Northern exposure sign | Radiology Signs

Answer:

The northern exposure sign is the appearance of a massively distended loop of large bowel whose apex lies superior to the transverse colon, often in the left upper quadrant, on the supine abdominal radiograph. It indicates caecal volvulus, in which a mobile caecum twists and displaces to a position far from the right iliac fossa. The radiograph typically shows a single large gas-filled viscus with a few haustral markings, dilated small bowel behind the obstruction and a collapsed distal colon, and the appearance is often described as a coffee bean or kidney-shaped loop. It must be separated from sigmoid volvulus, which is far commoner, occurs in older and institutionalised patients, and shows a loop arising from the pelvis whose apex points to the right upper quadrant with the coffee bean sign and an inferior convergence of the walls at the pelvis. CT is the definitive test, showing the whirl sign of the twisted mesentery, the transition point, the abnormal caecal position, the ileocaecal valve, and the signs of ischaemia including mural thinning or thickening, pneumatosis, poor enhancement and free fluid. Caecal volvulus is a surgical problem: unlike sigmoid volvulus it is not reliably treated endoscopically, and management is usually right hemicolectomy or caecopexy, with resection when the bowel is compromised.

Why is it called so?

The caecum is displaced far to the north of its expected position in the right iliac fossa.

Pathophysiology

Caecal volvulus requires a congenitally incomplete peritoneal fixation of the right colon, present in a minority of the population, which leaves the caecum and ascending colon on a mesentery long enough to permit rotation. The caecum twists about that mesentery, usually in an axial or organoaxial fashion, or folds anteriorly and superiorly in the variant called caecal bascule. The twist creates a closed loop, so gas and fluid accumulate and the loop distends rapidly, while the mesenteric vessels within the twist are compressed, first the veins and then the arteries. Distension plus vascular compromise leads to mural ischaemia, perforation and peritonitis, which is why delay carries high mortality and why a distended loop in an unexpected position warrants urgent CT.

Alternative names: Coffee bean sign of caecal volvulus

Other associated named signs: Coffee bean sign, the whirl sign of the twisted mesentery and the bird beak sign at the transition point

References

1. Rosenblat JM, Rozenblit AM, Wolf EL, DuBrow RA, Den EI, Levsky JM. Findings of cecal volvulus at CT. Radiology. 2010;256(1):169-75. PMID: 20574094.