Pain modulation in IBS and MPS patients: Deficiency of Diffuse Noxious Inhibitory Controls

Diffuse Noxious Inhibitory Controls (DNIC) is a type of endogenous pain modulation that is characterized by pain in a local area (experimental stimulus) being inhibited by a second pain in a remote area (conditioning stimulus). Previous psychophysical studies have suggested that DNIC is dysfunctional in chronic pain conditions. The purpose of this study was to examine the efficacy of DNIC with prolonged thermal stimulation in patients with Irritable Bowel Syndrome (IBS) and Myofascial Pain Disorder (MPD) compared to matched controls. Each session consisted of five 30-second trials in which the experimental stimuli (contact thermode) and the conditioning stimuli (water bath) were presented concurrently. Temperature intensities for the experimental and conditioning stimulus were individually tailored to produce a rating of 40-50 eVAS and 20-30 on a 0 (no pain) to 100 (most intolerable pain) scale during a training session, respectively. Participants experienced three sessions on different days including a session without the water bath, a control session using a 23.0 deg C water bath, and a DNIC session with the individualized temperature for the water bath. Analysis revealed that IBS and MPD patients experienced no pain inhibition during DNIC trials, whereas the control subjects reported less pain during DNIC trials. While IBS patients rate pain as the same for the three conditions, MPD patients rated significantly more pain during the DNIC session. No differences among the three groups were observed for sessions without the water bath and 23.0 deg C bath. Overall, this suggests that patients with chronic pain conditions process pain differently than pain-free individuals. DNIC appears to be an indicator of healthy pain inhibition. These findings suggest that patients with chronic pain conditions have reduced ability to inhibit pain, possibly because of dysfunction of their endogenous pain inhibition systems. Supported by the UF College of Dentistry Seed Grant.