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.
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.
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.
Key medicines
Oral budesonideFor active microscopic colitis, use the selected licensed formulation at 9 mg each morning for the SmPC-defined induction course, usually up to eight weeks.
Maintenance budesonideFor frequent relapse after successful induction, a gastroenterologist may use 6 mg each morning or the lowest effective licensed regimen with scheduled reassessment.