Necessity of phenotypic classification of inflammatory bowel disease.

Inflammatory bowel diseases (IBD) are classically divided in Crohn's disease (CD) and ulcerative colitis (UC). However, these two entities are still heterogeneous and a further classification in subphenotypes is necessary. Clinical subphenotypes are easy to use, do not necessitate complicated tests and can already give very important information for the management of the patients. In CD, clinical subphenotypes are based on age at diagnosis, disease location and disease behaviour. Age at diagnosis allows to differentiating paediatric CD, classical young adult onset and more seldom CD of the elderly. These categories are associated with a different risk of development of complications and disabling disease and may have partly different pathophysiology. The classification on disease behaviour, including stricturin, penetrating or uncomplicated disease may have an impact on reponse to medical treatment and need for surgery. Finally the classification based on location is particularly relevant since it has been associated with different types of complications. Particularly ileal disease has been associated with the risk of surgery and colonic (particularly rectal) disease, with the risk of perianal disease. In UC, the classification in subphenotypes is essentially based on disease location, distinguishing proctitis, left-sided colitis and extensive colitis. This subclassification also has a very significant clinical relevance since extensive colitis has been associated with and increased risk of colon cancer, colectomy and even in some studies, mortality.