Irritable bowel syndrome (IBS) is recognised by a characteristic pattern of abdominal pain with disturbed bowel habit, not by any single symptom.
Clinical features
The core symptoms are abdominal pain together with constipation, diarrhoea or an alternating pattern, and bloating with visible distension. The disturbance is chronic, and it may be accompanied by symptoms outside the colon.
Pain is essential: IBS should not be diagnosed without it. The pain is often relieved by defecation, and with its onset the stool may become more or less frequent and harder or looser. It can occur anywhere in the abdomen but is most common in the lower abdomen, and in many patients it flares with life events or difficulties.
The predominant bowel pattern may be constipation, diarrhoea or an alternating mixture; because the pattern can change over time, the predominant form can shift from one period to another. This is the basis of the classification into IBS-D (diarrhoea-predominant), IBS-C (constipation-predominant) and IBS-M (mixed), with a fourth unclassified group, IBS-U.
Bloating is a distressing feeling found in more than 60% of cases, but it is not essential for the diagnosis. No single symptom is sensitive or specific enough on its own, so IBS is identified from the pattern as a whole. Distension of the abdomen often worsens towards the end of the day, and the abdomen responds to it with a feedback pattern in which the intercostal muscles contract, the abdominal muscles relax and the diaphragm is pushed aside by the abdominal contents.
Associated conditions
IBS overlaps heavily with other gastrointestinal and non-gastrointestinal conditions, including dyspepsia, gastro-oesophageal reflux disease (GERD), fibromyalgia, headache, backache or joint pain, gynaecological conditions such as pelvic inflammatory disease and dysmenorrhoea, altered sleep, chronic fatigue and dizziness. Because of this overlap it is worth thinking broadly: in a patient with fibromyalgia or painful sexual intercourse, IBS should be considered.
Time course and the risk of misdiagnosis
For a confident diagnosis, the symptoms should be present for at least 6 months. Both over-diagnosis and under-diagnosis occur. One sign of over-diagnosis is that patients with IBS undergo surgery for gallstones more often than other groups, which suggests that symptoms are sometimes attributed to IBS when they are caused by an organic disorder such as gallstones.
Criteria
Several sets of criteria have been used to define IBS. The Manning criteria describe 6 symptoms that are more common in patients already diagnosed with IBS; 4 of them are statistically significant, and these are the first four:
- abdominal distension
- relief of abdominal pain after a bowel movement
- looser stools with the onset of abdominal pain
- more frequent bowel movements with the onset of abdominal pain
- a feeling of incomplete emptying
- mucus per rectum
The Kruis criteria are a scoring system that also uses the presence and duration of symptoms, laboratory findings and the physical examination.
From the Manning and Kruis criteria came the current consensus definition, Rome IV. It requires recurrent abdominal pain, on average at least 1 day per week in the last 3 months, together with two or more of the following:
- pain related to defecation
- pain associated with a change in the frequency of stool
- pain associated with a change in the shape of stool
The criteria must have been fulfilled for the last 3 months, with symptom onset at least 6 months before diagnosis. Rome IV also uses stool form to classify IBS into four main groups:
- IBS-D — 23%
- IBS-C — 22%
- IBS-M — 24%
- IBS-U (unclassified)