What is the difference between ulcerative colitis and Crohn's disease, and can one turn into the other?
Ulcerative colitis affects only the colon and rectum, with continuous inflammation in the inner lining. Crohn's disease can affect any part of the digestive tract, in patches, through the full thickness of the wall. They are two separate diagnoses, and one does not change into the other, although in a small number of people the diagnosis is later revised or labelled IBD-unclassified.
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Ulcerative colitis and Crohn's disease are the two main forms of inflammatory bowel disease (IBD). They share many symptoms, but they differ in three practical ways: where they affect the gut, the pattern of inflammation, and how deep it goes (Cleveland Clinic).
Where it happens
Ulcerative colitis is limited to the large intestine (colon and rectum). It usually starts in the rectum and can spread upward, but it stays in the colon. Crohn's disease can affect any part of the digestive tract, from the mouth to the anus, though it most often involves the end of the small intestine and the start of the colon (Cleveland Clinic).
The pattern and depth
In ulcerative colitis the inflammation is continuous and affects only the inner lining of the bowel. In Crohn's disease it tends to come in patches, with healthy tissue in between, and can reach through the full thickness of the bowel wall. That deeper involvement is why Crohn's is more likely to lead to narrowing, fistulas, and strictures (Mayo Clinic).
Can one turn into the other?
No. Ulcerative colitis and Crohn's disease are separate diagnoses, and one does not transform into the other. In a small number of people the features overlap and the diagnosis is hard to settle at first; doctors may call this IBD-unclassified (sometimes indeterminate colitis), and the diagnosis can be revised over time as the picture becomes clearer (Cleveland Clinic). That is a refinement of the diagnosis, not one disease becoming another.