Irritable bowel syndrome (IBS) is a common disorder with community prevalence estimated to be around 10-15% and Australian estimates suggest that IBS is managed at around 285,000 general practice consultations annually.

Women are 1.5 times more likely to be affected than men, with the most common age being between 20 to 40 years old. Onset of IBS after the age of 50 is uncommon.

There is much active research in the pathophysiology of IBS, although currently thinking indicates abnormalities involving gastric motility and hypersensitivity. There has been current work looking at the pathogenic role of eosinophils in the proximal small bowel. Depression and anxiety are commonly associated with IBS, but these factors do not appear to be causal.

The diagnosis is made based on clinical features in the absence of alarm features. The Rome Criteria is the best known set of diagnostic criteria, but this is generally only used in a research setting. A more simple definition of IBS is abdominal pain associated with altered bowel habit, being a change in stool form or frequency) over a period of at least 3 months. Rome III criteria describe four IBS subtypes: constipation-predominant, diarrhoea-predominant, mixed, and unsubtyped. Typical symptoms of IBS include abnormal stool frequency (more thean 3x/day or less than 3x/week), abnormal stool form (lumpy /hard, or loose/watery), straining, urgency or incomplete evacuation, mucous, and bloating.

There are certain alarm features which should prompt further investigation to exclude organic diseases such as inflammatory bowel disease, coeliac disease and bowel cancer. Anaemia and unintentional weight loss are often features of organic disease and should always prompt further investigation. Rectal bleeding and nocturnal symptoms are not features of IBS and should also prompt further investigation,although recent literature suggests these features are not helpful in separating IBS from patients with organic disease.

ROME III DIAGNOSTIC CRITERIA FOR IRRITABLE BOWEL SYNDROME

Recurrent abdominal pain or discomfort at least 3 days a month in the past 3 months with onset more that 6 months with at least two of the following:

- improvement with defecation

- onset associated with change in stool frequency

- onset associated with change in stool form

Alarm features which suggest other disease:

- age >50 years

- males

- short history of symptoms

- documented unintentional weight loss

- nocturnal symptoms

- rectal bleeding

- family history of colorectal cancer

- recent antibiotic use

EVALUATION

IBS is a functional disorder where colonic hypersensitivity and abnormal motility lead to abdominal discomfort associated with abnormal bowel habit. The diagnosis is made based on clinical features, however organic disease and IBS also share many clinical features. Patients who fulfill the diagnostic criteria for IBS but who also have alarm features will need further investigation.

Further evaluation with blood testing including assessment for anaemia, iron studies, inflammatory markers and coeliac serology. Stool culture for parasites is useful in people who have diarrhoea predominant symptoms. Testing for lactase deficiency is generally not required as this diagnosis should be made on clinical grounds. Testing for small bowel bacterial overgrowth is generally not recommended, in particular due to the difficulty in making an accurate diagnosis, however empirical treatment can be considered for high risk patients, such as patients with previous small bowel surgery or diabetics. Colonoscopy and random colon biopsy should be considered for patients aged >50, weight loss, nocturnal symptoms, rectal bleeding, diarrhoea predominant symptoms and people with a family history of colorectal cancer or inflammatory bowel disease.