If the inflammation just affects the rectum that is called proctitis, or very limited disease. This occurs in about 5% of children. Another 5% have inflammation that extends back through the sigmoid and that is called proctosigmoiditis. About 70% of children have inflammation that extends throughout the colon (pancolitis) and the remainder have inflammation that extends to the splenic flexure.
The cause of UC is not known. It is thought that there is an abnormal interaction between the intestinal immune system and the stool bacteria (microbiome) that occurs in genetically pre-disposed individuals. At times it appears as if a severe infection may trigger the disease but that is fairly unusual. Having a family member with ulcerative colitis or Crohn’s disease does increase a first degree relatives chances of getting ulcerative colitis.
Crohn’s disease is a related disorder in which there can be inflammation anyplace in the gastrointestinal tract, from the mouth to the rectum. In UC inflammation is limited to the lining of the colon whereas in Crohn’s disease the inflammation can affect not only the lining but the wall and surface of the intestine as well.