Authors: John Quintner and Milton Cohen

“How we come by our knowledge of another person’s pain is a nice study in communication. It has much in common with the sort of communication attempted by the painter, the poet and the musician – the conveying of moods and feelings.” [Parkhouse & Holmes 1963]

The popularity of “pain sensitivity”

The frequency with which the term “pain sensitivity” is found in the current pain literature testifies to its general acceptance by modern pain theorists, researchers and clinicians, if not also the lay community, e.g., Coderre & Melzack [1985], Mogil [1999], Bradley et al. [2000], Nielsen et al. [2009], Gracely & Ambrose [2011], Kim et al. [2017], and Beales et al. [2020].

“Pain sensitivity” appears to make sense because its connotation is “ability to tolerate pain”, in the same vein as “I have a high threshold for pain” or “it takes a lot of stimulus for me to acknowledge that I am hurting”.

When did it happen?

The term “pain sensitivity” was in use in the early years of the 20th century as evidenced by both Spearman [1906] and Whipple [1910] when discussing the skin’s “sensitivity to pain”.

Why “pain sensitivity” is a misnomer

In biology, the term “sensitivity” is a measure of how intense a physical stimulus has to be before a system reacts to it. Systems that react to physically weak stimuli are said to be more sensitive than those that do not react.

 “Pain sensitivity” is in fact a shorthand for “sensitivity to pain”, the basic premise of which is that pain is a stimulus. That assertion constitutes the fundamental error. By definition, pain is always an experience: it is a response (although of course not always to a definable stimulus) but never itself a physical “thing”. As “being sensitive” is itself a response, it follows that one cannot be “sensitive” to a response.

Accordingly, the term “pain sensitivity” constitutes an epistemological error, arising out of the conflation of pain as a response and pain as a stimulus.

How did this happen?

The pioneering American pain neurophysiologists, Hardy, Woolf and Goodell [1952] defined “pain sensitivity” as the level of experimental stimulation that produces pain. More specifically, “pain sensitivity” is the level of a noxious stimulus that evokes pain, which is the threshold for responding as “pain” (or, “pain threshold”).  This is a property of the stimulus, not of the response.

They also used the term “pain sensibility” when attempting to measure the intensity of pain experienced in response to suprathreshold stimuli. This experience terminates at “pain tolerance” when the subject says, “I cannot take any more stimulus.” “Pain sensibility” is a property of the response, not of the stimulus.

In their landmark publication, “Pain Sensations and Reactions”, Hardy et al. [1952] variously used the phrases “pain sensitivity retained”, “absence of pain sensitivity” and “alterations in pain sensitivity”.

Where did they go wrong?

It turns out that the language chosen by these pioneers was unfortunate. By using the terms “pain sensitivity” and “pain sensibility”, but not clearly identifying that one refers to a stimulus and the other to a response, the conflation was perpetuated. 

However there was another problem. The term “sensitivity”  – the ability of an organism to react to stimuli – not only comes from the same root (“sens-“) as  “sensibility”  – mental susceptibility or responsiveness – but also is very close in meaning to it. Because of this proximity, strictly speaking “sensitivity” is also a property of the response/responder, not of the stimulus.

Hardy et al’s concept of “pain sensitivity” was incorrect in terms of language: it is a misnomer.

An attempt to rectify the error?

Perhaps realising that he and his co-workers had fallen into error in their terminology, Hardy [1956] reconceptualised “sensitivity” in relation to noxious stimulation rather than pain. He went on to define “pain experience” broadly “as those combinations of reactions involving consciousness which have been observed to be highly correlated with noxious stimulation and pain (notwithstanding the circular argument).” (p.42) Moreover, he recognised that the sensation component of pain is but a part of the total pain “experience” and, “indeed, may not even be a major feature.”

 Perpetuating the error

The English physician Kenneth Keele [1957] was familiar with the body of research performed by Hardy et al. [1952] and set about determining the pain threshold in a population of 363 pain-free healthy persons. He used a pressure algometer to measure “sensitivity to pressure (-induced or -evoked) pain”.

Based upon their responses to a target of around 2 kg of pressure, Keele was able to classify his subjects as falling into one of three groups, hypersensitives, normosensitives and hyposensitives.  Because this was a stimulus-dependent exercise, the potential for stigmatisation of the responders, by putting them into “groups”, was established.

Clearly, as evidenced by the continued use of “pain sensitivity” in the peer-reviewed pain literature, this later attempt by Hardy to clarify the terminology was not heeded. Instead, pain research has continued to reinforce the attempts of the mid-nineteenth century researchers to conceptualise pain as a measurable, “normalising” and, thus, objectifiable phenomenon [Stahnisch 2015].

Regrettably not only is the misnomer “pain sensitivity” abundant but also technological advances in the neurosciences have served to perpetuate if not amplify the epistemological error in their quest to “objectify” pain.

Commencing with the advent of electroencephalography, and followed by increasingly sophisticated techniques of neuroimaging, and deep brain stimulation, neuroscientific research has produced a much more refined “objectification” of pain phenomena [Stahnisch 2015], which conveniently overlooks the fact that whatever may be observed to happen in the brain is not pain.

Conclusion

The moods and feelings conveyed by those who are experiencing pain sit rather uneasily beside the explosion of information produced by modern neuroscience, which is obsessed with producing objectifiable and reproducible knowledge. These people have become the subjects of Medicine’s new “way of seeing” them through the lens of modern neuroscience. The problems associated with the term “pain sensitivity” seem to have so far escaped the attention of pain theorists.

References

Beales D, Mitchell T, Moloney N, et al. Masterclass: A pragmatic approach to pain sensitivity in people with musculoskeletal disorders and implications for clinical management for musculoskeletal conditions. Musculoskelet Sci Pract 2020: Available online 18 July 2020, 10221.

Bradley LA, McKendree-Smith NL, Alberts KR, et al. Use of neuroimaging to understand abnormal pain sensitivity in fibromyalgia. Curr Rheumatol Rep 2000;2(2):141-148.

Coderre TJ, Melzack R. Increased pain sensitivity following heat injury involves a central mechanism. Behav Brain Res 1985;15(3):259-262.

Gracely RH, Ambrose KR. Neuroimaging in fibromyalgia. Best Pract Res Clin Rheumatol 2011;25(2):271-284.

Hardy JD. The nature of pain. J Chron Dis 1956;4(1):22-51.

Hardy JD, Wolff HG, Goodell H. Studies on pain: a new method for measuring pain threshold: observations of spatial summation of pain. J Clin Invest 1940;19:649-657.

Hardy JD, Wolff HG, Goodell H. Pain Sensations and Reactions. Baltimore MD: Williams & Wilkins, 1952.

Keele KD. Pain sensitivity and the pain pattern of cardiac infarction. Proc R Soc Med 1962;60:17-19.

Kim HJ, Yang GS, Greenspan JD, et al. Racial and ethnic differences in experimental pain sensitivity: systematic review and meta-analysis. Pain 2017;158(2):194-211.

Mogil JS. The genetic mediation of individual differences in sensitivity to pain and its inhibition. Proc Natl Acad Sci 1999;96:7744-7751.

Nielsen CS, Staud R, Price DD. Individual differences in pain sensitivity: measurement, causation and consequences. J Pain 2009;10(3):231-237.

Parkhouse J. Holmes CM. Assessing post-operative pain relief. Proc R Soc Med 1963;56:579-585.

Spearman C. ‘Footrule’ for measuring correlation. Br J Psychol 1906;2:89-108.

Stahnisch FW. Objectifying ‘pain’ in the modern neurosciences: a historical account of the visualization technologies used in the development of ‘algesiogenic pathology’, 1850-2000. Brain Sci 2015;5:521-545. DOI: 10.3390/brainsci5040521

Whipple GM. New instruments for testing discrimination of brightness and of pressure and sensitivity to pain. J Educ Psychol 1910;1(2):101-106.

About Australian Pain Society

The Australian Pain Society is a multidisciplinary body aiming to relieve pain and related suffering through leadership in clinical practice, education, research and public advocacy.

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