A 72-year-old woman with diverticular disease presents with acute left lower abdominal pain, fever, and peritonitis. CT imaging reveals free air in the pelvis and left paracolic gutter. Which segment of the colon is the most common site of perforation in diverticular disease?

Explanation

## Most Common Site of Diverticular Perforation **Key Point:** The sigmoid colon is the most common site of perforation in diverticular disease, accounting for >90% of perforated diverticula in Western populations. ### Why the Sigmoid Colon? #### Anatomical Factors 1. **Highest intraluminal pressure** - The sigmoid colon has the smallest luminal diameter - By Laplace's law: Wall tension = (Pressure × Radius) / 2 - Smaller radius → higher wall tension for the same pressure - Diverticula are more likely to form and rupture in high-pressure zones 2. **Taeniae coli arrangement** - The sigmoid colon has three longitudinal muscle bands (taeniae) - Diverticula form at weak points between taeniae where blood vessels penetrate - These weak points are abundant in the sigmoid region 3. **Increased muscular contractions** - The sigmoid colon exhibits more frequent and forceful contractions - This increases intraluminal pressure and mechanical stress on diverticula ### Comparison of Sites in Diverticular Disease | Site | Frequency | Reason for Rarity | |------|-----------|-------------------| | **Sigmoid colon** | >90% | Smallest diameter, highest pressure, most diverticula | | **Descending colon** | 5–10% | Larger diameter, lower pressure | | **Caecum** | <2% (Western); 70% (Asian) | Larger diameter; common in right-sided diverticulosis in Asia | | **Rectosigmoid junction** | <1% | Rectal muscular support | **Clinical Pearl:** In Asian populations, right-sided (caecal and ascending colon) diverticulosis is more common, but even there, when perforation occurs, the sigmoid is still involved in a significant proportion of cases. ### Imaging Findings in Sigmoid Perforation ```mermaid flowchart TD A[Acute diverticulitis with perforation]:::outcome --> B{Imaging modality?}:::decision B -->|CT abdomen/pelvis| C[Free air in pelvis and paracolic gutters]:::action B -->|Plain X-ray| D[Pneumoperitoneum on upright film]:::action C --> E[Sigmoid wall thickening + diverticula visible]:::outcome D --> F[Rigler's triad: free air, free fluid, bowel wall thickening]:::outcome E --> G[Surgical intervention indicated]:::urgent F --> G ``` **High-Yield:** Sigmoid perforation → left lower quadrant pain + left paracolic free air + peritonitis = surgical emergency. ### Why Other Sites Are Uncommon **Descending colon:** Larger diameter and lower intraluminal pressure make diverticular perforation rare. When diverticulosis affects the descending colon, it is usually uncomplicated. **Caecum:** In Western populations, caecal diverticulosis is rare (<5% of cases). In Asian populations, right-sided diverticulosis is more common, but perforation is still less frequent than sigmoid perforation. **Rectosigmoid junction:** The rectum has a different blood supply and muscular anatomy. Diverticula are uncommon here, and perforation is exceptionally rare. **Mnemonic:** **SIG-HIGH** = SIGmoid colon has HIGHest pressure and most perforations.