Everyone responds differently to pain. We know that. But can we meaningfully group people by how they respond? A recent study by Zheng et al. (2014) has found that by exposing a sample of healthy, pain free participants to a common, stable threat, and then measuring their pain response, we can differentiate an individual’s pain response into one of two categories: pain-adaptive, or pain-nonadaptive. Moreover, they have been able to identify that pain-adaptive individuals show local pain inhibition that lasts for longer than their counterparts.
So what does this all mean, and what on earth is a pain-adaptive person?
When exposed to a prolonged threat – in this case putting their hand in icy water (1-4°C) for 5 minutes, or placing a hot thermode (47°C) on their calf for 7 minutes – the pain-adaptive individual will report an increase in pain, followed by a reduction in pain approximately 2 minutes later. Conversely, the pain-nonadaptive person will either report the pain as continually increasing, or to increase and then plateau throughout the exposure. Zheng et al. found that 26% (11 out of n=42) of participants were pain-adaptive to the heat, while 39% (16 out of n=41) were pain-adaptive to the cold. Interestingly, being adaptive to heat stimuli was not correlated with being adaptive to cold stimuli or vice versa; in fact, some participants were adaptive to both and some did not adapt to either.
So why do we have these different pain responses?
Well, at this stage, we don’t know! Local pain inhibition and conditioned pain modulation (CPM– also known as Diffuse Noxious Inhibitory Control or DNIC) are two of our inbuilt mechanisms for reducing pain: in local pain inhibition, overall pain reduces when two threats are located in close proximity on the body. In CPM, overall pain reduces when a distant threat lessens the perception of another threat. CPM has been of particular interest recently as it has been shown to be altered in some chronic pain conditions, including osteoarthritis , fibromyalgia , and tension-type headache . In this study, Zheng et al. tested the participants’ endogenous pain inhibition and were able to identify differences in the length of time that local pain inhibition lasted between the two groups. While the participants were experiencing either cold pain from the icy water, or heat pain from the heat thermode, a second threat (in the form of a mechanical pressure stimulus) was introduced at sites that were either local or distant to the site of current pain – the local sites enabled the participants’ local pain inhibition to be tested, while the distant sites targeted the participants’ CPM. This second threat was then reapplied 20 minutes later.
The pain-adaptive individuals demonstrated longer lasting local pain inhibition than the pain-nonadaptive individuals. When pressure pain thresholds (PPT – the magnitude of pressure that when applied against your skin (using an algometer) is first perceived as painful) were re-assessed 20 minutes after the local stimulus (heat or cold) was removed, the pain-adaptive individuals had maintained their increased PPT. Whereas, the PPT of the pain-nonadaptive individuals, had returned to pre-testing levels by the 20 minute re-test. While these findings do not explain why some individuals are pain-adaptive, and some are not, they are the first step in exploring how these two groups of pain responses are different on a physiological level.
In summary, another group of elitists has possibly been identified!
Features of the pain-adaptive individual include:
1) Faster pain facilitation (time to peak pain was 33% and 50% less than the pain-nonadaptive group for the icy water and heat thermode, respectively) – so perhaps these individuals have a more robust pain response system;
2) Faster pain reduction – after approximately 2 minutes of exposure to a stable noxious stimulus, pain reports from pain-adaptive people dropped by at least 2 points on the 0 to 10 numerical pain scale – so these individuals would be the last ones standing in a charcoal walk challenge;
3) Prolonged local pain inhibition – with increased local PPT maintained for at least 20 minutes after exposure, these individuals exhibit a higher threshold to pressure pain in an area that has been exposed to noxious thermal stimulation – so perhaps you need to slap their sunburn extra forcefully if you want it to hurt them!
Beyond considering the individual advantages of pain adaptability, such categorisation of individuals may be clinically helpful when we are trying to develop pain therapies that are more appropriate to an individual’s pain response. Although there are many more factors (such as psychological and contextual influences) that need to be considered, categorisation of pain adaptability may be another step in the right direction for pain management – and, as Zheng et al. describe, we now have a model that allows us to distinguish these pain-adaptive and pain-nonadaptive individuals from one another. Importantly, it must be remembered that some individuals were only pain-adaptive to one of the noxious stimuli – either the heat, or the cold – and so perhaps further categorisation of the pain-adaptive individual is needed to describe if one is heat pain-adaptive, cold pain-adaptive, or heat and cold pain-adaptive. In saying this, we might also need to be cautious about how such a categorisation would be communicated to a patient, or if it should be communicated at all. I personally don’t know how well I would respond to being told I was a pain-nonadaptive person – it is not a very motivating or reassuring label!
For more info click on a full-text version of the article.
Em joined the BiM group when she started her honours degree in nociceptive spatial summation, while studying Physiotherapy. Em has just finished her degree and is currently working as a research assistant at BiM after being sucked into the pain world, and the antics of the BiM group. She is not yet sure what her future holds, but she knows she loves learning and loves working with people, so both further study, and clinical work is likely. When Em isn’t worrying about her future, you can find her baking BiM-approved allergen-free treats, climbing lofty, cycling the summit, kicking boxing bags, attempting to translate basketball into netball, or collapsing in a heap on the couch!
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