Montreal classification of inflammatory bowel disease

Last revised by Hao Ting (Jacky) Liao on 9 Jan 2023

The Montreal Classification of inflammatory bowel disease is used primarily to classify the severity of ulcerative colitis and Crohn disease based on specific categories.

This can then be used to guide treatment, discern risk of complications as well as facilitate epidemiological studies 1. There have been different classifications throughout the years, which have been revised to better capture the dynamic nature of inflammatory bowel disease.

The Montreal classification system's utility in Crohn disease lies mostly in epidemiological studies to classify the type of Crohn disease an individual is afflicted with. Clinically, it allows stratification of individuals with regard to their risk of disease progression and complications. Severity for treatment purposes is primarily based on the Crohn Disease Activity Index, which identifies disease activity based on a point system, as well as biochemical testing 2,3,

With regard to ulcerative colitis, its classification can be used to determine severity of disease which can be then used to guide treatment 4.

Crohn disease has three categories, including age of onset, location and behavior. Age of onset and location are descriptive in nature, whereas an increase in subcategory in behavior provides an indicator of severity of the disease and increases in sub-category reflects an increase in severity; similarly with ulcerative colitis.

  • age of onset

    • A1: ≤16 years

    • A2: 17-40 years

    • A3: > 40 years

  • location

    • L1: terminal ileum

    • L2 : colon

    • L3: ileocolon

    • L4: upper gastrointestinal

      • L4 is a modifier that can be added to L1,2 and L3 when concomitant upper gastrointestinal disease is present

  • behavior

    • B1: non-stricturing, non-penetrating

    • B2: stricturing

    • B3: penetrating

    • P: perianal

      • P is a modifier that can be added to B1, 2 and 3 when concomitant perianal disease is present

  • extent

    • E1: ulcerative proctitis; involvement limited to rectum (rectosigmoid junction)

    • E2: left sided ulcerative colitis: involvement limited to portion of colorectum distal to splenic flexure

    • E3: extensive ulcerative colitis: involvement extends proximal to splenic flexure

  • severity

    • S0: ulcerative colitis in clinical remission; no symptoms of UC

    • S1: mild UC ≤ 4 bloody stools daily, lack of fever, pulse <90 bpm, hemoglobin >105g/L, ESR < 30mm/hr

    • S2: moderate ulcerative colitis: > 4-5 stools daily but with minimal signs of systemic toxicity

    • S3: severe ulcerative colitis: ≥ 6 bloody stools daily, pulse > 90 bpm, temperatures > 37.5°C, hemoglobin < 105 g/L, ESR > 30 mm/hr

The Rome classification in 1991 proposed by the International Working Party suggested a classification system based on disease extent, anatomical location, behavior and operative history 5,6. This was subsequently revised in Vienna at the World Congress of Gastroenterology in 1998, which introduced age of onset as well as including anatomical location and disease behavior 6.

This was further refined in 2005 to be what the Montreal classification is today. The modifications made with regards to Crohn Disease included addition of an age group for <16 years, which allows classification for pediatric cases.

It also further defines location and behavior as it was found that upper gastrointestinal cases and perianal disease were not mutually exclusive and could exist in conjunction with other locations and disease behaviors 1,7.

Ulcerative colitis did not have specific sub-classifications listed in the Vienna and Rome classifications, which was introduced with the Montreal classification system. The existing limitation regarding these classifications is that it captures a snap-shot of the disease state as opposed to longitudinally, which is important given the dynamic nature of inflammatory bowel disease 1.

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