Editing Ulcerative colitis (section)

====Extent of involvement====
[[File:Classification of Colitis.jpg|thumb|Classification of colitis, often used in defining the extent of involvement of ulcerative colitis, with proctitis (blue), proctosigmoiditis (yellow), left sided colitis (orange) and pancolitis (red). All classes extend distally to the end of the rectum.]]
[[File:Severe ulcerative colitis.jpg|thumb|Gross pathology of normal colon (left) and severe ulcerative colitis (right), forming pseudopolyps (smaller than the cobblestoning typically seen in Crohn's disease), over a continuous area (rather than skip lesions of Crohn's disease), and with a relatively gradual transition from normal colon (while Crohn's is typically more abrupt).]]
In contrast to Crohn's disease, which can affect areas of the gastrointestinal tract outside of the colon, ulcerative colitis is usually confined to the colon. Inflammation in ulcerative colitis is usually continuous, typically involving the rectum, with involvement extending proximally (to [[sigmoid colon]], ascending colon, etc.).<ref name=ACG_Guidelines_2019 /> In contrast, inflammation with Crohn's disease is often patchy, with so-called "skip lesions" (intermittent regions of inflamed bowel).<ref name="Feuerstein_Crohns">{{cite journal | vauthors = Feuerstein JD, Cheifetz AS | title = Crohn Disease: Epidemiology, Diagnosis, and Management | journal = Mayo Clinic Proceedings | volume = 92 | issue = 7 | pages = 1088–1103 | date = July 2017 | pmid = 28601423 | doi = 10.1016/j.mayocp.2017.04.010 | s2cid = 20223406 | doi-access = free }}</ref>

The disease is classified by the extent of involvement, depending on how far the disease extends:<ref name=":1" /> [[proctitis]] (rectal inflammation), left sided colitis (inflammation extending to descending colon), and extensive colitis (inflammation proximal to the descending colon).<ref name=ACG_Guidelines_2019 /> Proctosigmoiditis describes inflammation of the rectum and sigmoid colon. Pancolitis describes involvement of the entire colon, extending from the rectum to the cecum. While usually associated with Crohn's disease, [[ileitis]] (inflammation of the ileum) also occurs in UC. About 17% of individuals with UC have ileitis.<ref>{{cite journal | vauthors = Haskell H, Andrews CW, Reddy SI, Dendrinos K, Farraye FA, Stucchi AF, Becker JM, Odze RD | title = Pathologic features and clinical significance of "backwash" ileitis in ulcerative colitis | journal = The American Journal of Surgical Pathology | volume = 29 | issue = 11 | pages = 1472–1481 | date = November 2005 | pmid = 16224214 | doi = 10.1097/01.pas.0000176435.19197.88 | s2cid = 42108108 }}</ref> Ileitis more commonly occurs in the setting of pancolitis (occurring in 20% of cases of pancolitis),<ref name=Ungaro /> and tends to correlate with the activity of colitis. This so-called "backwash ileitis" can occur in 10–20% of people with [[pancolitis]] and is believed to be of little clinical significance.<ref name="ISBN 0071599916">Fauci ''et al.'' ''Harrison's Internal Medicine'', 17th ed. New York: McGraw-Hill Medical, 2008. {{ISBN|978-0-07-159991-7}}</ref>