Contemporary morphologic definition of backwash ileitis in ulcerative colitis and features that distinguish it from Crohn disease.

Terminal ileum (TI) sections from 250 ulcerative colitis (UC) total colectomy specimens resected during 3 periods and endoscopic TI biopsy specimens from 100 contemporary chronic UC and 100 Crohn disease (CD) patients were reviewed. The respective proportions of cases resected during the 3 periods with moderately or markedly active cecal UC were 72%, 34%, and 2% and with moderate or marked backwash ileitis (BWI), 21%, 18%, and 0%. The activity level of BWI correlated with level of cecal UC. In contemporary initial endoscopic TI biopsy specimens, 6% of chronic UC patients had BWI, all with moderately to markedly active cecal chronic UC. In CD cases, 75% had chronic or active enteritis, consisting of patchy lamina propria edema containing mildly active inflammation, crypt disarray, and focally blunted or flattened villi. Mucous gland metaplasia was present in 27% of CD biopsy specimens. BWI should be restricted to active enteritis that involves the ileum in a contiguous pattern from the cecum that has a similar or greater degree of active inflammation. Mild BWI predominantly involves the superficial mucosa in a contiguous pattern. Focal isolated ileal erosions, mucous gland metaplasia, or patchy edema with mild active inflammation are features of CD.