Comb sign | Radiology Signs

Answer:

The comb sign is a row of dilated, tortuous vasa recta running perpendicular to the mesenteric border of an inflamed bowel loop, spaced like the teeth of a comb. On contrast-enhanced CT or MR enterography the engorged vessels are seen as multiple parallel tubular structures penetrating the mesenteric fat and entering the thickened bowel wall, with widened spacing between them because of accompanying fibrofatty proliferation. It is one of the most specific signs of active Crohn disease and correlates with endoscopic severity, histological inflammation and raised C-reactive protein, and it usually appears alongside mural hyperenhancement, wall thickening beyond 3 mm, mural stratification and increased mesenteric fat attenuation. Its main value is in separating active inflammatory disease, which may respond to medical therapy, from established fibrostenotic disease. It is not unique to Crohn disease and is reported in vasculitis, mesenteric venous thrombosis, strangulating obstruction and, less often, ulcerative colitis.

Why is it called so?

The parallel engorged vessels arrayed along the mesenteric border resemble the teeth of a comb.

Pathophysiology

Active transmural inflammation of the bowel wall generates local hyperaemia. The vasa recta supplying the affected segment dilate and elongate to carry the increased flow, and they become visible individually on contrast-enhanced imaging because their calibre now exceeds the resolution threshold. The associated fibrofatty proliferation of the mesentery separates the vessels from one another and displaces the loop away from its neighbours, so the engorged vessels stand out against a background of proliferated fat rather than being crowded together. Resolution of inflammation reverses the hyperaemia, which is why the sign tracks disease activity rather than chronic damage.

Alternative names: Vascular jejunisation, engorged vasa recta

Other associated named signs: Creeping fat and fibrofatty proliferation, target or double halo sign of mural stratification, string sign of Kantor, and the cobblestone appearance

References

1. Sakurai T, Katsuno T, Saito K, Yoshihama S, Nakagawa T, Koseki H, et al. Mesenteric findings of CT enterography are well correlated with the endoscopic severity of Crohn’s disease. Eur J Radiol. 2017;89:242-248. PMID: 28188060.