A 72-year-old woman with a history of diverticulosis presents with acute left lower quadrant pain and fever. CT abdomen shows free air in the peritoneal cavity. At what anatomical site is perforation most commonly seen in diverticulitis?

Explanation

## Most Common Site of Diverticular Perforation **Key Point:** The sigmoid colon is the most common site of diverticulitis and perforation, accounting for 90–95% of cases in Western populations due to its unique anatomical features. ### Anatomical Basis for Sigmoid Predilection 1. **Taeniae coli arrangement**: The three longitudinal muscle bands (taeniae) are narrowest at the sigmoid, creating areas of weakness (Spigelian fascia) where blood vessels penetrate. 2. **Highest intraluminal pressure**: Laplace's law dictates that smaller-diameter segments (sigmoid) generate higher wall tension for the same pressure. 3. **Narrowest luminal diameter**: The sigmoid colon has the smallest diameter, predisposing to stasis and bacterial overgrowth. 4. **Increased muscular contractions**: The sigmoid exhibits more pronounced peristalsis, leading to higher intramural pressure. ### Imaging Findings in Sigmoid Perforation **CT Abdomen (Gold Standard)** - Free air in peritoneal cavity (pneumoperitoneum) - Focal defect or disruption in sigmoid colon wall - Diverticula with wall thickening and pericolonic fat stranding - Abscess formation (may be loculated in pelvis) - Peritoneal fluid with debris **Plain Radiograph** - Pneumoperitoneum (Rigler's sign) - Less sensitive than CT **High-Yield:** In the **right colon** (cecum, ascending colon), diverticulitis is rare in Western populations (~5%) but more common in Asian populations. However, when right-sided diverticulitis occurs, perforation risk is higher because the wall is thinner. ### Geographic Variation in Diverticular Disease | Region | Most Common Site | Frequency | |--------|------------------|----------| | Western (Europe, North America) | Sigmoid colon | 90–95% | | Asia (Japan, Singapore) | Right colon (cecum) | 70% | | India | Sigmoid > right colon | Variable | **Clinical Pearl:** The phrase "**sigmoid is the culprit**" helps recall that sigmoid diverticulitis is the most common presentation. Conversely, right-sided diverticulitis in Western patients should raise suspicion for Crohn's disease or other pathology. **Mnemonic: SIGMOID** — **S**mallest diameter, **I**ncreased pressure (Laplace), **G**reatest muscular contractions, **M**ost common site, **O**ften perforates, **I**ntestinal weakness (Spigelian fascia), **D**iverticula predilection. [cite:Robbins 10e Ch 17]