IBS is a multi-faceted, heterogeneous disorder often referred to as a biopsychosocial disease. The precise cause of IBS has still to be confirmed; however, there is mounting evidence to suggest that IBS is a neurological bowel disease, associated with dysregulation of the bidirectional communication between the gut, the enteric nervous system, and the central nervous system (spinal chord and brain), the so-called ‘brain–gut axis’.(Talley 2001; Gershon 2003) The suggestion that dysregulation of this axis forms the underlying pathophysiological mechanism is consistent with the cardinal features of IBS: altered gastrointestinal motility and a heightened awareness of distension, contraction or discomfort of the bowel (visceral hypersensitivity). Modulation of this mechanism by other factors implicated in the development of IBS may explain the heterogeneity of the disorder.

Other hypothesis put forward to explain the pathophysiology of IBS implicate several inherent and environmental cause. Gastrointestinal infections or trauma precede IBS in approximately 25% of cases, and prior exposure of the gut lumen to pathogens or inflammation of the gut may cause chronic hypersensitivity to distension stimuli. This phenomenon may be mediated by an imbalance in the inflammatory response.(Talley 2001)

A genetic predisposition to IBS has been suggested by familial clustering and twin studies, (Morris-Yates, Talley et al. 1998) and is supported by the recent discovery of a significant association between a functional polymorphism in the serotonin transporter gene and IBS with diarrhoea predominance.(Yeo, Boyd et al. 2004)
 
Factors implicated in IBS

Episodes of IBS may be triggered or exacerbated for a number of reasons. An unsuitable diet may be one reason. Dietary factors commonly associated with IBS include poorly digested or absorbed sugar intake, aerophagia, and an excessively high-fibre diet (especially of wheat.(Dapoigny, Stockbrügger et al. 2003; Heitkemper, Jarrett et al. 2004)

The menstrual cycle is known to affect the severity of some IBS-symptoms, with many women suffering an exacerbation of bowel symptoms, especially increased pain, bloating and intestinal gas, during menses.(Whitehead, Cheskin et al. 1990; Heitkemper, Jarrett et al. 2004)

Stress may be a trigger for or a consequence of IBS symptoms. Patients with IBS appear to report more lifetime and daily stressful events and have greater reactivity to stress than normal individuals. Stress-induced visceral hyperalgesia has been identified as a potential mediator of visceral hypersensitivity in IBS patients.(AGA-position-statement 2002; Drossman, Camilleri et al. 2002)

Psychological and psychiatric comorbidity is common in IBS.(Drossman, Camilleri et al. 2002) However, patients with IBS are also more likely to report a history of physical or sexual abuse as children, and this may in part explain the high psychiatric comorbidity associated with IBS.(Heitkemper, Jarrett et al. 2004)