Crohn's disease can be so different in the way it behaves in different individuals. Someone with inflammation in one area of the large intestine, someone with a fistula and a child with small bowel narrowing have such different patterns that they may need very different treatment. For doctors to have a better understanding of how to treat each pattern more precisely, we now classify these Crohn's patterns by it's initial location and it's behavior (stricturing, penetrating or just inflammatory) as well as by the patient's age. 

Since some patients or family members may read or hear about these classifications, Nutrition4IBD details them for you. The original Montreal Classification was modified by the Paris Classification, mainly to be able to look at the children with Crohn's disease and their growth. It's likely that one day, the Paris Classification will be replaced by others that also add information about each person's genetics,  microbiome and what may trigger their symptoms, so that treatment can be individualized further. For right now, you can learn about the Paris Classification.

Paris Age Classification (when Crohn's disease was diagnosed)

A1a   up to 10 years old A1b   10 to 17 years old A2      17 to 40 years old A3      over 40 years old

And then did the disease affect someone's height (how tall they grew)

G1 Growth interfered with G0    Never a problem with growth 

Where the disease begins and how it behaves can then be categorized for each age group. 

What Parts of the Bowel Are Affected?

Montreal and Paris Disease Locations (when the disease was first diagnosed)

L1 is the terminal ileum (the end of the small intestine, where Dr Crohn and co-workers originally described the disease), the inflammation there is known as ileitis (sometimes the first part of the large intestine, the cecum, is also involved)

L2 is when the disease is only in the large intestine or colon (hence, this is colitis); 

L3 is when the disease is spread over both of those locations (ileocolitis); 

L4 is when the disease only occurs in the small intestine, but above the last section. 

In the Paris classification, L4 is further broken into 

L4a, Crohn's in just the esophagus, stomach and duodenum; 

L4b, Crohn's in the small intestine between the duodenum and terminal ileum.   

The Pattern of Crohn's Disease

Another important factor in understanding Crohn's disease is how it behaves, because that affects treatment decisions, perhaps more than any other disease feature. The classical way most disease shows up at the beginning is with ulcers and inflammation (Behavior B1, or non-penetrating, non-stricturing disease). Narrowing can develop with strictures forming (B2) and penetrating fistulas (B3) and the combination of both B2B3 can develop as well. 


B1     Inflammation only  (No strictures or fistulas) B2     Strictures or narrowing    B3     Fistulas  (penetrating disease) B2B3   Both strictures and fistulas

Of particular importance, the location and behavior can change over the years


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