In this issue of the Scandinavian Journal of Pain (SJP), Waller and coauthors report normative data for pressure and cold pain thresholds in a cohort of young pain-free adults [1].
1 The use of experimental pain models in human pain research
Experimental pain tests provide a safe and reversible means of inducing pain in a dose dependent manner in the laboratory. These methods have been used in human pain research since the 1950s with experiments covering a wide range of topics such as psychophysics (pain measurement), pharmacological interventions, psychological mechanisms, placebo effects, and brain imaging. Most of these studies have been conducted as true experiments, with between-group or crossover designs, making causal conclusions valid.
2 Individual differences in experimental pain sensitivity
In the past ten years or so, the focus of experimental pain research has gradually shifted away from studying effects of experimental conditions, towards studying experimental pain sensitivity as a baseline characteristic of the individual. Three factors have contributed towards this development: First, the observation that there are huge individual differences in sensitivity to experimental pain stimuli [2]. In fact, these differences are so great that stimuli that lie well below pain threshold for some subjects may be well above pain tolerance for others. Second, it was demonstrated that experimental pain sensitivity is heritable, and for some stimulus modalities the genetic contribution accounts for half the variance or more [3, 4].This is important because it shows that pain sensitivity is a stable innate characteristic of the individual. Finally, a growing number of studies have found that chronic pain patients had increased sensitivity to pain (hyperalgesia) e.g. Staud et al. [5, 6], Stabell et al. [7]. This suggests that hyperalgesia may be a contributing factor to the development and/or maintenance of chronic pain.
3 Experimental pain models in epidemiology
In response to the above observations, several large prospective studies of representative samples have been initiated, among them the Tromsø Study, where 10,500 participants were assessed withthe cold-pressortest [8], and the Rotterdam study, where heat pain thresholds have been assessed in 4000 subjects [9]. Among the topics addressed in these and similar studies are the genetics of pain sensitivity, the relationship between pain sensitivity and established risk factors for chronic pain, and cross-sectional and longitudinal associations between experimental pain sensitivity and chronic pain.
4 Comparability is crucial to the aggregation of data across studies
Waller and coworkers add to this emerging field by performing pressure (4 sites) and cold pain threshold tests in a sample of 970 individuals recruited from Western Australian Pregnancy Cohort Study. In this issue of the SJP they report normative threshold data for 617 individuals (280 females, 337 males) who were pain free at the time of study [1]. Using multi-variable regression, they also report on the association between experimental pain thresholds and a number of factors that are associated with clinical pain and/or general health in many studies: sex, smoking, obesity, physical activity, mental health and health related quality of life.
Though it remains to be shown that reference values for experimental pain tests have clinical utility outside the field of neuropathic pain, their immediate importance lies in providing a basis for comparing data across studies and countries. Genome wide association studies in particular are strongly dependent on aggregating data across study populations, and an obvious prerequisite is that the pain measures are comparable. Publishing reference values and associations with key covariates is an important step in this direction.
DOI of refers to article: http://dx.doi.org/10.1016/j.sjpain.2016.08.003.
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Conflict of interest: None declared.
References
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