Ulcerative colitis versus Crohn’s disease: pattern and depth
Ulcerative colitis and Crohn’s disease are the two main inflammatory bowel diseases (long-term gut inflammation), and they differ in spread and depth.
Remember
Ulcerative colitis is unbroken and mainly superficial, usually starting in the rectum, whereas Crohn’s disease is patchy and can run through the whole bowel wall.
Ulcerative colitis (UC) and Crohn’s disease are both types of inflammatory bowel disease, meaning long-term inflammation of the gut. To tell them apart, think about the pattern of disease (continuous or patchy) and then its depth (surface or full thickness).
UC usually begins in the rectum (the last part of the large bowel, just above the anus) and spreads upwards through the colon in one unbroken stretch, so there are no healthy gaps along the way. The inflammation sits mainly in the mucosa (the inner lining) and the submucosa (the layer just beneath it), and the muscle layer is drawn in only when the disease is severe. Because UC inflammation is more superficial, fistulae (abnormal tunnels joining the bowel to another structure) are not a typical feature.
Crohn’s disease behaves differently. It is patchy: diseased segments, called skip lesions, are separated by stretches of normal bowel. It can affect any part of the gut from mouth to anus, most often the terminal ileum (the last part of the small bowel). In Crohn’s disease of the colon, the rectum is often spared.
The inflammation is also transmural, meaning it passes through the full thickness of the bowel wall (trans = across, mural = wall). That depth lets it break through into neighbouring structures such as other loops of bowel, the bladder, the vagina or the skin, forming fistulae, and scarring can narrow the bowel into strictures (narrowings).
Key facts
- UC pattern
- it usually begins in the rectum and spreads upwards continuously, with no skip lesions, and is confined to the colon and rectum apart from backwash ileitis (inflammation of the terminal ileum) in some patients.
- UC depth
- inflammation is mainly in the mucosa and submucosa, and the muscle layer joins in only when disease is severe.
- Crohn’s pattern
- it is patchy, with skip lesions, and can affect any part of the gut from mouth to anus, most often the terminal ileum.
- Crohn’s depth
- inflammation is transmural, which is why fistulae and strictures can develop.
- Crohn’s appearance
- through a colonoscope (which sees only the inner surface), the characteristic finding is skip lesions with deep ulcers and a cobblestone appearance (swollen lining bulging up between the deep ulcers).
- Perianal disease
- perianal (around the anus) fistulas and abscesses occur in about one in five people with Crohn’s disease, whereas UC typically presents without perianal inflammation.
- Granulomas
- these small rounded clusters of immune cells favour Crohn’s disease over UC when present, but are not found in most patients with Crohn’s disease.
Exam traps
- Granulomas do not mark out ulcerative colitis: they favour Crohn’s disease over ulcerative colitis, but they are not found in most patients with Crohn’s disease, so their absence does not exclude it.
- Continuous mucosal inflammation alone does not point to ulcerative colitis if it starts in the terminal ileum, because ulcerative colitis usually starts in the rectum and the terminal ileum is the commonest site of Crohn’s disease, which is not continuous.
- Patchy disease with normal bowel in between describes how the disease is spread, not how deep it goes, so it does not explain a fistula, which needs inflammation through the full thickness of the wall.
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