A 58-year-old woman with a history of colorectal cancer presents with progressive abdominal distension and constipation. CT abdomen shows dilated colon with a transition point at the rectosigmoid junction. What is the most common site of colonic obstruction?

Explanation

## Most Common Site of Colonic Obstruction **Key Point:** The rectosigmoid junction is the most common site of colonic obstruction, accounting for approximately 50–60% of all large bowel obstructions. This is due to the narrowest luminal diameter and the highest intraluminal pressures in the colon. ### Anatomical Basis for Rectosigmoid Predominance 1. **Narrowest luminal diameter** — The rectosigmoid region has the smallest diameter of the entire colon 2. **Highest intraluminal pressures** — According to Laplace's law, pressure is inversely proportional to radius; smaller diameter = higher pressure 3. **Increased muscular tone** — The rectosigmoid has more pronounced muscular contractions 4. **Angulation** — The rectosigmoid junction has a natural anatomical bend that predisposes to obstruction ### Sites of Colonic Obstruction: Frequency Distribution | Site | Frequency | Common Causes | |------|-----------|---------------| | **Rectosigmoid junction** | 50–60% | Colorectal cancer, diverticulitis, stricture | | **Splenic flexure** | 10–15% | Volvulus, external compression | | **Hepatic flexure** | 5–10% | Malignancy, adhesions | | **Caecum** | 10–15% | Volvulus, ileocaecal pathology | ### Clinical Pearl **High-Yield:** The rectosigmoid junction is not only the most common site of obstruction but also the most common site of **volvulus** in the colon. Sigmoid volvulus accounts for 5–10% of all large bowel obstructions and is particularly common in elderly patients with chronic constipation. ### Imaging Features at Rectosigmoid Obstruction - **Dilated proximal colon** — Caecum may dilate to >12 cm (risk of perforation) - **Transition point** — Abrupt narrowing at rectosigmoid junction - **Collapsed distal rectum** — Suggests mechanical obstruction - **"Bird's beak" appearance** — Characteristic of volvulus on contrast studies - **Haustra** — Preserved proximal to obstruction, lost in transition zone ### Mnemonic: SHFC **S** — **Sigmoid/Rectosigmoid** (most common, 50–60%) **H** — Hepatic flexure (5–10%) **F** — Splenic flexure (10–15%) **C** — Caecum (10–15%) **Tip:** When you see colonic obstruction on imaging, always look for the transition point. If it is at the rectosigmoid junction, think of colorectal cancer (especially in older patients) or sigmoid volvulus (especially in elderly with constipation). The narrower the lumen at the obstruction site, the higher the risk of perforation and the more urgent the intervention. [cite:Harrison 21e Ch 298]