Conditioned Pain Modulation (CPM) Test: Reliability, Feasibility, and Normal Values

Abstract

Pain is the common characteristic associated with conditions such as chronic headache, arthritis, herpes zoster, diabetic neuropathy, and temporomandibular joint disorders (TMD). Typically, nerve impulses produced peripherally by a stimulus travel centrally to the spinal cord and the brain stem where they are elaborated before they reach various regions in the cortex, resulting in pain perception. Impulses may go through a modulation process -through the endogenous pain inhibitory pathways in humans- where alterations of the pain signals lead to either their inhibition or enhancement before they reach the higher centers of the brain. Currently, the best way to assess the efficiency of endogenous pain inhibition in humans is by performing a test that assesses pain modulation. Pain modulation can occur by several mechanisms and previous work has shown that the application of a noxious stimulus in one part of the body can inhibit pain in another part of the body, presumably by activating the descending inhibitory pain system. This phenomenon was previously termed ‘Diffuse Noxious Inhibitory Control’ (DNIC), but recently the term Conditioned Pain Modulation (CPM) is recommended for humans. The laboratory assessment of CPM involves a subject rating the pain intensity of a “test stimulus” (TS), during and after the application of a second painful “conditioning stimulus” (CS) to a distant region of the body. However, the reliability of this test has been investigated only by a few studies, and questions remain unanswered about its reproducibility, as well as its feasibility. Different types of conditioning and test stimuli have been employed. Our systematic review has shown that using cold pressor test (CPT) as a CS is one of most efficient methods to induce CPM when combined with pressure pain threshold (PPT) test stimulus. Previous studies have shown that chronic pain is associated with decreased endogenous analgesia which is indicated by less efficient CPM. Only a few studies have explored the reliability of the CPM test to assess the alterations in pain modulation in chronic pain patients. As a diagnostic test, it is essential that any CPM assessment yields consistent and reliable results. Therefore, in this dissertation we aimed to: 1-Dedicate the 1st chapter to provide a background about suggested pain modulation mechanisms including CPM, and factors that may affect this phenomenon. 2-Report the reliability of the CPM assessment paradigm by performing a systematic review of published studies on this topic. One assessor searched multiple electronic databases including: PubMed, EMbase and Web of Science, from inception to December 2015 inclusive. Inclusion criteria were comprised of original research, human subjects, and CPM tested identically at least twice with at least a 60 minutes interval between testing sessions. The main outcome was reliability. Reliability of CPM was reported as the Intra-class Correlation Coefficient (ICC), Coefficient of Variation (CV), and Coefficient of Repeatability (CR). 3-Assess the reliability of CPM through intra-session reliability and inter-session reliability. Healthy volunteers were tested three times (three visits) within a two-week period; each visit included two test sessions. In each session, PPT was performed identically on the test sites of the dominant side of the body in a non-overlapping fashion: the masseter muscle (side of the face), hand (thenar eminence), and foot (dorsum of the foot) three times. The PPT (the TS) was applied during and before the CPT (the CS) to calculate the CPM value. The CPM test stability was assessed by assessing the change in CPM scores from the baseline visit to the end of the follow-up visit. 4- The development of CPM normal values and its relationship at three body sites by age/gender. The difference between PPT values before and during the CPT is the magnitude of CPM. We aimed to determine the relationships between CPM at three body sites and age/gender in a healthy population, and second, to begin the development of a normative database for CPM. Specifically, the mean, median, lowest and highest pain inhibition scores were described separately for males, females, younger, middle and older age individuals, and by the anatomical sites of the pain stimulus – foot, hand and face. 5-Measure feasibility of clinical CPM testing: patient experience, and any adverse events. Although subjects were tested multiple times during each testing session, the proposed actual test procedure in the clinic is the PPT at one of the three tested sites during and before the CPT. Subjects answered a brief post-testing questionnaire that included questions on their tolerance for the test, the perceived safety of the test, as well as any experienced side effects. From the totality of our findings, our final recommendation for a reliable and feasible clinical CPM testing protocol includes performing the test on the hand, employing PPT using a manual algometer as a TS applied to the dominant hand, and CPT as a CS applied to the non- dominant hand. We recommend performing the test in the form of three rounds of PPT with 20s in between before CPT on the dominant hand, followed by immersion of non-dominant hand in cold water (CPT) at temperature of 5-7oC. When pain from CPT reaches 7 out of 10 according to numerical pain scale- where 0 is no pain and 10 is severe pain, the PPT should be applied simultaneously during the CS at this point while the non-dominant hand remaining in the cold water. At the beginning of the visit, we recommend performing a training session of the pain tests (CPT and PPT) on the volunteer’s hand 5 minutes before starting the testing session.