By Haley Masterson

The two types of Inflammatory Bowel Disease are both idiopathic diseases of the immune system. However, they differ in several commonly tested ways that can be explained using a simple mnemonic: Only GALS can be Crones (Crohn’s).

Granulomas – While a lack of granulomas won’t rule out Crohn’s Disease, if a test question mentions them, you should be thinking about Crohn’s. Granulomas don’t appear in Ulcerative Colitis.

All Layers – Crohn’s affects all of the layers of the bowel– it’s a transmural disease, whereas Ulcerative Colitis is confined to the mucosa (with occasional submucosa inflammation). Only Crohn’s has muscularis and serosa involvement.

You can also think of “All Layers” as being “All levels” of the bowel – Crohn’s can affect anything from the ileum (the most commonly involved segment) to the sigmoid colon, whereas Ulcerative Colitis is generally limited to colonic lesions. This becomes easy to remember when you consider that Ulcerative Colitis has “colon” right in its name.

Skip lesions – When skip lesions are either described or pictured on a gross specimen, you should be thinking of Crohn’s disease. Ulcerative Colitis is a continuous disease, whereas only Crohn’s can be present in multiple areas of the bowel with sharply demarcated areas in-between.

There are, of course, other differences between the two diseases. For example, Crohn’s can cause fistulas. But, to remember this, just consider the transmural (“all layers”) involvement of Crohn’s – after all, in order to cause a fistula, there must be complete penetration of the intestinal layers. In addition, Crohn’s shows rectal sparing, whereas Ulcerative Colitis always involves the rectum.

So if an IBD question refers to a continuous lesion with no granulomas and only mucosal involvement, you should think about Ulcerative Colitis. But if the lesion contains granulomas, has transmural involvement (all layers), and sharply demarcated “skip lesions”? Think Crohn’s.

 

Bibliography

1. Kumar, Vinay, and Stanley L. Robbins. Robbins Basic Pathology. 8th ed. Philadelphia, PA: Saunders/Elsevier, 2007.

2. Le, Tao, and Vikas Bhushan. First Aid for the USMLE Step 1 2010. New York: McGraw-Hill Medical, 2010.