** | Visible focal outpouching with air inside | Diagnostic hallmark | | **Wall thickening** | Focal at diverticular site | Indicates inflammation/perforation | | **Fat stranding** | Adjacent to affected colon | Pericolonic inflammation | | **Fluid collection** | Small to moderate, localized | Abscess formation | | **Mesenteric involvement** | Usually limited to affected segment | Distinguishes from diffuse perforation | ### Why This Case Is Perforated Diverticulitis 1. **Age and risk factors**: Elderly patient with known diverticulosis (major risk factor). 2. **Location**: Left lower quadrant pain and left-sided findings (sigmoid colon is most common site for diverticulosis in Western populations). 3. **Imaging hallmark**: The **focal diverticular outpouching** with air inside is pathognomonic — this is the actual perforated diverticulum. 4. **Paracolic air**: Air in the paracolic gutter (rather than anterior to liver or diffusely distributed) is characteristic of colonic perforation. 5. **Localized inflammation**: Small fluid collection and fat stranding indicate contained perforation, typical of diverticulitis. ### Differential Imaging Patterns: Perforation Sites ```mermaid flowchart TD A[Pneumoperitoneum on Imaging]:::outcome --> B{Air location & morphology?}:::decision B -->|Anterior to liver<br/>Focal antral/pyloric defect| C[Perforated PUD]:::action B -->|Paracolic gutter<br/>Diverticular outpouching<br/>Colonic wall thickening| D[Perforated Diverticulitis]:::action B -->|Right paracolic gutter<br/>Appendiceal wall defect<br/>Periappendiceal collection| E[Perforated Appendicitis]:::action B -->|Diffuse peritoneal air<br/>Multiple dilated loops<br/>Mesenteric stranding| F[Perforated Small Bowel<br/>or Trauma]:::action C --> G[Hemodynamically stable?]:::decision D --> H[Contained perforation?]:::decision G -->|Yes| I[Conservative management<br/>possible]:::action H -->|Yes| J[Percutaneous drainage<br/>+ antibiotics]:::action ``` ### High-Yield Imaging Distinctions **High-Yield:** The **diverticular outpouching** visible on CT is the single most specific finding for diverticulitis perforation. Unlike peptic ulcer perforation (which shows a focal wall defect in the gastric antrum), diverticulitis shows an actual outpouching of the colon with air inside. **Clinical Pearl:** Perforated diverticulitis with a **small, localized fluid collection** (< 3 cm) and **contained air** can often be managed conservatively with IV antibiotics and percutaneous drainage if needed. This is in contrast to free perforation with diffuse peritonitis, which requires urgent surgical intervention. ### Imaging Findings by Perforation Site | Perforation Site | Air Location | Wall Finding | Associated Features | |------------------|--------------|--------------|---------------------| | **Gastric/Duodenal (PUD)** | Anterior to liver, lesser sac | Focal antral/pyloric defect | Sentinel clot, lesser omental air | | **Sigmoid/Descending Colon (Diverticulitis)** | Left paracolic gutter | Diverticular outpouching | Fat stranding, small abscess | | **Appendix** | Right paracolic gutter | Appendiceal wall defect | Periappendiceal collection, RLQ pain | | **Small Bowel** | Diffuse peritoneal air | Focal bowel wall defect or none | Mesenteric stranding, dilated loops | | **Colon (non-diverticular)** | Paracolic/pelvic air | Focal colonic wall defect | Feculent material, large abscess | **Warning:** Do not confuse **pneumatosis intestinalis** (air in the bowel wall, seen in ischemia or necrotizing enterocolitis) with **pneumoperitoneum** (free air in the peritoneal cavity). Pneumatosis appears as lucency within the bowel wall itself, not as free air.