I’m going to discuss one of the more complex and debatable subjects among health care professionals, pain.
Essentially I’m setting myself up to have a very rocky start to this site. However, considering the main reason people see a physiotherapist for is pain somewhere in their body, particularly in my area of work -the musculoskeletal system (fancy jargon for things relating to bone, cartilage, muscle, tendons and ligaments) – as well being the main cause of disability globally (Hurwitz et al, 2018), I feel it is a subject we need to try to at least reach a common ground on from the start.
- Curiosity: According to the 2015 Global Burden of Disease Study, low back pain together with neck pain, were the leading cause of years lived with disability globally, and have remained as such since 1990 (Hurwitz et al, 2018).
So what is pain? This thing that basically everyone has experienced one time or another in their life, but when we try putting it into words, it doesn’t really come easy. I say mostly because there are actually some people who are born without the ability to experience pain – keep that in mind, as it will come up later.
We can mostly agree pain is unpleasant, although there are some people who appear to actually find it quite enjoyable, usually named masochists (if you don’t know what that means, type it in, Google’s your friend as Jay Z said – just make sure your grandma isn’t nearby). We can agree we feel pain somewhere in our body, although some people sometimes also experience in limbs that they are missing, an experience known as ‘phantom limb pain’.
- Curiosity: phantom limb pain termed coined by American Civil War military battlefield surgeon Silas Weir Mitchell (Collins et al, 2018).
Sorry, it doesn’t seem like I’m making this any easier for us. It will be philosophically lacking if we start talking about a subject without having defined well what we’re actually talking about, so my suggestion is that we start by discussing briefly how our understanding and definition of pain evolved throughout the ages.
The first attempts at explaining pain were mainly mystical in nature (Stillwell and Harman, 2018). However in the seventeenth century, as a direct result from the renascence and its focus on leaving behind mystical and religious explanations to the world, came the man who doubted everything, René Descartes. He said “I’m going to create an idea so influential and controversial, that it will distort our understanding of pain and the relationship between the human body and mind for generations to come”. As he said, he did, and Cartesian Dualism was born. This is a theory that argued the existence of a dualism of body and mind, the two being distinct: the mind being immaterial and the body being material (Stillwell and Harman, 2018; Russel 2004). Under this theory Monseigneur Descartes explained that when a person received a particular stimulus such as suffering a cut or being burnt, this would cause the tugging of physical tubes that travelled to the brain, where they would cause the release of animal spirits that caused pain – which he deemed to be directly proportional to the amount of tissue damage – and a consequent response of moving the affected body part away from the stimulus (Stillwell and Harman, 2018).
When you first hear about this theory, you may think it is outdated, if not also ridiculous. I mean, animal spirits being released in the brain.
It took two centuries for the concept of animal spirits to stop being used. Despite this dualistic theory of body and mind, and linear relationship between something called noxious (potentially nocive) stimulus and pain still remains the most prevalent one throughout most of society (Stillwell and Harman, 2018), although many sectors of healthcare are making a significant effort to change this.
- Curiosity: Noxious stimuli are potentially dangerous stimuli captured by specialized peripheral receptors that fall into one of three categories: mechanical (for e.g. pressure, growth, incision), thermal (hot or cold) and chemical (reduced oxygenation, infection). These receptors capture stimulus fitting these categories and convey them in the form of electrical nerve signals. These stimulus are not harmful per se, however when the these signal reach a certain intensity they are perceived by the body as being potentially dangerous (Potter, 2007). Think about how both light touch and pinching, or warm and boiling water, belong to the same class of stimulus, but at different intensities, and as such being perceived differently in normal physiological function.
The problem with this type of linear relationship was that it was not able to provide an explanation for cases where pain was present without a clear physical cause, like the previously mentioned phantom limb pain or most cases of non-traumatic lower back (Lim et al, 2018; Lewis and Sullivan, 2018). In an attempt to improve on this, in the 1960’s scientists focused more on the possibility of the brain having an active role in pain modulation, as opposed to its so far attributed role of a simple passive receiver (Stillwell and Harman, 2018).
It was at this time when the well known and still vastly utilised gate control theory of pain was presented by Melzack and Wall. In simple terms, the argued that there was some type of neurophysiological “gate” in the spinal cord that could be closed by non-noxious stimuli, resulting in actually nociceptive inputs to be blocked from ascending into the brain (Stillwell and Harman, 2018).
Melzak and Wall are also the first ones to propose that the brain could stop nociceptive stimulus through a process called Descending Inhibition. Despite we now knowing that gate theory is incorrect, the idea that pain could be as much a top-down as a bottom –up process was a big breakthrough in pain research and continues to be build upon by modern research (Stillwell and Harman, 2018).
However, as often occurs with scientific breakthroughs, they lead to inaccurate extrapolations. A couple of decades after Melzack and Wall we start seeing an emergence of brain-centric theories of pain. The most influential of these was actually proposed again by Melzack: the neuromatrix theory(Stillwell and Harman, 2018). This theory states that sensory, affective and cognitive-related brain regions provide inputs to a widely distributed neural network in the brain, the body-self neuromatrix, which result in outputs to other brain areas that subsequently produce the perception of pain, pain related action programmes (quickly moving your foot away from the Lego piece you just stepped on), and stress regulation programs (immune response) (Stillwell and Harman, 2018).
By now you may be thinking – “But what is the problem of the brain having a central role in pain? I thought that’s were all our experiences come from. It could also explain those cases in which people have pain without structural causes or on missing body parts!”.
Well, your reasoning sounds logical. However, consider the following: if the brain is the source of pain, then when we experience pain, the previously mentioned areas of the pain should be active, and when we are not experiencing pain, those areas will be inactive. The empirical conclusion of this would be that if we monitor those areas, let’s say using some type of electrodes or even the fancier functional MRI (fMRI) while someone is having a pain experience, then we could confirm if those areas are indeed behind the experience of pain. Salomons and their team tried exactly that and published the results in the JAMA Neurology journal (Salomons, 2016), but the results were quite surprising – they were able to locate and record the theorized pain-related activity of certain brain areas, but, this same brain centers displayed the same pain-related signatures were present and recorded in people who had congenital insensitivity to pain. People who literally can’t feel pain – I told you before that detail was going to be important.
So how can people who can’t experience pain have the same pain related areas and signatures in their brain? Is pain not in the brain? Where is it and how does it work? It sounds like we still have as many questions as when we started, if not even more.
Maybe because we’re looking at pain as a simple cause-effect relationship and it is actually more complex than that. As stated by Manzotti (2016), in all known cases of pain a brain is present, but also are bodies, stimuli, tissue changes, behaviours of that body and brain, social interactions between those bodies and brains, and an environment in which all those exist.
If you want to find what this means for our understanding of pain, don’t miss the second part of this explanation!
Collins, K. L. et al. 2018, ‘A review of current theories and treatments for phantom limb pain’, Journal of Clinical Investigation, 128(6), pp. 2168–2176. doi: 10.1172/JCI94003
Hurwitz, E.L., Randhawa, K., Yu, H., Côté, P., Haldeman, S. 2018, The Global Spine Care Initiative: a summary of the global burden of low back and neck pain studies. European Spine Journal, [e-journal]27, pp.796-801. Available through: https://link.springer.com/article/10.1007/s00586-017-5432-9 [Accessed 9 January 2021].
Lewis, J. and Sullivan, P. O. 2018 ‘Is it time to reframe how we care for people with non-traumatic musculoskeletal pain ?’BRr J Sports Med 0(0), pp. 1–2. doi: 10.1136/bjsports-2018-099198.
Lim, Y. Z. et al. 2019 ‘People with low back pain want clear, consistent and personalised information on prognosis, treatment options and self-management strategies: a systematic review’, Journal of Physiotherapy. Elsevier B.V., 65(3), pp. 124–135. doi: 10.1016/j.jphys.2019.05.010.
Potter, J. F.,Titman, H.M. 2007. Persistent Pain. In: R.J. Ham, P.D. Sloane, G.A. Warsaw, M.A. Bernard, E. Flaherty, eds. 2007 Primary Care Geriatrics. Mosby, pp.350-360
Russel, B. 2004. History of Western Philosophy. New York: Routledge Classics
Salomons, T. V., Iannetti, G. D., Liang, M., &Wood, J. N. 2016. The Bpain matrix^ in pain-free individuals. JAMA Neurology, 73, 4–5. https://doi.org/10.1001/jamaneurol.2016.0653.
Stilwell, P., Harman, K. 2018. An enactive approach to pain: beyond the biopsychosocial model. Phenomenology and the Cognitive Siences,[e-journal] 18, pp.637-655. Available through: https://doi.org/10.1007/s11097-019-09624-7