A 68-year-old woman with a 3-day history of progressive abdominal distension and constipation undergoes CT abdomen. The radiologist reports a transition point at the rectosigmoid junction with a 'bird's beak' appearance and a proximal colon dilated to 8 cm. Which imaging finding best distinguishes sigmoid volvulus from large bowel obstruction due to malignancy?

Explanation

## Sigmoid Volvulus vs Malignant Large Bowel Obstruction: Key Discriminators ### The Coffee Bean Sign: Pathognomonic for Volvulus **Key Point:** The **coffee bean sign** (or **omega sign**) is the most specific radiological finding for sigmoid volvulus and is virtually absent in malignant obstruction. ### Imaging Characteristics Comparison | Feature | Sigmoid Volvulus | Malignant LBO | |---------|-----------------|---------------| | **Coffee bean sign** | Present (pathognomonic) | Absent | | **Apex direction** | Points away from obstruction | N/A | | **Transition point** | Tight, twisted | Shouldered, irregular | | **Proximal dilatation** | Marked (>10 cm common) | Moderate (6–8 cm) | | **Wall enhancement** | Uniform, preserved | Asymmetric, may show shouldering | | **Onset** | Acute, episodic | Insidious, progressive | | **Cause** | Mechanical twist of mesentery | Infiltrating tumor | ### Understanding the Coffee Bean Sign 1. **Anatomical basis**: In sigmoid volvulus, the sigmoid colon twists on its mesentery, creating a characteristic twisted, closed-loop appearance that resembles a coffee bean on axial CT. 2. **Orientation**: The apex (point) of the coffee bean points AWAY from the site of maximal obstruction (the twisted junction). 3. **Specificity**: This sign is virtually pathognomonic for volvulus and is not seen in malignant obstruction, where the transition is typically shouldered and irregular. **High-Yield:** Coffee bean sign = sigmoid volvulus; shouldered transition = malignancy. ### Why Other Features Are Non-Discriminatory **Clinical Pearl:** While both conditions present with a transition point, the CHARACTER of that transition is the key discriminator: - **Volvulus**: Tight, twisted transition with the coffee bean sign. - **Malignancy**: Shouldered, irregular transition with shouldering (asymmetric narrowing) and possible shouldering at the margins. ### Clinical Implications ```mermaid flowchart TD A[Large Bowel Obstruction]:::outcome --> B{Coffee bean sign present?}:::decision B -->|Yes| C[Sigmoid Volvulus]:::outcome B -->|No| D{Shouldering at transition?}:::decision D -->|Yes| E[Malignancy]:::outcome D -->|No| F[Other causes: stricture, diverticulitis]:::outcome C --> G[Endoscopic decompression first-line]:::action E --> H[Surgical resection]:::action ``` **Mnemonic:** **VOLVULUS** = **V**olvulus shows **O**mega/**C**offee **B**ean; **M**alignancy shows **S**houldering. [cite:Harrison 21e Ch 297; Robbins 10e Ch 17]