Is finding a pukka neurosignature for pain on the top of our discovery list?

Earlier this month, Nature Reviews Neurology published a Consensus Statement from a presidential taskforce of the International Association for the Study of Pain (IASP), on the use of brain imaging tests for chronic pain. The statement answers the question of whether chronic pain can be identified ‘objectively’. Whether or not brain imaging can provide a ‘neurosignature for chronic pain’ (i.e. provide diagnostic differentiation) has been hotly debated over recent years; the medicolegal and ethical implications of this are not trivial. Overall, the authors of this Consensus Statement emphasise that, while brain imaging has the potential to increase our understanding of the neural mechanisms underlying pain, predict individual outcomes from pain management interventions, and find targets for therapeutic agents, its use in the context of diagnosis is still in a discovery phase. While we think that this is a realistic and responsible conclusion for the present, a few concerns about the statement remain.

Our first concern is that although the authors themselves state that brain imaging should be used as an adjunct, not a replacement, for pain reports, they go on to suggest that neuroimaging could be “particularly valuable for nonverbal populations (…disabled, very young or have dementia…), cases in which the evidence from verbal reports and behavior is conflicting…” We find this a worrying take-home message. Once upon a time we used back x-rays to tell us whether someone really had back pain. If we aren’t careful we may find ourselves coming full circle, using a brain image of proof of someone’s pain report. Just as a back x-ray did not tell the full story, neither does a brain image, in our opinion.

Why might brain imaging fail to tell us the whole story? Well there is one big issue that was not raised in the statement, and we were surprised by its absence. The issue is ‘the hard problem’ [1] of how and why physical processes might give rise to experience. Surely this argument needs to take place before any questions of technology or methods of analysis are asked.

We argue that brain imaging can provide us with, at best, objective correlates of some of the subjective experience of pain. But, can you find features such as time, knowledge, meaning, and social context in the physical properties of the brain – with objective measures such as brain regional blood flow, or varying connectivity of neurons? Even if you take the point of view that pain subjectivity is partly physical (some reports of abolition of pain with brain injury might support this), and partly a social construct involving experience, et cetera, the idea that we might discover a pukka brain biomarker of pain might need a revolution in a few other fields first.

We acknowledge that such a big philosophical question has the field divided. Some people would argue that pain – a subjective experience – exists outside the brain – that the brain is necessary, but not sufficient for pain (Derbyshire [2]). Others believe that the hard problem will resolve if we find out how the brain works. They argue that more fine-grained analysis tools such as MVPA [3], big data sets, or technological advancement will enable us to  identify the neurological mechanisms that generate pain. This concept, of objective correlates of a subjective experience, is crucially important to this debate and we believe it deserved at least a paragraph of airplay.

Another interesting omission for us in this discussion is that of suffering. We don’t try to image suffering – why is that? Pain and suffering are not the same, but is it not the suffering that we are trying to ease? If a person presents with pain and no suffering we perhaps don’t feel the same urge as a clinician or caring health professional to ‘do something’; one of the imperatives that drives this discussion is our desire as researchers and clinicians to better understand pain and help people who suffer from pain.  Would an objective measure be able to account for this component of the pain story?

Overall we think that the risks associated with using brain imaging as a test for chronic pain are considerable, and not worth taking with what we have available to us at this stage. We wish for a broader debate about the appropriateness and validity of chasing a diagnostic measure for chronic pain. We must take responsibility as scientists. The authors state, as part justification for this Consensus Statement, that “Patients seek objective testing to demonstrate the reality of an invisible condition that is sometimes subject to doubt…”. Indeed chronic pain may be yet invisible, but with a few exceptions it is not inaudible. That is, a patient’s report of pain should not be subject to doubt. And in using brain imaging, as it stands, as a test for pain we run the risk of buying into this doubt. We believe we are better off empowering patients with education and explanations of the complexities of pain and its subjectivity than buying into the temptation to demonstrate that someone is in pain.

About Carolyn Berryman

Carolyn finished her PhD with BiM in 2015 exploring chronic pain, somatic hyper vigilance and cognitive function and is pursuing her research and teaching interests in chronic pain and cognitive function with clinical application.

Carolyn has masters degrees in physiotherapy and medical science (pain management). Before winning an Australian Postgraduate Award to return to study she taught pain sciences to under and post graduate physiotherapy students at UniSA with the Noisters and ran her own clinic for a couple of decades. Way back in 1995 she was co-convenor of the inaugural Moving in on Pain conference in Adelaide. Now she uses that experience to inspire and mentor the next generation of interest in pain sciences at Uni SA.

About Flavia Di Pietro

Flavia Di Pietro BodyInMindFlavia Di Pietro completed her PhD with the Body in Mind Sydney group early in 2014. Her project used functional MRI to investigate the brain’s patterns of activity in people with complex regional pain syndrome (CRPS) of the upper limb. She now works as a post-doctoral researcher at the University of Sydney with Associate Professor Luke Henderson. They are researching the electrical and chemical function of the brain in people with chronic orofacial pain.

References

[1] Chalmers, D., The conscious mind: in search of a fundamental theory. 1996, Oxford, UK: Oxford University Press.

[2] Derbyshire, S., Pain and the dangers of objectivity. First ed. Meanings of Pain, ed. S. van Rysewyk. 2016: Springer International Publishing AG.

[3] Koch, C., et al., Neural correlates of consciousness: progress and problems. Nature Reviews Neuroscience, 2016. 17: p. 307 – 321.