Is Hyperalgesia Limited to the Gut in Irritable Bowel Syndrome

The first studies to investigate visceral sensitivity in IBS concluded that enhanced sensitivity in IBS was limited to the gut (43-46). Interestingly, two prior studies have examined cutaneous pain in IBS patients using electrocutaneous stimulation (43,44). In one study, 13 patients with Crohn's disease, 13 control subjects, and 12 patients with IBS had electrodes positioned on the

Figure 3 IBS patients' and normal control subjects' M-VAS pain Intensity ratings of rectal distension pressures of 35 and 55mmHg (left panel) and of thermal stimulation of the hand and foot (right panel). Note that patients with IBS rate pain intensity much higher than controls (p < 0.001) at both temperatures and skin sites. Values are represented as meansĀ± SD, n = 12 IBS patients, 17 controls. Abbreviations: VAS, visual analog scale; IBS, irritable bowel syndrome. Source: From Ref. 40.

Figure 3 IBS patients' and normal control subjects' M-VAS pain Intensity ratings of rectal distension pressures of 35 and 55mmHg (left panel) and of thermal stimulation of the hand and foot (right panel). Note that patients with IBS rate pain intensity much higher than controls (p < 0.001) at both temperatures and skin sites. Values are represented as meansĀ± SD, n = 12 IBS patients, 17 controls. Abbreviations: VAS, visual analog scale; IBS, irritable bowel syndrome. Source: From Ref. 40.

skin of their hands (44). Touch threshold, defined as the current just detectable by the subject, and pain threshold, defined as the current at which the subject first described the stimulus as painful, were significantly higher in both IBS and Crohn's disease, compared to normal subjects. A later study compared somatic transcutaneous electrical nerve stimulation in 17 patients with IBS and 15 healthy controls (43). The perception threshold and threshold for discomfort were both higher in the IBS subjects than controls. A possible limitation common to both studies is that the thresholds for perception and discomfort to electrical stimulation may not have necessarily involved stimulation of nociceptive receptors. Thresholds for detection of electric shock and discomfort thresholds may be below that required to activate nociceptive receptors (11). Furthermore, tactile input and perception are inhibited by nocicep-tive input. This may account for the higher thresholds in IBS patients if, unlike control subjects, they have ongoing visceral nociceptive input.

Thus, less agreement has been reached with regard to secondary cutaneous hyperalgesia in IBS patients (40,47-51). One study suggests that IBS patients exhibit cutaneous hypersensitivity only when they have fibromyalgia as a comorbid condition (48). However, some investigators have acquired evidence that patients with IBS but without other chronic pain conditions including fibromyalgia have both visceral hypersensitivity and cutaneous hyper-sensitivity in response to experimental stimuli (40,47,51). A number of these studies have compared results of both clinically relevant painful rectal distension and painful cutaneous heat stimulation in IBS patients with age- or sex-matched normal control subjects (40,49,51).

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