IBD Curriculum Topic 3.2

Investigations are performed at diagnosis, and subsequent intervals, to establish disease extent and activity, noting whether stenosis or penetrating disease (fistula) have occurred.

5% of patients diagnosed with Crohn’s disease will be re-classified as having ulcerative colitis within 12 months; colitis cannot be classified in about 10% of patients with IBD, also known as indeterminate colitis.

The Montréal classification, disease activity (e.g. CDAI or Harvey-Bradshaw index) and predictors of a complicated natural history should be defined at disease onset to guide initial therapy. 


  • Disease activity - classified as mild, moderate or severe
  • Remission – CDAI < 150
  • Response – reduction in CDAI >/= 100
  • Relapse – increase in CDAI by at least 70-100 points, to a level >150
  • Steroid-refractory – active disease despite >4 weeks high dose steroid therapy
  • Steroid-dependent – requirement for at least prednisolone 10mg/d or budesonide 3mg/d for at least 3 months to maintain remission, or relapse within 3 months of stopping steroid therapy.
  • Recurrence – re-appearance of lesions following surgical resection. Endoscopic recurrence in the neo-terminal ileum is classified using the Rutgeert’s score.
  • Localised disease – a total of </= 30cm of disease; usually ileo-caecal
  • Extensive disease – a total of >/= 100cm of disease

Fistulating perianal disease

There is no consensus for classifying perianal fistulae in Crohn’s disease. In clinical practice most experts use a classification of simple or complex. Simple fistulas are superficial or inter-sphincteric; and complex fistulae are trans-, supra- or extra-sphincteric.

Although complex fistula may have multiple external openings, MRI is usually required to accurately classify the fistula. When examining the perineum, also note the presence of skin tags, anal fissures, suspected abscesses and ano-rectal strictures.

Consider the possibility of malignancy in a chronic non-healing fistula, requiring biopsy of the tract.

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