Microscopic colitis

What microscopic colitis is

The colon is inflamed, but the inflammation is at a scale the camera cannot resolve. At colonoscopy the lining looks normal, or near enough to normal that nothing stands out. The changes appear when biopsies of that normal-looking lining are stained and examined, which is where the name comes from.

It is not a rare condition and it is not a minor one.

The one in eight figure is 12.8 per cent pooled across good-quality studies, and it is the number worth holding on to. It is the reason biopsies are taken from a colon that looks entirely normal.

The three subtypes

The distinction is made by the pathologist, not by anything felt or seen, and it matters less than the name suggests. All three are treated the same way.

• Collagenous colitis. A thickened band of collagen sits immediately under the surface lining, along with extra inflammatory cells in the tissue beneath.
• Lymphocytic colitis. The collagen band is normal, but the surface lining is infiltrated by lymphocytes, at a density well above what is normally present.
• Incomplete microscopic colitis. The changes are present but fall short of the thresholds for either of the other two. People in this group look very much like the other two in age, risk factors and response to treatment, and a 2026 meta-analysis of 1,370 such patients concluded that the formal criteria may be drawn too tightly.

Occasionally one subtype is found on a later set of biopsies where the other was found first. That is a recognised feature of the condition rather than a change of diagnosis.

What it feels like

The dominant symptom is watery diarrhoea without blood, reported by between 84 and 100 per cent of people across studies. In a large Danish series the average was six to seven bowel actions a day. What goes with it is as telling as the diarrhoea itself.

• Urgency, in about 55 per cent
• Needing to open the bowels overnight, in about 35 per cent, which is one of the features that separates this from irritable bowel syndrome
• Loss of control, in about 26 per cent, rarely volunteered and worth mentioning because it changes how urgently things are done
• Abdominal pain, bloating and weight loss, which occur but are less consistent

In about a third of people the diarrhoea starts abruptly, which is why it is so often put down to a gastro bug that never settled. The course tends to come and go rather than run steadily, and that pattern is part of why it goes unrecognised. In a European registry, 43 per cent of people had symptoms for six months or more before the diagnosis was made.

Who gets it

Three things stand out, and all three are well established.

• Age. Typically diagnosed after 60, and people over 65 are around five times more likely to have it than younger people
• Sex. Women are affected about two and a half times as often as men, and made up close to 72 per cent of patients in the three largest cohorts
• Smoking. Current smokers are about three times as likely to develop it as people who have never smoked, and they tend to develop it about a decade earlier
• Other autoimmune conditions, which are over-represented, and which is part of why coeliac disease and thyroid function are checked

Why it gets missed

Four reasons, and they compound.

• The colon looks normal. An endoscopist who is not thinking about microscopic colitis has no visual prompt to take biopsies, and without biopsies the procedure cannot make the diagnosis.
• The stool test that reassures everyone does not cover it. Faecal calprotectin is genuinely useful for Crohn's disease and ulcerative colitis. The European guideline states plainly that it is not useful for excluding or monitoring microscopic colitis. Normal calprotectin, normal blood tests and a normal-looking colon are all entirely compatible with the diagnosis.
• It looks exactly like irritable bowel syndrome. Two meta-analyses found microscopic colitis in about 9 per cent of people who met the criteria for diarrhoea-predominant functional bowel disease. The guideline recommends actively ruling it out in anyone carrying that label who has the risk factors or who has not responded to treatment.
• The age at which it appears is an age at which change gets accepted. People in their sixties and seventies are often told, or tell themselves, that bowels simply change.

How the diagnosis is made

By biopsy, and by biopsies taken from the right places. The European guideline recommends a full examination of the colon, including the last part of the small bowel, with biopsies from at least the right and the left side of the colon. Where both sides are sampled, the characteristic changes are present in both in 95 to 98 per cent of cases. Biopsies taken only from the rectum are not adequate, which is why a full colonoscopy is preferred to a shorter look at the left colon alone when this diagnosis is a possibility.

Nothing about the procedure itself is different. The preparation, the sedation and the recovery are the same as for any colonoscopy. The difference is in what is done during it and in waiting for the pathology, which takes a couple of weeks rather than being available on the day.

Visible abnormalities of some kind are reported in just under 40 per cent of cases, things like an irregular vascular pattern or mucosal fragility, but they are non-specific and are not what the diagnosis rests on.

Your care pathway

Medicines linked to it

Several commonly used medicines are associated with microscopic colitis. A 2022 meta-analysis of twelve case-control studies reported increased odds with anti-inflammatory painkillers (odds ratio 1.64), acid-suppressing tablets of the proton pump inhibitor class (2.36), one widely prescribed group of antidepressants (2.16) and aspirin (2.84).

Two things need saying about those numbers. The first is that association is not cause. The European guideline makes the point explicitly, and the authors of the meta-analysis note that the findings for two of the four classes may partly reflect how the comparison groups were chosen. The second is that the size of the effect differs between them, and none of it tells you whether a particular person's medicine is responsible.

What follows in practice is a review rather than an assumption. Where a medicine was started close to the time the diarrhoea began, stopping it or changing to something else is reasonable and is what the guideline suggests. Any change is a decision for the doctor who prescribed it. Bringing the current list, including anything bought over the counter, is the useful thing to do.

What gets checked alongside it

Two conditions sit close enough to microscopic colitis that they are looked for as a matter of course.

Coeliac disease

Screening for coeliac disease is recommended in everyone diagnosed with microscopic colitis. It is found in roughly 2 to 4 per cent, against about 0.4 per cent in comparison groups. It matters because it is treated completely differently and because missing it means the diarrhoea will not settle.

Bile acid diarrhoea

Bile acid diarrhoea produces symptoms that are indistinguishable from microscopic colitis, and the two coexist in around 14 per cent of cases. It is not part of the routine work-up at diagnosis. It becomes the obvious next question in someone who does not respond to treatment, and where both are present the medicines that bind bile acids are added. There is more on this on the chronic diarrhoea page.

How it is treated

Treatment works, which is the main thing to take from this section.

The first-line treatment

A steroid that acts locally in the bowel and is largely broken down by the liver before it reaches the rest of the body. That is why it is tolerated far better than conventional steroid tablets, and it is the treatment both the European and the American guidelines recommend first. In the randomised trials, after six to eight weeks of treatment:

• Collagenous colitis. 81 per cent responded, against 36 per cent on placebo
• Lymphocytic colitis. 84 per cent reached remission, against 43 per cent on placebo

Pooled across the trials, side effect rates were no different from placebo. Where treatment is continued for a long time, calcium and vitamin D and some attention to bone density are reasonable, particularly in anyone who already has reason to be concerned about their bones.

What works less well

• The anti-inflammatory tablets used for ulcerative colitis. Head to head in collagenous colitis, remission rates were 80 per cent for the local steroid, 44 per cent for those tablets and 38 per cent for placebo.
• Conventional steroid tablets. Tested, and they work less well, with high relapse rates afterwards.
• Probiotics. Both guidelines recommend against them for this.
• Antibiotics. Not enough evidence to recommend them.

The two guidelines part company slightly on the first two. The European guideline recommends against them outright. The American guideline keeps them as fallback options for people who cannot have the local steroid. Either way they are second choices, not starting points.

Other things that have a place

Loperamide, commonly known as Imodium, is not a treatment for the inflammation, but expert opinion supports using it for mild symptoms and as needed. Bile acid binders are added where bile acid diarrhoea is present too. For the small number of people whose disease does not respond to the local steroid, the guideline supports tablets that damp the immune response, and the injected biologic treatments used in Crohn's disease and ulcerative colitis. Surgery is a last resort in a very small number of people and is almost never needed.

What happens over the longer term

Most people respond to the first course. The question that then matters is what happens when it stops, and relapse is common enough that it should be planned for rather than discovered.

In the maintenance trials, people who had responded were randomised to continue on a low dose or to stop. 68 per cent of those who continued stayed in remission, against 20 per cent of those who stopped. At six months the figures were 75 per cent and 25 per cent. A lower dose, or alternating doses, held up about as well as a higher one.

In practice that produces three patterns, and which one a person falls into is only known with time. Some need one course and nothing further. Some have occasional flares treated as they come. Some do best on the lowest dose that keeps things quiet, continued for as long as it is needed. Whether and when to come off treatment is a decision to make together rather than a fixed rule.

Repeat biopsies to check whether the inflammation has settled are specifically not recommended. The microscopic picture and how a person feels correlate poorly, so treatment is judged on symptoms. Remission is usually defined as fewer than three bowel actions a day on average, and fewer than one watery one, measured over a week.

Bowel cancer risk

This is the first question most people ask after being told they have colitis, and the answer is a good one. Microscopic colitis does not increase the risk of bowel cancer or of polyps. A meta-analysis of five studies found rates no higher than in people without it, and if anything slightly lower. On that basis the European guideline recommends against placing people with microscopic colitis on a special surveillance colonoscopy programme.

That is a meaningful difference from extensive ulcerative colitis or Crohn's colitis, where long-standing inflammation does raise the risk and where surveillance colonoscopy is part of care. It does not apply here.

Ordinary bowel cancer screening for your age still applies, as it does for everyone.

Getting the most out of the appointment

Four things make the first consultation more useful.

• A week of notes on the bowels. How many times a day, how watery, whether anything wakes you overnight, and whether there has been any loss of control. Those four details carry more weight than a general description, and they are also how response to treatment is measured later.
• The full medicine list, prescription and over the counter, with a rough idea of when each was started. This is the part people most often leave out and it is directly relevant.
• Any previous results. Blood tests, stool tests, and in particular the pathology report from any earlier colonoscopy. Knowing whether biopsies were taken, and from where, often answers the question on its own.
• Whether anything has been tried, including diets, and what happened.

If you have had a colonoscopy described as normal and the diarrhoea has not settled, bring the report. The useful question is not what the bowel looked like but whether anyone took biopsies from the right and the left colon.

Where microscopic colitis is assessed in Sydney

A/Prof Viraj Kariyawasam is a consultant gastroenterologist and hepatologist whose practice is focused on inflammatory bowel disease, the family of conditions microscopic colitis belongs to. He sees people with chronic diarrhoea and microscopic colitis for consultation at Norwest, Rouse Hill, Pennant Hills and Macquarie Park.

The colonoscopy that makes the diagnosis, with biopsies from the right and the left colon, is performed at Norwest Private Hospital, Macquarie University Hospital and Pennant Hills Day Surgery. A referral from your GP is needed for a consultation.