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Pain measurement: Measuring an experience is like holding water


Measurement in pain is complicated. Firstly it’s an experience, so inherently subjective – how do we measure “taste”, for example? Or “joy”? Secondly, there’s so much riding on its measurement: how much pain relief a person gets, whether a treatment has been successful, whether a person is thought sick enough to be excused from working, whether a person even gets treatment at all…

And even more than these, given it’s so important and we have to use surrogate ways to measure the unmeasurable, we have the language of assessment. In physiotherapy practice, what the person says is called “subjective” while the measurements the clinician takes are called “objective” – as if, by them being conducted by a clinician and by using instruments, they’re not biased or “not influenced by personal feelings or opinions in considering and representing facts”. Subjective, in this instance, is defined by Merriam Webster as “ relating to the way a person experiences things in his or her own mind. : based on feelings or opinions rather than facts.”  Of course, we know that variability exists between clinicians even when carrying out seemingly “objective” tests of, for example, range of movement, muscle strength, or interpreting radiological images or even conducting a Timed Up and Go test (take a look here at a very good review of this common functional test – click)

In the latest issue of Pain, Professor Stephen Morley reflects on bias and reliability in pain ratings, reminding us that “measurement of psychological variables is an interaction between the individual, the test material, and the context in which the measure is taken” (Morley, 2016). While there are many ways formal testing can be standardised to reduce the amount of bias, it doesn’t completely remove the variability inherent in a measurement situation.

Morley was providing commentary on a study published in the same journal, a study in which participants were given training and prompts each day when they were asked to rate their pain. Actually, three groups were compared: a group without training, a group with training but no prompts, and a group with training and daily prompts (Smith, Amtmann, Askew, Gewandter et al, 2016). The hypothesis was that people given training would provide more consistent pain ratings than those who weren’t. But no, in another twist to the pain story, the results showed that during the first post-training week, participants with training were less reliable than those who simply gave a rating as usual.

Morley considers two possible explanations for this – the first relates to the whole notion of reliability. Reliability is about identifying how much of the variability is due to the test being a bit inaccurate, vs how much variability is due to the variability of the actual thing being measured, assuming that errors or variability are only random. So perhaps one problem is that pain intensity does vary a great deal from day-to-day.  The second reason is related to the way people make judgements about their own pain intensity. Smith and colleagues identify two main biases (bias = systematic errors) – scale anchoring effects (that by giving people a set word or concept to “anchor” their ratings, the tendency to wander off and report pain based only on emotion or setting or memory might be reduced), and that daily variations in context might also influence pain. Smith and colleagues believed that by providing anchors between least and “worst imaginable pain”, they’d be able to guide people to reflect on these same imagined experiences each day, that these imagined experiences would be pretty stable, and that people could compare what they were actually experiencing at the time with these imagined pain intensities.

But, and it’s a big but, how do people scale and remember pain? And as Morley asks, “What aspect of the imagined pain is reimagined and used as an anchor at the point of rating?” He points out that re-experiencing the somatosensory-intensity aspect of pain is rare (though people can remember the context in which they experienced that pain, and they can give a summative evaluative assessment such as “oh it was horrible”). Smith and colleagues’ study attempted to control for contextual effects by asking people to reflect only on intensity and duration, and only on pain intensity rather than other associated experiences such as fatigue or stress. This, it must be said, is pretty darned impossible, and Morley again points out that “peak-end” phenomenon (which means that our estimate of pain intensity depends a great deal on how long we think an experience might go on, disparities between what we expect and what we actually feel, and differences between each of us) will bias self-report.

Smith et al (2016) carefully review and discuss their findings, and I strongly encourage readers to read the entire paper themselves. This is important stuff – even though this was an approach designed to help improve pain intensity measurement within treatment trials, what it tells us is that our understanding of pain intensity measurement needs more work, and that some of our assumptions about measuring our pain experience using a simple numeric rating scale might be challenged. The study used people living with chronic pain, and their experiences may be different from those with acute pain (eg post-surgical pain). The training did appear to help people correctly rank their pain in terms of least pain, average pain, and worst pain daily ratings.

What can we learn from this study? I think it’s a good reminder to us to think about our assumptions about ANY kind of measurement in pain. Including what we observe, what we do when carrying out pain assessments, and the influences we don’t yet know about on pain intensity ratings.

Morley, S. (2016). Bias and reliability in pain ratings. Pain, 157(5), 993-994.

Smith, S. M., Amtmann, D., Askew, R. L., Gewandter, J. S., Hunsinger, M., Jensen, M. P., . . . Dworkin, R. H. (2016). Pain intensity rating training: Results from an exploratory study of the acttion protecct system. Pain, 157(5), 1056-1064.

2 comments

  1. While pain itself is an experience, it is not one that arises and exists out an ethereal psychological vapor! Pain is a physiologic experience that results from a complex set electronic events that present some as yet undefined electrical signal array to the conscious centers of the brain. Those same electronic signals also have output to other neurophysiologic events.

    To put it in more understandable terms, the same electronic events and processes that lead to the “cognitive experience” of pain also alter functions such as motor reflexes. Either sensitivity to stimuli that trigger reflexes, and/or the amplitude of those reflexes can be heightened in at least some important reflexes such as the flexor withdrawal reflex. These abnormal reflexes can and do lead to pathologic objectively measurable events such as falls. These falls can and do lead to significant injuries and even death…..and they occur in the absence of opioids so the falls cannot be blamed on opioids. There are a number of high quality papers showing a correlation between pain and falls. For a more detailed description and a more comprehensive bibliography see the article on Chronic Pain and Falls in Practical Pain Management September 2015 issue. (http://www.practicalpainmanagement.com/pain/other/co-morbidities/chronic-pain-falls)

    Indeed many patients with chronic pain have a crossed adductor reflex which is generally considered a pathologic reflex. How are all of these correlated with pain? Unfortunately the medical profession as a whole has acquiesced in the believe that the experience of pain is some mystical brain based event. It is more than that. And Pain Patients have a real complex and poorly understood neurophysiologic disease.

    1. Hi Rick, I don’t think that pointing out that the experience of pain is multifactorial and that we can’t directly measure it stops us from also understanding that pain as an experience can also contribute to a whole bunch of physiological responses. I also think that our knowledge of pain neurobiology means we MUST be careful about assuming that pain is only biological. Pain is a psychological experience underpinned by many many neurobiological processes that we don’t fully understand. I personally don’t believe it’s some ethereal psychological vapour – and work very hard to make sure that I include the bio in my biopsychosocial formulations (these develop in collaboration with the person experiencing the pain) – but for far too long (since Descartes at least) pain has been treated as merely a symptom of underlying tissue pathology without ANY attention being paid to the person who is experiencing that pain. For my money I’d rather have someone spend time listening to me and my experience than someone who only attends to my sore body parts. Chronic pain is a neurobiological problem, but ALSO an experience that arises out of social and psychological processes. It’s a case of levels of analysis – from microcellular to macro-social – and all of these are needed.
      BTW I’m not at all surprised that people experiencing pain are more prone to falling – but I don’t think it’s just a matter of increased reflex response.

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