‘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 (eg, 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 twentieth 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 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?

Pioneering American pain neurophysiologists Hardy, Wolff and Goodell (1952) defined ‘pain sensitivity’ as the level of experimental stimulation that produces pain.

John Quintner.

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, they perpetuated conflation of the two.

However, there was another problem.

The word ‘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 to it in meaning.

Because of this proximity, strictly speaking ‘sensitivity’ is also a property of the response/responder, not of the stimulus.

It turns out that Hardy et al’s concept of ‘pain sensitivity’ was incorrect in terms of language—it is a misnomer.

Attempting 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)’.

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

English physician Kenneth Keele (1967) was familiar with the body of research produced 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’.

Professor Milton Cohen. 

Based on their responses to a target of around two kilograms of pressure, he 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 mid-nineteenth-century researchers to conceptualise pain as a measurable, ‘normalising’ and therefore 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, followed by increasingly sophisticated techniques for 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.

This letter has previously appeared online on the Australian Pain Society website (blog).

>> John Quintner is a consultant physician in pain medicine and rheumatology (retired) and works as a volunteer in pain education at Arthritis & Osteoporosis WA.

>> Professor Milton Cohen is a specialist pain medicine physician, rheumatologist and adjunct professor at St Vincent’s Clinical School, UNSW Medicine, UNSW Sydney.

REPLY

I thank John Quintner and Milton Cohen for their response to the InMotion series of articles concerning ‘pain sensitivity’.

This is not the first time that these luminaries and deep thinkers have reminded the pain community of the apparent errors in terminology and the potential for us as clinicians to objectify pain in an unreasonable manner.

Quintner and Cohen have raised a number of reasonable concerns about the term ‘pain sensitivity’.

The history of terms such as ‘sensitivity’ and ‘sensibility’ reminds us that pain terminology has always been a little fraught.

A quick perusal of the International Association for the Study of Pain’s list of pain terms and their definitions also shows us that they don’t all make perfect sense and that they may contain contradictions in terms or propose circular arguments (see nociceptive pain as one example).

Another example is the recent introduction of the term ‘nociplastic’.

This has not been without its critics, related both to semantics and to how the term makes us view patients and their symptoms (Kosek et al 2016 and following correspondence).

The challenge of ‘objectifying’ pain has existed arguably for as long as pain itself.

Elaine Scarry, in her book The Body in Pain (1985), puts it quite succinctly—‘Pain differs from the [psychic, somatic and perceptual] events... by not having an object in the external world’—meaning that pain resides within the sufferer.

As such, Scarry asserts that this internal experience of pain renders language very difficult, if not impossible, to reliably employ in order to explain it.

This is what we meet in the clinical encounter.

Patients allow us into their realm as they try to describe their ‘sensory and emotional experience’ of pain.

They desire us to alter this experience in some way.

Modern pain education sits somewhere in this clinical encounter.

Clinicians have their language to describe their ‘world’, just as patients have theirs.

Therefore, a space exists where these two worlds (and perhaps world views) observe and relate to each other.

Sometimes, as clinicians, we see these worlds collide.

Quintner and Cohen have written much on this topic in the past (eg, Quintner et al 2008).

What Quintner and Cohen’s response does not suggest is how we might otherwise explain to ourselves or our patients the phenomena that are seen in a so-called sensitised pain state.

Tim Austin FACP.

The previous (pre-2016) classification system of pain, which included only nociceptive and neuropathic pain types, excluded (by definition alone) many experiences of pain.

Patients frequently want a label for their pain (as opposed to the pejorative labelling of patients to fit preconceived criteria).

The constructs (and they truly are constructs) of nociplastic pain, pain sensitivity or for that matter chronic primary pain have allowed many patients to be heard, validated and understood and for progress to be made in the journey towards pain relief or contented management.

Mark Sullivan’s delightful paper ‘What do we owe patients with chronic pain?’ (2019) addresses many of our patients’ desires—for a diagnosis or cause of their pain, for a fix for the pain, for understanding and acceptance.

He notes that ‘pain may be relieved through a change in its meaning rather than a change in intensity’.

It seems almost too crude to call this conceptual change education, but that is what it is.

Sullivan goes on to describe pain management as ‘increasing the capacity of patients to be agents in their own lives’.

Perhaps pain education is the intersection of a clinician’s terminology and that of the patient.

This is what Darren Beales and Tim Mitchell’s InMotion series on pain sensitivity reminds us constantly.

The first article in the series emphasised the importance of person-centred care and of responding to the person where they are at any moment.

Perhaps the issue here is not with the term ‘pain sensitivity’—a fraught one, as correctly stated by Quintner and Cohen.

Maybe, as with so many terms in the field of pain management, it is more important how we use the term in our own minds and as we interact with our patients.

>> Tim Austin FACP is a Specialist Pain Physiotherapist (as awarded by the Australian College of Physiotherapists in 2021) and the national chair of the APA Pain group.

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 1967;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.