Deficiency in endogenous modulation of prolonged heat pain in patients with Irritable Bowel Syndrome and Temporomandibular Disorder

Abstract

Females with Irritable Bowel Syndrome (IBS) and Temporomandibular Disorder (TMD) are characterized by enhanced sensitivity to experimental pain. One possible explanation for this observation is deficiencies in pain modulation systems such as Diffuse Noxious Inhibitory Control (DNIC). In a few studies that used brief stimuli, chronic pain patients demonstrate reduced DNIC. The purpose of this study was to compare sensitivity to prolonged heat pain and the efficacy of DNIC in controls to IBS and TMD patients. Heat pain (experimental stimulus; 44.0-49.0 degrees C), which was applied to left palm, was continuously rated during three 30-s trials across three separate testing sessions under the following conditions: without a conditioning stimulus; during concurrent immersion of the right foot in a 23.0 degrees C (control); and during noxious cold immersion in a (DNIC; 8.0-16.0 degrees C) water bath. Compared to controls, IBS and TMD patients reported an increased sensitivity to heat pain and failed to demonstrate pain inhibition due to DNIC. Controls showed a significant reduction in pain during the DNIC session. These findings support the idea that chronic pain patients are not only more pain sensitive but also demonstrate reduced pain inhibition by pain, possibly because of dysfunction of endogenous pain inhibition systems.

Figure 1

Average continuous ratings of pain intensity during testing sessions with either no water bath, a 23.0°C water bath, or DNIC water bath for (a) control subjects, (b) IBS patients, and (c) TMD patients. For each group, pain intensity gradually increased during the first 15 seconds followed by a brief decay.

Figure 2

Average PPR (a) and AUC (b) for control subjects and patients with IBS and TMD during a session with no water bath (open bar), a session with 23.0°C water bath (gray bar), and a session with the DNIC water bath (close bar). PPR and AUC values were obtained during continuous ratings of pain intensity and are represented as means ±SD. * P <0.01 indicates differences between the DNIC session and No bath/23°C sessions. Peak pain ratings (PPR); Area under the curve (AUC); Electronic visual analogue scale (eVAS).