Individual differences in pain: understanding the mosaic that makes pain personal

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It has long been appreciated that individuals differ from each other in important ways. More than 2000 years ago, Plato said: “No 2 persons are born exactly alike; but each differs from the other in natural endowments (360 B.C.).” Such individual differences are a hallmark of the experience of pain and have been a topic of keen interest to pain researchers for many years. Indeed, more than 70 years ago, in describing the rationale for their psychophysical study of pain sensitivity in healthy adults, Chapman and Jones12 stated that “A striking variation in the intensity of pain, experienced in diseases with apparently similar lesions, is a common observation.” Historically, this interindividual variability in pain response was more often viewed as a nuisance than a fruitful area of scientific inquiry; however, the genomic revolution and the ensuing promise of precision medicine have reinvigorated and legitimized scientific interest in individual differences.11,17,20,52 The purpose of this article is to provide an overview of factors contributing to individual differences in pain. Given the abundance of potential individual difference factors, I will not attempt a comprehensive review of this field, rather provide examples of individual differences from our own research as well as the work of other investigators. First, I will introduce the topic of individual differences in responses to pain and its treatment, including a biopsychosocial context for conceptualizing individual differences. Then, I will present findings regarding demographic factors that are associated with individual differences in pain. Next, I will discuss genetic and psychosocial contributions to individual differences, and I will present examples of interactions among these multiple individual difference factors. I will describe the clinical implications of individual differences in pain, followed by conclusions and recommendations for future research.
By definition pain is a subjective and highly personal experience, which presents challenges for both the researcher and clinician. A well-recognized challenge resulting from the subjective nature of pain is that direct measurement of pain is impossible, rather we must rely on individuals' self-report, and to some extent their behavior, to provide a glimpse into their experience. However, an equally important but less often discussed challenge results from the highly personal nature of the pain experience; the experience of pain is sculpted by a mosaic of factors unique to the person, which renders the pain experience completely individualized. That is, there are pervasive and important individual differences in pain, and these individual differences produce pain experiences that are completely unique to the person experiencing them (ie, they make the pain personal). For purposes of this article, I will define individual differences in pain as between-person differences in the pain experience that are independent of the initiating stimulus. Perhaps the simplest manifestation of individual differences is that an experimental stimulus delivered at a standardized intensity elicits subjective pain reports that vary dramatically between individuals (Fig. 1), as noted decades ago by Chapman and Jones12 and more recently by others.15,25,71,86 Interestingly, these differences in self-reported pain are corroborated by interindividual differences in cerebral activation evoked by the same painful stimulus13 and are in part predicted by individual differences in brain morphology,24 suggesting that these individual differences are not simply a product of idiosyncrasies in the reporting of pain. Such individual differences also emerge in the clinical environment. For example, pain reports following the same surgical procedure vary greatly across patients.6,42,83 Similarly, responses to pain treatments are characterized by robust individual differences2,4,10,52; however, a discussion of factors contributing to variability in treatment responses is beyond the scope of this article, which will focus on the individual difference factors impacting the experience of pain.
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