Synopsis
Recognise microscopic colitis as a treatable cause of chronic watery diarrhoea, obtain diagnostic biopsies despite normal mucosa, exclude important mimics and use budesonide safely.
- Think of microscopic colitis in persistent watery, usually non-bloody diarrhoea with urgency, nocturnal stools or faecal incontinence, particularly in an older adult.
- A macroscopically normal colonoscopy does not exclude the diagnosis; histology from appropriately labelled right- and left-colon biopsies is the decisive investigation.
- Collagenous colitis shows a thickened subepithelial collagen band, while lymphocytic colitis shows increased intraepithelial lymphocytes; clinical management overlaps substantially.
- Review proton-pump inhibitors, NSAIDs, selective serotonin reuptake inhibitors and other temporally related medicines, but describe associations carefully and do not stop essential treatment without agreement.
- Exclude infection, coeliac disease and bile acid diarrhoea where the history supports them, because coexistence or a convincing mimic can explain incomplete response.
- Oral budesonide is first-line induction for active disease; a relevant UK product licence specifies 9 mg each morning, generally for a time-limited course.
- Relapse after withdrawal is common. Reconfirm the diagnosis and competing causes before a specialist uses the lowest effective maintenance strategy.
- Microscopic colitis is not managed with the colorectal dysplasia-surveillance programme used for longstanding ulcerative colitis solely because the word colitis appears in its name.
Key red flags
Visible bleeding, progressive iron deficiency, marked inflammatory response, abdominal mass or persistent unexplained weight loss is atypical and requires investigation for cancer, conventional IBD, ischaemia or another organic disorder.
Orthostatic symptoms, oliguria, confusion, tachycardia or muscle weakness suggest volume or electrolyte depletion from high stool output. Assess urgently and correct deficits while determining whether an acute alternative is present.
Investigation priorities
Obtain diagnostic tissue and exclude macroscopic colitis, neoplasia or another lower-gastrointestinal cause.
Management branches
Watery diarrhoea persists and initial assessment has not shown an obvious infectious or structural cause.
- Characterise frequency, nocturnal symptoms, blood, urgency, weight change, exposures, previous surgery and medicines, while checking hydration and warning features.
- Send targeted stool and blood tests, including coeliac serology, and arrange lower-GI endoscopy when microscopic colitis remains plausible.
- Request multiple right- and left-colon biopsies even if mucosa appears normal, clearly stating the clinical suspicion to endoscopy and pathology teams.
- Reconcile histology with symptoms and exclude a stronger alternative before attaching a durable diagnosis or starting recurrent steroid treatment.