Due to the overlap in the clinical presentation of Crohn disease (CD) and ulcerative colitis (UC), imaging often has a role to play in distinguishing the two. Distinguishing features include:
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bowel involved
CD: small bowel 70-80%, only 15-20% have only colonic involvement
UC: rectal involvement 95%, with terminal ileum only involved in pancolitis (backwash ileitis)
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distribution
CD: skip lesions typical
UC: continuous disease from rectum up
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sex
CD: no sex preference
UC: male predilection
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colonic wall
fat halo sign is seen in 61% of chronic UC cases, but only in 8% of CD 2
the bowel wall is thicker in CD than in UC (when colon involved) 2
serosal surface smooth in UC (95%), irregular in CD (80%) 2
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UC: although rectal involvement is very common, perianal complications are not as frequently seen
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CD: common, seen in 82% of patients 2
stranding of ischiorectal fossa/perirectal fat (73%)
fistulas/sinus tracts (22%)
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CD: common in chronic cases
UC: not seen, as small bowel not involved
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abscess formation
CD: common, eventually seen in 15-20% of patients
UC: uncommon
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extraintestinal complications
gallstones: seen in 30-50% of CD patients 2
primary sclerosing cholangitis: more common in UC
hepatic abscess: seen in CD
pancreatitis: more common in CD
cachexia: common in CD, uncommon in UC
erythema nodosum: more common in CD compared to UC 3
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