Topography of the Sensory Dysfunction

Several lines of evidence indicate that altered sensitivity in patients with functional gut syndromes affects exclusively the visceral territory. Somatic sensitivity, both to the cold pressure test and to transcutaneous electrical nerve stimulation, is normal or even reduced both in dyspeptic and IBS patients (6,40,47,48). This increased tolerance of somatic pain has been related to the pain reporting behavior characteristic of painful conditions. In contrast to these data showing a selective visceral sensory dysfunction, it seems that patients with IBS have an increased incidence of somatic pain disorders, such as fibromyalgia and various myofascial pain syndromes (49-52). The reason for this association is unknown. It remains to be established whether patients with IBS and concomitant fibromyalgia are different than those with irritable bowel alone.

Several studies have attempted to define the regions of the gut and the specific pathways affected in different subsets of patients, and there seems to be a region specificity (6). Increased gastric but normal duodenal sensitivity was shown in a specific subset of patients with motility-like dyspepsia predominantly complaining of postcibal bloating (48). In this study, dyspeptic patients invariably recognized that gastric distension, but not duodenal distension, reproduced their customary symptoms, whereas in healthy subjects both stimuli were perceived alike.

In IBS patients, colonic hypersensitivity to distension has been well documented (22,40, 53-56), and it has been further demonstrated that other regions of the gut, such as the jejunum and even the esophagus also display heightened perception, suggesting a widespread sensory dysfunction (6,57,58). However, the sensory dysfunction in IBS does not affect all types of afferents, but exhibits fiber specificity. Studies using both mechanical stimuli and transmucosal nerve stimulation have shown that patients with IBS have increased perception of mechanical stimuli (distension) with normal perception of electrical stimulation (6). These data suggest that small bowel hypersensitivity in IBS is related to a selective alteration of mechanosensitive pathways. The level of the afferent dysfunction has not been established, but using these techniques, a response bias can be reasonably excluded. It has been postulated that patients with noncardiac chest pain tend to overinterpret esophageal stimuli as painful (59). However, in IBS patients, transmucosal electrical nerve stimulation induces normal perception, even though electrical and mechanical stimuli produce similar, undistinguishable sensations in most tests (6).

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