Microscopic colitis: symptoms, treatment and diagnosis

The review describes chronic or intermittent watery diarrhoea without blood as the characteristic symptom. It can start suddenly or gradually. People commonly pass 4 to 9 stools a day, and often more than 10.

Diarrhoea was present in 96% to 100% of patients in the studies it cites. Weight loss and abdominal pain were present in over 40% of cases, and night-time diarrhoea and fatigue are also prominent. Quality of life is reduced compared with matched people without the condition, although effective treatment can bring it back towards normal.

The course varies. For some people symptoms settle on their own or after a causative medicine is stopped; some have a single episode, some a relapsing-remitting course and others continuous symptoms.

Microscopic colitis is a collective term for two conditions with similar symptoms, findings and treatment: lymphocytic colitis and collagenous colitis. The name reflects that the inflammation is only visible when biopsies are looked at under a microscope.

The cause is not yet clear. The review says it is likely to be multifactorial, with an overactive immune response in the bowel to substances in the gut in people who are predisposed. It is a different condition from ulcerative colitis and Crohn's disease, although the incidence is similar to each of them.

The review says the first step is removing exacerbating factors: smoking, and medicines such as PPIs, SSRIs and NSAIDs where they are thought to be involved. It notes that there is not yet clear evidence on how well withdrawing a suspected medicine alters the course, and you should never stop a prescribed medicine without checking with your doctor.

Budesonide is the mainstay of treatment for starting and maintaining remission. It is a steroid that is largely broken down on its first pass through the body, and the review describes it as the only treatment studied in randomised controlled trials. In trials of 9 mg once daily for 8 weeks, clinical remission was reported in 80% of people with collagenous colitis and 79% in lymphocytic colitis, and mesalazine was no better than placebo.

If symptoms are mild, loperamide, cholestyramine, mesalazine or bismuth may be considered, but the review says these are less effective. Cholestyramine may be especially helpful where bile acid malabsorption is also present. If symptoms are truly refractory, immune-modulating treatment such as azathioprine, methotrexate or anti-TNF drugs may be considered, although the review says controlled studies are still needed.

Relapse is common once budesonide stops. In one cohort of 33 people with collagenous colitis who had responded, 61% relapsed, and 88% of those relapses were within 3 months of stopping. In three trials, a lower daily dose of 4.5 to 6 mg kept remission going in 61% to 77% of people over 6 to 12 months.

The authors suggest reassessing after a relapse, retreating, and aiming for the lowest effective maintenance dose. Anyone needing maintenance budesonide should be considered for calcium and vitamin D and a bone density check, because long-term use has been linked in one study to lower bone density.

For a first presentation with chronic diarrhoea, the review says routine blood tests (full blood count, urea and electrolytes, CRP, thyroid function and coeliac serology) are useful, and stool culture if infection is suspected. If microscopic colitis is suspected, colonoscopy with biopsies is needed, as the lining usually looks normal or near-normal.

The review says biopsies should ideally be taken from several parts of the colon, because diagnostic changes are more often found in the right colon and a flexible sigmoidoscopy is not enough to exclude it. Faecal calprotectin can be raised but is not useful for diagnosing, excluding or following microscopic colitis.

Microscopic colitis is the diagnosis in about 19% of people who have a colonoscopy for chronic non-bloody diarrhoea, making it the most frequent finding in that group, according to the review.

Several associations are described. The medicines with most evidence are NSAIDs, PPIs and SSRIs, and smoking increases risk. It is also associated with autoimmune conditions, most often coeliac disease: in a cohort of 1,009 people with coeliac disease, 4.4% had microscopic colitis, which was 45 times higher than in people without it.

Bile acid malabsorption was found in 43% of one cohort of 57 people with microscopic colitis. Doctors therefore check for coeliac disease and consider bile acid malabsorption if treatment and stopping a medicine do not help.

The review does not describe a diet treatment for microscopic colitis. Its dietary relevance is that lactose malabsorption and coeliac disease are among the conditions to exclude if symptoms persist. Do not cut out food groups without advice, as weight loss is already common. A GP or dietitian can help if you suspect particular foods make your diarrhoea worse.

See a GP if you have ongoing watery diarrhoea, particularly if it wakes you at night or you are losing weight. The NHS advises calling 111 if diarrhoea lasts more than 7 days, if you have bloody diarrhoea or bleeding from the bottom, or if you have signs of dehydration that do not improve. Blood in the stool is not a feature of microscopic colitis, so it points to another cause that needs checking.

The NHS says to call 999 or go to A&E if you vomit blood, have a sudden severe tummy ache, or are confused or not responding as usual. Older people and those with other health conditions are at higher risk of dehydration and should seek advice earlier.

If symptoms continue despite treatment, the review says to reconsider the diagnosis and review medicines and smoking, and to look for coeliac disease, bile acid malabsorption, lactose malabsorption and small bowel bacterial overgrowth.

On the NHS, chronic diarrhoea is usually assessed by a GP, who may arrange blood and stool tests and a referral to gastroenterology for colonoscopy with biopsies. Private gastroenterology services can offer the same colonoscopy and biopsy diagnostic route. Either way, if biopsies were not taken from several parts of the colon, it is reasonable to ask whether microscopic colitis has been considered.