Dr John Wright

Gastroenterologist
MbChB MRCP(UK) PhD

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Diarrhoea is never caused by irritable bowel syndrome: (True or False)

Dr John P Wright
Updated: 12 December 2011

The short answer is “True”. The long answer is “False”.

The question revolves around the definition of diarrhoea. A watery stool that could be poured, if one had the inclination, is a diarrhoea stool. If only one such stool is passed with normal soft stools before and after, one would be hard pressed to diagnose diarrhoea as a condition. The only time a single watery stool is passed is usually as a result of a “gastrointestinal panic attack”. This can be equated with the diarrhoea caused by severe stress as might occur when a dog has a sudden fright. In humans a similar situation can occur quite unexpectedly while for instance shopping over Christmas. This attack is characterised by great urgency, flatus and a return to normality immediate after defecation. This is functional diarrhoea and is one of the two examples of irritable bowel causing diarrhoea.

The second variety of diarrhoea in irritable bowel syndrome is really increased frequency rather than true watery stools. The patient wakes in the morning with an immediate necessity to pass stool. This is usually formed. A few minutes later a second stool is passed which is usually slightly softer. Depending on the severity of the condition a number of stools are then passed at increasing intervals, in smaller volumes and with increasing softness, verging to diarrhoea. The episode is completed by mid-morning at latest. Some patients are unable to leave their homes until they are sure they have emptied themselves out completely. They characteristically have strong feelings of incomplete evacuation particularly towards the end of the session.

The definition of diarrhoea normally includes a timing/duration element. In the context of irritable bowel syndrome a total duration of three months would be required. The intraday variation discussed above is also important. Pathological diarrhoea occurs throughout the day and particularly after eating. The final sign that diarrhoea is pathological is waking up from sleep at night to pass stool.

The commonest causes of pathological diarrhoea masquerading as irritable bowel are post-infectious diarrhoea, lactose intolerance, drug induced diarrhoea, inflammatory bowel disease and coeliac disease.

Patients who have had a viral or bacterial enteritis sometimes continue with many months of diarrhoea. Colonic biopsies are usually mildly abnormal and differentiate these patients from those with inflammatory bowel disease.

Patients with significant lactose intolerance may be identified with a simple test of drinking a glass of skimmed milk. Intolerant patients will have symptoms of bloating, cramps and diarrhoea within two hours.

Drug induced diarrhoea can be very deceptive and careful history taking is needed. Long-standing chronic medication may still produce diarrhoea as a side effect even after many years of regular use.

The term inflammatory bowel disease includes ulcerative colitis and Crohn's disease. More importantly it includes microscopic colitis which is at least as common as the traditional diagnoses but is not usually visible to the naked eye at endoscopy. Of critical importance therefore are adequate colonic biopsies when patients are being investigated for chronic diarrhoea. If biopsies are not taken the colonoscopy is a waste of time and money.

Finally coeliac disease is perhaps the most contentious cause of chronic diarrhoea. With 1 in 400 Northern Europeans thought to have some degree of coeliac disease, the rarity of the diagnosis in South Africa appears strange. The three diagnostic modalities of serology, small bowel biopsy and capsule video endoscopy are struggling to find their place in the diagnostic process. Serology, tissue transglutaminase and anti-endomysial antibodies, are reported positive in 80% of patients. Initially the sensitivity was thought to be higher but appears to be falling progressively in Europe. Our experience of true positives in South Africa has been disappointing. The reason is not clear. As far as a firm diagnosis the duodenal loop histology remains the gold standard but is under threat from capsule videoendoscopy in which virtually each villus can be individually assessed. The jury is still out.

Our question, “Is diarrhoea ever caused by irritable bowel syndrome?” has an uncertain answer. An early morning rush or a sudden unexpected diarrhoeal stool are typical of functional bowel / irritable bowel syndrome. More persistent symptoms need investigation which includes histology. In these patients an underlying pathological process is probably present although we can not always find the cause.