Inflammatory Bowel Disease - Classification

Classification

The main forms of IBD are Crohn's disease and ulcerative colitis (UC).

Accounting for far fewer cases are other forms of IBD, which are not always classified as typical IBD:

• Collagenous colitis
• Lymphocytic colitis
• Ischaemic colitis
• Diversion colitis
• Behçet's disease
• Indeterminate colitis

The main difference between Crohn's disease and UC is the location and nature of the inflammatory changes. Crohn's can affect any part of the gastrointestinal tract, from mouth to anus (skip lesions), although a majority of the cases start in the terminal ileum. Ulcerative colitis, in contrast, is restricted to the colon and the rectum.

Microscopically, ulcerative colitis is restricted to the mucosa (epithelial lining of the gut), while Crohn's disease affects the whole bowel wall ("transmural lesions").

Finally, Crohn's disease and ulcerative colitis present with extra-intestinal manifestations (such as liver problems, arthritis, skin manifestations and eye problems) in different proportions.

Rarely, a definitive diagnosis of neither Crohn's disease nor ulcerative colitis can be made because of idiosyncrasies in the presentation. In this case, a diagnosis of indeterminate colitis may be made. Although a recognised definition, not all centres refer to this.