On CT imaging of small bowel obstruction, which finding indicates transition from dilated to collapsed bowel and helps identify the site of obstruction?

Explanation

## Transition Point in Small Bowel Obstruction — CT Imaging **Key Point:** The **transition point** is the critical CT finding that identifies the anatomical site of small bowel obstruction, marked by the abrupt change from dilated proximal bowel to collapsed distal bowel. ### Definition and Significance The transition point represents the location where the obstructing lesion is present. On CT, it appears as: - Abrupt change in bowel caliber - Proximal bowel dilated (>3 cm for jejunum, >2.5 cm for ileum) - Distal bowel collapsed (<1.5 cm) - Sharp demarcation between the two zones ### Clinical Importance **High-Yield:** Identification of the transition point on CT is essential for: 1. **Confirming the diagnosis** of mechanical bowel obstruction (vs. ileus) 2. **Localizing the obstruction** — helps determine the likely cause and guides surgical planning 3. **Assessing for complications** — allows evaluation for ischaemia, perforation, or closed-loop obstruction ### CT Findings at the Transition Point | Finding | Implication | |---------|-------------| | **Single transition point** | Simple obstruction (adhesion, hernia, stricture) | | **Multiple transition points** | Closed-loop obstruction or volvulus | | **Transition with fat stranding** | Suggests ischaemia or inflammation | | **Transition with wall thickening** | May indicate malignancy or inflammatory cause | ### Other Volvulus-Specific Signs (Not the Transition Point) **Whirlpool sign** — spiral arrangement of mesentery and bowel loops seen in volvulus (a specific cause of obstruction, not the transition point itself). **Barber pole sign** — alternating bands of dilated and collapsed bowel in sigmoid volvulus. **Cascade sign** — waterfall-like appearance of dilated small bowel loops. **Clinical Pearl:** While plain X-ray may show signs of obstruction (air-fluid levels, dilated loops), only CT can reliably identify the transition point and determine the cause of obstruction in most cases. [cite:Robbins 10e Ch 17]