In my last post I discussed some of the mechanisms thought to be involved in central sensitisation, and while many of the details remain pretty unknown, I think the general conclusion is that yes, it really is a thing. What do I mean by central sensitisation? Well, it’s curious, it can refer to the processes at spinal and brain levels that seem to reduce the usual descending inhibitory mechanisms, expand the areas in which neural activity takes place, and allows increased information flow to eventually reach conscious awareness. At the same time it can refer to the experience in which a person feels greater pain than anticipated, given the degree of input; pain that is distributed more widely than anticipated, given the degree of input; and/or pain that lasts longer than we’d expect, given the degree of input (Woolf, 2011). BTW most of this post is derived from talks given by Pro Jo Nijs at the recent New Zealand Pain Society Conference.
The question now is whether this really matters. After all, nociceptor inputs can trigger a prolonged but reversible increase in central nociceptive pathways – if they’re reversible, just eliminate the original nociceptive input, and voila! The sensitisation is gone. What we know, however, is that in many cases the tendency towards having long-term increased sensitivity remains, or was perhaps always present.
Well, unfortunately if someone does tend to have greater activity in the central nervous system, then it has the potential to add enormously to poor outcomes if he or she decides to have surgery. For example, individuals with this tendency experience poorer outcomes after total knee replacement; and after shoulder surgery; but not after hip-joint replacement surgery. Testing in these cases was conducted using conditioned pain modulation which involves people undergoing painful testing – when they’re already in pain! Brave souls. You can see why it’s not a popular testing procedure in mainstream surgical situations.
Adding to the view that central sensitisation matters clinically, Ferrandiz and colleagues (2016) found that central sensitisation mediates the treatment effects in people with low back pain; Jull and colleagues (2007) found the same for neck pain after whiplash; Coombes and colleagues (2015) found the same for people with chronic tennis elbow. It seems that central sensitisation is associated with greater pain catastrophising, slower movements, higher pain reporting, poorer functioning, increased perception of pain, and fear of moving.
The question now is how best to assess for the presence of this phenomenon. Given that most people won’t want to undergo conditioned pain modulation (not to mention the need for testing equipment and skilled technicians to administer the test!), what’s needed is a reasonably simple way to identify those who have the characteristics of central sensitisation so we can plan for, and manage it, more effectively.
Nijs and the Pain in Motion Research Group published the first set of criteria in 2014, from an epidemiological perspective. This classification approach involves first excluding neuropathic pain – and the group propose using IASP diagnostic criteria for neuropathic pain (see Haanpaa & Treede, 2010). If the problem is neuropathic pain (where there is a clear lesion of the nerve), then it’s managed accordingly (although we really don’t have great treatments for this kind of pain, either!).
Then they propose an algorithm which helps to clarify whether the problem is central sensitisation or “something else”.
The first question is whether the person identifies they have “disproportionate pain experience” – now this I have a problem with, because what is a “proportionate” pain experience? Given how fluid our experience of pain can be, and how poorly the experience correlates with what’s going on in the tissues, I find this a bit tough to use as a clear-cut indicator. Nevertheless, it’s the first question asked in this algorithm…
The next question relates to the person experiencing diffuse pain distribution (or, perhaps, wider spread than expected). If this is the case, eg someone has a grazed knee, but pain is experienced all over the entire leg, then it’s identified as central sensitisation. If the result is more like pain just above the knee to just below, then it’s somewhat equivocal, so the authors suggest the person completes the Central Sensitisation Inventory. This is a questionnaire I’ve discussed in the past. I’m no nearer to establishing whether it really is a useful measure than when I wrote that blog, but the measure continues to be used, and research is ongoing. Certainly, Jo Nijs and group seem to think the measure holds promise and might help to classify those at greater risk of developing problems with pain if they proceed to surgery.
So, to summarise, while the mechanisms involved in central sensitisation are still being discovered, and it’s challenging to know where normal processes end and abnormal ones begin, it definitely seems to be a clinical phenomenon affecting not only those without peripheral nociception (eg migraine), but also those with clearcut peripheral problems like osteoarthritis. Central sensitisation processes seem to underpin some of the most problematic pain problems we know of, and can get in the way of recovery even when peripheral nociceptive input has been removed – 18% of total knee-joint replacements are revised because of ongoing pain, and this pain doesn’t seem to improve after subsequent surgery (NZ National Joint Registry). There’s confusion about language – does the term refer to the mechanisms thought to be involved, or does it refer to the experience described by people? And assessing it is challenging – either go through complex and painful testing, or complete a questionnaire that may confound distress about health (and subsequent hypervigilance about body symptoms) with pain and other responses that might represent the presence of central sensitisation processes being invoked.
More challenging still is what do we do once central sensitisation is identified? Are our treatments any good? That, folks, will be explored in the next enthralling episode on Healthskills!
Baert, I., Lluch, E., Mulder, T., Nijs, J., Noten, S., & Meeus, M. (2016). Does pre-surgical central modulation of pain influence outcome after total knee replacement? A systematic review. Osteoarthritis and Cartilage, 24(2), 213-223.
Haanpää M, Treede RD. Diagnosis and classification of neuropathic pain. Pain Clinical Updates 2010; XVII.
Nijs, J., Torres-Cueco, R., van Wilgen, P., Lluch Girbés, E., Struyf, F., Roussel, N., . . . Vanderweeën, L. (2014). Applying modern pain neuroscience in clinical practice: Criteria for the classification of central sensitization pain. Pain Physician, 17(5), 447-457.
Valencia, C., Fillingim, R. B., Bishop, M., Wu, S. S., Wright, T. W., Moser, M., . . . George, S. Z. (2014). Investigation of central pain processing in post-operative shoulder pain and disability. The Clinical Journal of Pain, 30(9), 775.
Woolf, C. J. (2011). Central sensitization: Implications for the diagnosis and treatment of pain. Pain, 152(3 Suppl), S2-15.
Wylde, V., Sayers, A., Odutola, A., Gooberman‐Hill, R., Dieppe, P., & Blom, A. (2017). Central sensitization as a determinant of patients’ benefit from total hip and knee replacement. European Journal of Pain, 21(2), 357-365.