explanations

The “onion ring” model of pain


Clinicians constantly search for a better way to describe the tangled mess that constitutes ways to explore pain. Today I’m hoping to add another way, but hopefully one that might help disentangle certain aspects of pain for ease of learning. And as usual, it’s largely not my own model, but one first developed by Professor John Loeser, eminent neurologist and neurosurgeon and Director of the Multidisciplinary Pain Center from 1982-1997 at the University of Washington.

There are many different versions of the ‘Onion ring’ model – Gordon Waddell, orthopaedic surgeon and contemporary of Loeser also developed one, and more recently we’ve seen a version from Lorimer Moseley and colleagues in NOI publications. I’m going back to Loeser’s one because I think it’s useful – and in the case of conceptual models like this utility is the measure by which we decide to adopt a model or not. You be the judge. This is my public announcement that this is not intended to be a scientific model for generating and testing hypotheses: it’s meant to be an explanatory metaphor, if you like.

OK, so what is this model?

Like any onion, the model has inner to outer layers, but unlike an onion, these layers are permeable, and slightly fuzzy. They interact with one another, and the resultant whole is intended to reflect the experience of pain, along with the aspects that you and I might see – and includes various factors thought to influence the experience. It’s incomplete because much of what is known about pain is incomplete. It can’t explain everything, because no metaphor can – but it does provide some hooks for our minds to grab onto when we’re accessing new information and we want to establish relevance and recognition.

Loeser’s Onion Ring Model (1983)

The purple ring in the centre is all about neurobiology for me. Loeser’s original model labeled this “nociception”, but since 1983 we’ve learned a great deal more about the neurobiology of pain and we know that pain in the absence of nociception is probably a product of something gone awry in the way our nociceptive system is interpreting information. It could be neuropathic pain (where there is an identifiable lesion of the somatosensory system), or it might be nociplastic pain ( “pain that arises from altered nociception despite no clear evidence of actual or threatened tissue damage causing the activation of peripheral nociceptors or evidence for disease or lesion of the somatosensory system causing the pain.” – click). At this level of the model this is not pain. This inner ring refers only to biological processes prior to conscious awareness.

The next ring (dark blue) refers to the conscious experience we have of pain. This is the part we personally experience – it’s subjective, unpleasant, sensory and emotional, and we learn to associate this experience with potential or actual tissue damage, or we describe it in similar ways. In many respects this is the quale – the quality of what-it-is-like to experience pain – although others would argue it is an aporia (In philosophy, Aporia means literally ‘impasse, difficulty in passage, lack of resources, puzzlement’). However we like to define it, this part of Loeser’s model refers to the experience once our brain/mind has deemed it relevant to our predicament.

But, as the saying goes, wait! There’s more!

Because this dark blue ring is experiential, we can’t share it, or even know about another’s experience unless we do something about it, and before we do something about it, we appraise or judge it. With some provisos (told you this was a metaphor not a testable model!).

Drawing from cognitive models, Loeser then wraps another ring around the experience “pain” – this is what he described as suffering, but I prefer to describe as “judgement” or “appraisal”. Suffering is a judgement that this experience is threatening our essential self, our future (Cassel, 1999). So while there are certain behaviours that occur prior to awareness or judgement (see this) as soon as we are consciously aware of pain we’re judging that experience. And probably, because brains don’t just sit there waiting for information to come towards it, there is a good deal of permeability between the neurobiology ring, the pain-experience ring, and this ring. But for simplicity’s sake, let’s take it that when we experience “ouch” we typically check it out and interpret the meaning of that ouch in context of where we are, what we’re currently doing, who we’re with, and our past experiences. This interpretation or judgement phase can augment the meaning of pain to increase its threat value, or vice versa (OMG that was a snake bite! or Oh that was a bruise I didn’t need).

Wrapping around that “judgement” ring is a further ring – and this is possibly the one we most need to come to grips with. This ring is the behavioural response to our appraised experience. Pain behaviour or what we do when we recognise and judge our experience of pain is complex. It’s complex because all human behaviour is complex. It’s also complicated because we naively judge one another on the basis of what we see – and our own assumptions about what that behaviour might mean.

Behaviours include nocifensive responses, but don’t stop there. As we develop and mature from babyhood to adulthood, we embroider and alter our behavioural response to pain, just as we do with our appraisals. As babies we’re likely to scream our lungs out at the heel prick test at birth. I hope we don’t do that when we get a flu jab (and I truly hope you DO get a flu jab, and if you’re in Christchurch New Zealand that you get a measles immunisation pronto). We learn what to do from watching others (social learning), from others responses to us (operant conditioning), and from events that occur at the same time as our pain occurs (classical conditioning). Social learning is powerful – within different cultural groups, peer groups and family groups, we learn what is normal and OK to do when we’re sore. We also get rewarded (or not) for the way we behave. Little kids get told “stop that crying, it’s nothing” when they stub a toe, or they might get cuddled instead. Footballers get extra time if they roll around on the ground with an injury during a match; rugby players get adulation when they carry on playing despite a rib fracture or two. And for some people, associating a movement with pain can lead to longstanding limitations and avoiding that same movement in case it brings the pain on.

Pain behaviours include language and even that old “pain rating scale”. We use language and nonverbal behaviour to communicate. So when someone says “my pain is 12/10” what they’re really saying is “this is more than I can bear, help me”. We do not have a pure measure of how intense a pain is – and any measure of intensity is likely filtered through a process of judgement “what does this mean for me?” and communication “what will happen if I say X number?” So stop judging someone if they say their pain is 12/10 – it means they’re freaking out, and need comfort.

If you’re smart you’ll notice that I’ve sneakily been discussing the final onion ring, and to be fair, Loeser didn’t include this in his version – it’s one that Waddell, Main, and others have added and I think it’s integral to understanding what’s going on so I’ve added it too. The outer ring refers to the social context because this influences what people do (pain behaviours) as I’ve just outlined. It also includes social factors such as the workplace and compensation, legislation covering what is and isn’t covered in insurance plans, our community attitudes towards people who are experiencing pain, stigma and social isolation and sense of online community and such.

Loeser’s onion ring provides me with some nice ways to separate parts of my understanding of pain so I can explain how and why we need to examine them and influence them separately. Health professionals are always and inevitably influencing the judgement, behaviour, and social aspects of pain. Sometimes we get to influence the neurobiology and through interactions between all these layers, sometimes the experience of pain is reduced. Other times it is not. At the same time, if we can begin to shift the judgements and what we do about pain and yes, the social contexts in which experiencing weird unexplained pain is viewed as a moral failing or attempt to “get secondary gain”, maybe then we can help people live better lives despite their pain.

Cassell, E. J. (1999). Diagnosing Suffering: A Perspective. Annals of Internal Medicine, 131(7), 531-534. doi:10.7326/0003-4819-131-7-199910050-00009

Loeser JD, Ford WE. Chronic Pain. In: Carr JE, Dengerink HA, (eds). Behavioral Science in the Practice of Medicine. New York: Elsevier Biomedical:1983:331-345

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