Inflammatory bowel diseases (IBD) are commonly defined as either Crohn’s disease or ulcerative colitis. There is substantial overlap, however, in clinical symptoms, genetics, and treatment response between the two. As such, it is not uncommon for your physician to change your diagnosis based on development of new clinical information over time. There are several factors that can help distinguish between the two diseases. In ulcerative colitis, inflammation only occurs in the colon; the area of inflammation is also continuous extending from the anus to an area in the colon where the inflammation ceases and there is normal-appearing colon. Crohn’s disease may also involve only the colon, termed Crohn’s colitis, which makes distinguishing between the two diseases difficult. In patients who have features of both Crohn’s colitis and ulcerative colitis, they are often given the diagnosis of “indeterminate colitis.” Crohn’s disease is more likely; however, if there is inflammation in the small bowel or upper GI tract, the inflammation occurs in a patchy distribution, or a patient has perianal disease (anal fistulas, fissures, or inflamed skin tags). In addition, long-standing bowel inflammation in Crohn’s disease may result in bowel narrowings, termed strictures, or abscesses adjacent to the bowel from connections, termed. There is also an interesting association between smoking and IBD. Although the majority of patients with Crohn’s disease are nonsmokers, patients who smoke and are diagnosed with IBD are much more likely to have Crohn’s disease, whereas ulcerative colitis is more common in former smokers and nonsmokers.

Traditionally, inflammatory bowel disease (IBD) has been subgrouped into two diseases, Crohn’s disease (CD) and ulcerative colitis (UC). As the clinical presentation, endoscopic findings, disease course, and treatment response are heterogeneous in both CD and UC with significant overlap in both diseases, however, grouping IBD into two subtypes is likely an oversimplification of numerous distinct, yet related diseases. This is reflected in genome-wide association studies which have identified over 160 loci associated with IBD, many of which overlap between CD and UC [1]. Despite the disease heterogeneity, there are several clinical, endoscopic, histologic, and serologic clues to help distinguish between CD and UC. Still, approximately 10 % of patients whose disease cannot be differentiated between the two are diagnosed with indeterminate colitis. Although the term “indeterminate colitis” was originally proposed as a pathological diagnosis for colectomy specimens which could not discriminate between Crohn’s disease and ulcerative colitis, it has been widely adapted into a clinical classification [2].

In patients who have isolated colitis, differentiating between Crohn’s disease and ulcerative colitis can be difficult, although there are several endoscopic and histologic clues that can help set apart the two. Perianal disease involving fistulas, abscesses, fissures, or inflamed skin tags is more common in Crohn’s disease. Findings of upper GI involvement are suggestive of Crohn’s disease, although several studies have demonstrated that inflammation in the esophagus, stomach, and duodenum can be seen at the time of diagnosis in patients with UC [3]. Inflammation involving the terminal ileum is also common in CD. However, inflammation in the terminal ileum in a patient with pan-colitis is not diagnostic of CD as approximately 20 % of patients with pan-UC can have backwash ileitis [4]. Ulceration or stenosis of the ileocecal valve, ulcerations in the ileum, and granulomas on histology can aide in distinguishing between CD of the ileum and UC with backwash ileitis. Although disease distribution by endoscopy or pathology can be confounded by treatment, patchy inflammatory activity, granulomas, and inflammation extending deeper than the mucosa are also suggestive of Crohn’s disease.