Microscopic Colitis

Microscopic colitis is a chronic inflammatory disease of the colon causing watery diarrhoea with normal colonoscopic appearance and characteristic histology. It comprises collagenous colitis and lymphocytic colitis.

• Incidence is about 10 to 25 per 100,000, with a female predominance and peak onset at 60 to 70 years.
• Associations: smoking, celiac disease, autoimmune thyroid disease, rheumatoid arthritis, type 1 diabetes.
• Drug triggers: NSAIDs, proton pump inhibitors (lansoprazole in particular), SSRIs (sertraline), ranitidine, ticlopidine, statins.
• Mechanism is an abnormal mucosal immune response to luminal antigens, with bile acid malabsorption in up to 40%.
• Course is relapsing in 60 to 80%, without malignancy risk.

• Chronic or intermittent non bloody watery diarrhoea, often 4 to 9 stools daily, with nocturnal stools and urgency.
• Faecal incontinence, abdominal pain, weight loss, fatigue, arthralgia.
• Examination is usually normal. Blood in the stool is not a feature.
• Differential diagnosis includes celiac disease, bile acid diarrhoea, infection, irritable bowel syndrome, Crohn disease, medication effect, lactose intolerance.

• Colonoscopy is normal or shows minor erythema and oedema. Take multiple biopsies from the right and left colon (at least 2 from each of right, transverse and left).
• Histology defines the subtype (see collagenous and lymphocytic colitis).
• Exclude infection (stool culture, Clostridioides difficile toxin), celiac serology, thyroid function, faecal calprotectin (mildly raised in some), and review medications.
• Consider SeHCAT scan or serum C4 for associated bile acid diarrhoea.

• Stop triggers: NSAIDs, PPIs where possible, SSRIs, and encourage smoking cessation.
• Induction: budesonide 9 mg once daily for 6 to 8 weeks, with clinical remission in about 80 to 90%.
• Taper over 2 to 4 weeks to reduce relapse, or give maintenance budesonide 6 mg daily or 3 mg every other day for responders who relapse (up to 12 months, with bone and glucose monitoring).
• Mild disease: loperamide 2 to 4 mg up to four times daily, or bismuth subsalicylate 262 mg (2 tablets) three times daily for 8 weeks.
• Bile acid diarrhoea component: cholestyramine 4 g once to three times daily, separated from other drugs.
• Steroid dependent or refractory disease: azathioprine 2 to 2.5 mg/kg, methotrexate 15 to 25 mg weekly, or anti TNF therapy (adalimumab 40 mg every 2 weeks). Vedolizumab 300 mg IV is an option.
• Surgery (ileostomy or colectomy) for refractory disease is rare.
• Review at 3 months, then yearly. Prognosis is good with long term remission in many patients.