Inflammation or ulceration of mucosa initiates from the superjacent area of the anal canal, and is distributed in a continuous and symmetrical manner. Skip lesions are not commonly found. The extent of lesions may vary from those limited in the rectum to those extended to the entire colon. The boundary between the lesion area and the normal mucosa is clearly delineated.

According to a recent study, among the prospective cohort of patients with ulcerative colitis who showed no signs of Crohn’s disease, approximately 44% developed skip lesions and about 13% showed lesions that did not involve the rectum [3]. In addition, there were other cases in which patients with ulcerative proctitis or left colitis have inflammation only in the region surrounding the appendiceal orifice, without involving the rest of the colon. The appendiceal orifice inflammation (AOI) also shows erythema, edema, granularity, fragility, and ulceration. Among the several reports to evaluate the prevalence of AOI in total colonoscopy, Byeon et al. reported 51% (48/98) prevalence in newly-diagnosed distal UC patients [4]. The clinical significance of how inflammation around the appendiceal orifice varies is controversial [5].

Generally, the small intestine is not involved in UC, but backwash ileitis, which is defined by endoscopic and/or histological inflammation extending from the cecum into the terminal ileum, commonly by a few centimeters, is observed among 10% of patients with diffuse colitis. Although its etiology is still unknown, its association with concurrent primary sclerosing cholangitis has been suggested [6]. Whether patients who have backwash ileitis are more likely to develop colon cancer is still controversial [6].

Despite the recent high-quality endoscopic imaging, pathological findings still play an important role in diagnosing IBD. Neumann et al. introduced endocytoscopy as a new endoscopic imaging modality to evaluate in-vivo microscopic imaging within the mucosal layer of the IBD patients at a magnification up to 1,400-fold. They reported high concordance between endocytoscopy and histopathology for grading intestinal disease activity [7].