The link between overactive bladder (OAB) and irritable bowel syndrome (IBS) suggests that it is worth GPs asking about bowel symptoms when assessing patients with symptoms suggestive of OAB, a study from Japan concludes.

The researchers investigated the prevalence of OAB and IBS using a large-scale internet-based survey. Questionnaires were sent to 10,000 patients, 5,000 men and 5,000 women, with 1,000 of each gender represented from each decade from 20-29 through to 60 and over.

The severity of OAB symptoms was assessed using the OAB symptom score for the first time in a general population. Higher scores indicate greater severity: 3-5 mild; 6-11 moderate; 12-15 severe.

IBS symptoms were assessed using the well established Rome III criteria for functional gastrointestinal disorders. This also allowed classification according to subtype, i.e. diarrhoea- or constipation- predominant IBS.

Overall prevalence of OAB was 9.3% (9.7% in men and 8.9% in women) with prevalence increasing with age. Of those affected, 59% reported mild symptoms, 40% moderate symptoms and 1% severe symptoms.

The prevalence of IBS was greater, with 21.2% of subjects reporting symptoms (18.6% of men and 23.9% of women). The incidence of IBS declined with age.

Overall, 33% of patients with OAB also had IBS and this relationship was consistent in both men and women. Furthermore, 20% of patients without OAB were found to have symptoms of IBS according to the Rome criteria. IBS was reported in 33.3%, 32.8% and 38.5% of patients with mild, moderate and severe OAB, respectively. Prevalence of OAB was 14.6% in subjects with IBS and 7.9% in those without.

Several previous studies have demonstrated that the two disorders often occur concurrently, raising the possibility that these poorly understood conditions may share common pathological features.

The bowel and bladder are closely associated anatomically, share very similar innervation and have similar functions in terms of storage and voiding of bodily waste products.

Several theories have been put forward to explain the concurrence of OAB and IBS.

One is cross-organ sensitisation, whereby sensory innervation of the bladder and bowel interact. These interactions can occur at multiple levels, from the central nervous system to the periphery.

The link between these two conditions suggests that it is worth GPs asking about bowel symptoms when assessing patients with storage urinary symptoms suggestive of OAB.

To a lesser extent, it may be worth asking about urinary symptoms when assessing those with functional gastrointestinal symptoms suggestive of IBS.

Dr Jonathan Rees