Northern exposure sign | Radiology Signs

Answer:

The northern exposure sign is the appearance of a massively distended loop of sigmoid colon whose apex extends superior to (cephalad to) the transverse colon on the supine abdominal radiograph. It indicates sigmoid volvulus, the commonest colonic volvulus, in which a redundant sigmoid loop with a long, narrow-based mesocolon twists on itself and obstructs. In the paper that first described it, 30 patients with confirmed sigmoid volvulus were compared with 28 patients who had a dilated colon from another cause; the sign was 86% sensitive and 100% specific for sigmoid volvulus. The radiograph typically also shows the coffee bean (or bent inner-tube) appearance, a massively dilated, ahaustral loop with three converging walls that taper toward the pelvis, and a collapsed distal colon and rectum. It must be separated from caecal volvulus, which is considerably less common, occurs in younger patients with a congenitally mobile caecum rather than chronic constipation, and produces a distended loop whose walls converge toward the right iliac fossa even when the gas-filled viscus itself has migrated to the left upper quadrant or mid-abdomen. CT is the definitive test, showing the whirl sign of the twisted sigmoid mesocolon, the transition point, and, when ischaemia is present, mural thinning or thickening, pneumatosis, poor wall enhancement and free fluid. Sigmoid volvulus without evidence of perforation, peritonitis or ischaemia is treated first by endoscopic detorsion and decompression; because recurrence after decompression alone is common, elective sigmoid resection is recommended during the same admission, while urgent surgery is reserved for failed decompression or signs of gangrene or perforation.

Why is it called so?

The sigmoid loop is displaced far to the north of the pelvis, its apex rising above the transverse colon.

Pathophysiology

Sigmoid volvulus requires a redundant, hypermobile sigmoid colon on a mesocolon whose base has narrowed, most often from chronic constipation, laxative overuse, a high-residue diet, pregnancy, or chronic neuropsychiatric illness and institutionalised immobility, all of which predispose the elongated loop to twist on its narrow pedicle. As the loop rotates, usually counterclockwise, it forms a closed-loop obstruction: gas and fluid accumulate rapidly and the loop distends massively, while the mesenteric vessels running through the twisted pedicle are compressed, veins before arteries. Progressive distension and vascular compromise cause mural ischaemia, perforation and faecal peritonitis, which is why a markedly dilated ahaustral loop rising above the transverse colon warrants urgent decompression.

Alternative names: Coffee bean sign of sigmoid volvulus

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

References

1. Javors BR, Baker SR, Miller JA. The northern exposure sign: a newly described finding in sigmoid volvulus. AJR Am J Roentgenol. 1999;173(3):571-4. PMID: 10470881.