Ileitis is Crohn's Disease
Ileitis (inflammation of the ileum) is characteristically associated with Crohn's disease, not ulcerative colitis. Ulcerative colitis is restricted to the colon and does not primarily involve the ileum 1.
Anatomical Distribution Distinguishes the Two Diseases
Crohn's disease can affect any part of the gastrointestinal tract, including the ileum, while ulcerative colitis is limited to the colon 1. The key anatomical differences include:
- Crohn's disease may be defined by location as terminal ileal, colonic, ileocolic, or upper gastrointestinal, with approximately 25% having ileitis only, 25% colitis only, and 50% ileocolitis 2, 3
- Ulcerative colitis is characterized by diffuse mucosal inflammation limited to the colon, with disease extent ranging from proctitis to pancolitis, but never primarily involving the ileum 1
The Exception: Backwash Ileitis
The only scenario where ulcerative colitis involves the ileum is "backwash ileitis," which occurs in approximately 20% of patients with extensive UC or pancolitis 1, 4. However, this is critically different from primary ileitis:
- Backwash ileitis represents histological inflammation extending from the cecum into the most distal ileum in the setting of severe pancolitis 1
- This is not primary ileal disease but rather secondary involvement from severe colonic inflammation 4
- The presence of backwash ileitis does not exclude a diagnosis of UC and should not be confused with the primary terminal ileitis characteristic of Crohn's disease 1, 4
Pathological Features Confirm the Distinction
The microscopic characteristics further distinguish Crohn's ileitis from UC:
- Crohn's disease shows transmural inflammation (affecting all layers of the intestinal wall), patchy distribution with skip lesions, and may demonstrate granulomas 1, 4
- Ulcerative colitis shows mucosal/submucosal inflammation only, continuous diffuse involvement, and absence of granulomas 4, 5
- In Crohn's ileitis, inflammation is often associated with extra-ileal manifestations elsewhere in the gastrointestinal tract, with granulomas found in at least one site in 40% of cases 6
Clinical Pitfall to Avoid
Do not diagnose Crohn's disease solely based on finding ileal inflammation. While ileitis is most commonly caused by Crohn's disease, other etiologies exist including infectious diseases, spondyloarthropathies, vasculitides, ischemia, medication-induced inflammation, and neoplasms 7. The diagnosis requires integration of clinical, radiographic, endoscopic, and pathological findings demonstrating focal, asymmetric, transmural, or granulomatous features 2.