Microscopic colitis is a chronic inflammatory condition of the gut. It is approximately as common as the 'classic' inflammatory bowel diseases (ulcerative colitis and Crohn's disease) and is considered a separate entity.

Risk factors

• smoking
• drugs: NSAIDs, PPIs and SSRIs

Microscopic colitis (MC) is a chronic inflammatory bowel disease. It can be split into two types:

• Collagenous colitis (CC)
• Lymphocytic colitis (LC)

For both types of MC, there is an increased number of intraepithelial and lamina propria lymphocytes seen on microscopy. In CC specifically, there is a thickened subepithelial collagen band.

Diarrhoea in MC is thought to be related to the inflammatory response in the bowel wall, with a greater intensity of inflammation associated with increased severity of diarrhoea.

Abnormalities in electrolyte secretion and absorption have also been noted in MC patients. Specifically, sodium and chloride absorption seem to be impaired alongside increased chloride secretion.

Initial investigations for chronic diarrhoea may include:

• Blood tests: FBC, CRP, TFTs and coeliac serology
• Stool samples: to exclude infective causes and for faecal calprotectin levels

If malignancy is suspected then a 2-week wait referral should be considered (see NICE guidelines for further information).

Laboratory results are generally non-specific and non-diagnostic. CRP and ESR may be normal or mildly raised and mild anaemia may be seen. Around half of patients are positive for autoantibodies such as antinuclear and antimitochondrial antibodies.

Guidelines written by the British Society for Gastroenterology advise that when microscopic colitis is suspected, patients should be referred for colonoscopy even before assessing faecal calprotectin levels.

This contradicts NICE guidance for suspected IBD, which recommends that colonoscopy referrals should be based on a faecal calprotectin level of ≥100 μg/g. However faecal calprotectin levels are thought to be <100 μg/g in over half of patients with MC.

Microscopic colitis can only be diagnosed by histology examination of biopsied tissue taken during colonoscopy. Macroscopic changes are not usually seen unlike in other types of IBD. The characteristic microscopic findings are:

• Lymphocytic colitis: increased number of intraepithelial and lamina propria lymphocytes (>20 per 100 cells)
• Collagenous colitis: as above, along with a thickened collagenous band in the subepithelial layer (>10μm)

The symptoms of microscopic colitis frequently overlap with those of irritable bowel syndrome and it is thought that many patients with MC are misdiagnosed with IBS. Features that can help distinguish MC from IBS include:

• Older age at onset (usually > 50 years)
• Weight loss
• Nocturnal stools
• Stools are consistently watery/soft
• Imperative urgency that can lead to faecal incontinence
• Association with other autoimmune conditions (e.g. thyroid disease, rheumatic disease, diabetes mellitus and coeliac disease)
• Not associated with feelings of fullness/bloating or incomplete bowel evacuation

Differentials that should also be considered include the other inflammatory bowel diseases:

• Ulcerative colitis
  • Typically bloody diarrhoea
  • Abdominal pain in the left lower quadrant
  • Urgency and tenesmus
• Crohn's disease
  • Associated with weight loss
  • May feel a palpable mass in the right iliac fossa on examination
  • Mouth ulcers may be present
  • Perianal lesions such as abscesses or fistulae may be present

Another differential to consider is coeliac disease (CD), especially if the diarrhoea seems to be associated with gluten-containing foods, although CD can also co-exist alongside MC.

Lifestyle factors that can exacerbate MC should first be addressed. These include:

• Stopping smoking
• Stopping medications such as NSAIDs, PPIs and SSRIs where possible
• Decreasing caffeine intake
• Decreasing dairy intake (in patients with lactose intolerance)
• Decreasing alcohol consumption

If lifestyle factors are unsuccessful in managing symptoms then pharmacological interventions may be considered. For mild cases, anti-diarrhoeal drugs such as loperamide may be effective in achieving symptomatic relief. However it is important to note that clinical remission is seldom achieved with loperamide monotherapy.

Current treatment guidelines for MC recommend budesonide, a corticosteroid shown to be effective in the induction and maintenance of remission. A typical dosage is 9mg daily for 8 weeks and the medication then stopped to assess response. If symptoms recur, then re-initiation of budesonide may be necessary.

In patients who do not respond to budesonide, other medications that may be tried include immunomodulators (e.g. azathioprine) and biologics (e.g. anti-TNF-alpha drugs).

The main complication of MC is reduced quality of life, especially in those who have misdiagnosed with conditions such as IBS. Debilitating symptoms that may be experienced include:

• Abdominal pain
• Fatigue
• Arthralgia
• Myalgia
• Faecal urgency
• Faecal incontinence
• Nocturnal defecation
• Weight loss

Studies have found there is no increased risk of colon cancer associated with MC and some have even shown decreased risk.