Crohn’s disease may involve the entire gastrointestinal tract from mouth to perianal area and there can be areas of involved bowel separated by normal bowel, known as skip lesions.
The full thickness of the bowel wall can be involved, and this can result in scarring and narrowing of the bowel and tracts communicating between the bowel and other areas (fistulae and perforations).
The majority of patients with Crohn’s have small bowel involvement, usually in the end part of the small bowel known as the terminal ileum.
One third of patients have inflammation limited to this area, 50 percent of patients have involvement of both the ileum and colon, and 20 percent have disease limited to the colon.
A small percentage of patients have predominant involvement of the mouth, oesophagus, stomach or upper small bowel (duodenum). Crohn’s disease affects the area around the anus in one third of patients.