Patients with IBS or other disorders with visceral hypersensitivity report referred somatic hypersensitivity in the dermatomes where referred pain is perceived. However, outside the area of referred pain, as long as there is not a codiagnosis of another ailment such as fibro-myalgia, patients report normal or hyposensitivity to noxious (electric and mechanical) somatic stimuli (4,126-131). In contrast, thermal stimulation is more painful in IBS patients compared to controls, although this cutaneous hypersensitivity decreases as the site of stimulation moves away from the region of referred pain (132). In healthy volunteers, slow ramp CRD inhibits the RIII nocifensive reflex in both the arm and leg although the fast ramp disten-tion facilitates the reflex in the leg (133). These data suggest that acute or chronic colorectal pain can modulate somatic sensitivity to noxious stimuli.
Viscerosomatic inhibition (also called nocigenic inhibition or counterirritation) can be modeled in animal studies. Pelvic nerve stimulation in the rat inhibits the withdrawal response to noxious pinch of the foot (134). Noxious stimulation of the colon or urinary bladder inhibits populations of dorsal horn neurons in the cervical and upper thoracic spinal segments that respond to thoracic somatic and/or visceral stimuli (135-137). Additionally, CRD inhibits the response of dorsal horn neurons in the lumbar spinal cord to thermal stimulation of the hindpaw (136) and inhibits the tail flick reflex in lightly anesthetized rats (138). Colonic inflammation also inhibits paw withdrawal from noxious thermal stimuli (139,140). These studies suggest that noxious stimulation of pelvic viscera can inhibit sensory neuron processing of somatic stimuli and somatic reflexes evoked outside the presumptive region of referred pain comparable to the somatic hyposensitivity in IBS patients.
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