Microscopic Colitis
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Direct answer
A normal-looking colonoscopy with disabling watery diarrhoea is the signature of microscopic colitis — the diagnosis lives in the biopsies, never in the endoscopist's eyes. It affects middle-aged and elderly people, women more than men, and comes as lymphocytic or collagenous disease distinguished purely by histology. Stop the offending drug if one exists, and treat with budesonide, which is first-line and highly effective; relapses are managed with tapering or maintenance doses.
What you must remember
- Two histologies: lymphocytic colitis — ≥20 intraepithelial lymphocytes per 100 surface epithelial cells; collagenous colitis — a thickened subepithelial collagen band exceeding 10 µm; both show surface epithelial damage and a chronic inflammatory lamina propria.
- Endoscopy: normal or near-normal mucosa, occasionally a subtle mosaic or oedematous pattern; multiple biopsies from right and left colon are required — histology, not the view, makes the diagnosis.
- Clinical picture: chronic, non-bloody, often nocturnal watery diarrhoea with urgency and weight loss; no fever; normal CRP usually.
- Associations: coeliac disease — a meaningful minority, so check anti-tTG IgA — autoimmune thyroid disease, diabetes, and rheumatoid arthritis.
- Drug triggers: NSAIDs, proton pump inhibitors, SSRIs (also statins, beta-blockers, acarbose) — withdrawal alone may remit the disease.
- Smoking: an established risk factor — the inverse of its (temporary) protective effect in ulcerative colitis, a favourite contrast question.
- Treatment: budesonide 9 mg daily for 6–8 weeks is first-line induction, then taper to the lowest effective dose or alternate-day maintenance; alternatives include bismuth subsalicylate; refractory disease needs azathioprine or anti-TNF therapy.
- Prognosis: no increased colorectal cancer risk in most cohorts, so no routine surveillance colonoscopy — unlike IBD.
The drug history is the diagnosis
A 62-year-old woman has had a year of urgent, watery, night-waking diarrhoea and has lost 4 kg. Colonoscopy: the endoscopist reports an unremarkable colon. If biopsies were not taken, the case would have been archived as IBS — which is exactly how most of these patients spent years. Her biopsies show a 15 µm subepithelial collagen band: collagenous colitis. Now the part that changes management rather than merely labels her — the drug list. She takes a proton pump inhibitor bought over the counter for "acidity" and an NSAID for knee osteoarthritis. Both are suspects; the PPI is stopped, the NSAID is replaced with paracetamol, and her stools improve within weeks. When symptoms persist, budesonide 9 mg daily induces remission in a clear majority, and because relapse is common on withdrawal, the taper is slow — 6 mg, then 3 mg, with maintenance on the lowest dose that keeps the stool formed. Throughout, her anti-tTG is checked, because coeliac disease hides behind this label in a real minority and needs its own gluten-free diet.
The IBS label that hid for years
The exam's central trap is exactly the normal colonoscopy: candidates learn "normal endoscopy" and forget that the entire diagnosis depends on the forceps. The rule worth quoting — in chronic watery non-bloody diarrhoea in a middle-aged or older patient, biopsies must be taken even from a visually normal colon, from both right and left sides, because collagenous change can be patchy. A second favourite is the treatment list: unlike IBD, 5-aminosalicylates perform poorly here, and the star is budesonide — topical-release steroid designed for the ileocaecal region, yet it works in a diffuse colitis, which itself gets asked. Third, know the histological numbers: 20 intraepithelial lymphocytes per 100 cells and a collagen band over 10 µm — these thresholds turn vague prose into marks.
Frequently asked questions
How is microscopic colitis diagnosed?
By histology — ≥20 intraepithelial lymphocytes per 100 epithelial cells (lymphocytic type) or a subepithelial collagen band >10 µm (collagenous type) — on biopsies from a normal-appearing colon.
Which drug is first-line for induction?
Budesonide 9 mg daily for 6–8 weeks, with tapering to the lowest effective maintenance dose given the tendency to relapse.
Why is coeliac disease screened for in these patients?
A significant minority of microscopic colitis patients have coeliac disease, and it requires separate dietary treatment; anti-tTG IgA is the screen.
Which drugs are the classic triggers?
NSAIDs, proton pump inhibitors and SSRIs, among others; stopping the culprit drug can itself induce remission.
Does microscopic colitis increase colorectal cancer risk?
No meaningful increase is recognised in most studies, so routine surveillance colonoscopy is not recommended, unlike in IBD.