Diversion colitis, also known as diversional colitis, describes non-specific inflammation of segments of colon and/or rectum which have been surgically diverted from the faecal stream after colostomy or ileostomy.

A similar condition, diversion pouchitis, manifesting after formation of continent pouches (e.g. following ileal pouch-anal anastomosis) has a very similar presentation and likely exists due to the same pathophysiology. It should be noted this entity is distinct from an acute infective pouchitis.

Epidemiology

Diversion colitis is thought to be very common following surgical diversion, affecting over 90% of patients to some degree when assessed histologically ^1,2. The condition is more likely to be symptomatic if surgery was indicated due to ulcerative colitis ³.

Clinical presentation

Most patients with diversion colitis or pouchitis are asymptomatic ³. In the ~30% of patients with symptoms, the clinical presentation can vary significantly.

When symptomatic, described clinical features include ²:

• abdominal or pelvic pain or discomfort

• anorectal pain or discomfort

• tenesmus

• anorectal discharge (mucous or bloody)

The diagnosis, grade of severity, and presence of complications are readily made on endoscopy, which is the investigation of choice in patients with suspected diversion colitis or pouchitis ^2,3. However, the endoscopic severity may not correlate well with the clinical severity ².

Complications

• stricture ^3,4

• malignancy: in patients with long-standing disease, there has found to be a low incidence of colorectal cancer in the diverted segment ^2,4

Pathology

Although the pathogenesis is not fully elucidated, one prevailing hypothesis proposes that diversion colitis and pouchitis are most likely caused a deficiency of luminal short-chain fatty acids (SCFAs) and other normal gut-produced nutrients ^3,5. SCFAs, among other nutrients, are the products of carbohydrate metabolism by normal anaerobic intestinal flora and have multiple functions within the bowel, including being an energy source for colonocytes, playing a role in mucosal blood flow, and being important for normal colonic motility ^3,5.

In patients with diversion colitis or pouchitis, it is thought that a deficiency of these compounds develops due to the absence of the normal faecal stream, which leads to negative down-stream effects on colonic function, predisposing it to inflammation ^3,5. However, while this theory is popular, it is likely the pathogenesis involves multiple other factors as well ^3,5.

Microscopic appearance

Colonic mucosa in diversion colitis reveals overt lymphoid follicular hyperplasia with prominent germinal centres which is the most characteristic finding. Crypt architectural changes like distortion or bending of crypts, crypts branching and crypts loss happen due to lymphoid aggregates. Lamina propria expansion by lymphoplasmacytic infiltration, mostly in the upper half of the mucosa (against the ulcerative colitis), mucosal ulceration, crypt abscesses, cryptitis, crypt rupture associated granulomas are not uncommon. Features of chronicity like paneth cell metaplasia, loss of goblet cells as well as villiform change of surface epithelium may also be present ⁹.

Radiographic features

Although endoscopy is the diagnostic investigation of choice, diversion colitis or pouchitis may be appreciated on various imaging modalities. However, the primary role of imaging in these patients is more-so to exclude features of other competing differential diagnoses.

Fluoroscopy

Double contrast barium enema (DCBE) may reveal a wide variety of mucosal abnormalities, from isolated inflammatory pseudopolyps to diffuse mucosal nodularity ^5,6.

CT

Features on CT are those of non-specific colitis of the diverted segment, such as colonic wall thickening, wall enhancement, and adjacent fat stranding ⁵.

Treatment and prognosis

The treatment of choice is surgical reanastomosis ^2,3. In patients who are not surgical candidates, medical therapies which have shown benefit include SCFA enemas, immunosuppression (e.g. topical mesalazine, systemic glucocorticoids), and faecal microbiota transplantation ^2,3,7,8. Empiric antibiotics may also be used, especially in cases of pouchitis which may initially be thought to be infective and thus antibiotic-responsive, although this is not the case in true diversion pouchitis which typically does not respond to antibiotics ⁸.

Differential diagnosis

The differential diagnosis is of other causes of colitis, such as infectious colitis or recurrence of inflammatory bowel disease (in patients with a history of such).