A number of animal and human studies have assessed the role of spinal nociceptive processes using DNIC paradigms. Recently, Coffin et al. assessed the spinal process of nociceptive signals in IBS patients by analyzing the effects of rectal distensions on electromyographic recordings of the somatic nociceptive flexion (RIII) reflex (107). They reported a significant progressive inhibition of the RIII reflex in healthy volunteers during slow ramp distension, with biphasic effects (facilitation and inhibition) observed during rapid distensions. In contrast, the RIII reflex was significantly facilitated in IBS patients during slow ramp distension and inhibitions induced by rapid distensions were significantly reduced, suggesting hyperexcitability of spinal nociceptive processes in a subgroup of IBS patients.
Mayer studied the perceptual responses to rectosigmoid distension in IBS patients and controls with functional brain imaging using H215O positron emission tomography and found that following a train of repetitive sigmoid distensions, control subjects demonstrated greater activation of the PAG and thalamic regions compared to IBS patients (108). This effect was seen both during actual rectal distension and the expectation of the stimulus, despite its absence. As has been outlined, the PAG is an important structure involved in the modulation of spinal pain processing, and the above finding suggests that a proportion of IBS patients have inadequate activation of brain regions involved with antinociception.
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