Reliability and validity of a simple and clinically applicable pain stimulus: Sustained mechanical pressure with a spring-clamp

Abstract

Background: Generalized hyperalgesia, a widespread increased sensitivity to painful stimuli, has been demonstrated in a range of chronic pain conditions including low-back pain. The evidence suggests, that generalized hyperalgesia may be an important factor in the development of chronicity, but it is not commonly assessed in clinical practice. Whereas a range of tools and procedures for the quantitative sensory testing of pain sensitivity is available for laboratory pain research, most experimental pain stimuli are not well suited for clinical practice. In the current study, a simple and inexpensive mechanical spring-clamp was tested as a potential experimental pain stimulus.Methods: Ten seconds of mechanical pressure was applied to the thumb of 242 study participants: Healthy volunteers and low-back-pain patients (hospital and primary care). Pain intensity was measured by visual analogue scale before and after conditioned pain modulation by cold-pressor test (CPT). Correlation to pressure pain threshold (PPT) of the infraspinatus muscle and cold-pressor test pain intensity, time to pain onset and time to non-tolerance, was examined. Test/re-test reliability of clamp pain was also assessed and the stimulus-response relationship was examined with a set of 6 different clamps.Conclusions: Clamp pain was sensitive to changes in pain sensitivity provoked by conditioned pain modulation (CPM). Test/re-test reliability of the spring-clamp pain was better for healthy volunteers over a period of days, than for hospital patients over a period of weeks. A strong correlation (ρ = 0.73, P > 0.0001) was found between clamp force and evoked pain intensity. Correlation to other pain stimuli varied, with a strong correlation between pre and post-CPM clamp pain intensity (ρ = 0.81, P > 0.0001), moderate correlation between clamp pain and PPT (ρ = -0.43, P > 0.0001), as well as CPT pain intensity (ρ = 0.32, P > 0.0001) and a weak correlation with time to onset of CPT pain (ρ = -0.14, P < 0.05).The application of mechanical pressure is commonly employed to assess general pain sensitivity, and a simple spring-clamp seems a potentially useful instrument for quantitative sensory testing in a clinical setting.