A deeper exploration of pressure pain threshold in neck pain

In May of 2013 I pulled a conference double-date, moving between Montreal, Canada for the Canadian Physiotherapy Association’s Annual Congress and Toronto, Canada for the IASP Neuropathic Pain SIG International Congress in the same week.  It was hectic, but as usual took plenty of good things away from both congresses.  While at the NeuPSIG meeting in Toronto, I had the good fortune of sitting in on a session entitled ‘Central sensitization in neuropathic and non-neuropathic pain conditions’ presented by people I consider pioneers in the field: Dr. Clifford Woolf, Dr. Per Hansson and Dr. Michael Sullivan.  The session was very interesting but it was the Q & A session at the end that was quite lively (incidentally, I wish PT conferences had more lively Q & A sessions as well, for some reason we all seem quite subdued when it comes time to talk to the presenters).  In a very small nutshell, the discussion that interested me most was around this idea of quantitative sensory testing (e.g. mechanical, thermal, electrical pain thresholds) and how, if at all, it could be used to a) identify the presence of centrally sensitized nociceptive pathways, and b) determine the mechanism behind the sensitization.  Those of you who are familiar with the presenters will not be surprised to learn that the mechanisms put forth ranged from the cellular (genetics) to the psychosocial (esp. catastrophizing).  It was this aspect of Q & A that got me most engaged, with many posing different versions of the question ‘so, what is the mechanism behind QST sensitivity?’  As I sat in the audience (and after I had my time with the mic), I got thinking that back in my office here in London, Ontario I had a whole database of pressure pain threshold measures and concurrent pain-related cognitive scores that might at least be able to shed some light on that aspect of this discussion.  Concurrently, I had also been getting increasing questions from clinicians around the application of PPT in the clinical context as a result of the this series of 3 papers in JOSPT in 2011.  One of the common questions I kept getting was ‘do I have to do it 3 times in each location?’, which admittedly means the full test, if you were to do it across 4 sites (bilateral neck, bilateral tib. Ant.) takes nearly 15 minutes.  Finally, I was curious to know if PPT was actually a tool that could be used not only for prognostic purposes but also as an evaluative tool to track change over time.

So, for these reasons (that was quite the build up wasn’t it?) I set about working on a new paper on PPT in neck pain that explored these 3 questions: What are the measurement properties like between 1, 2 or 3 repetitions?  What is the responsiveness of PPT?  What are the associations between PPT and common pain-related cognitive or emotional scales?  What’s great for me about these papers also is that I get the chance to offer authorship to some of our clinicians who pass through or Master’s of Clinical Science program here at Western, which is often their first publication and gives them a foot in the door of scientific writing.

To summarize, here are my interpretations of the responses to the 3 questions we set to answer:

1.  Do you have to do 3 repetitions at each site?  Answer: Yes with a caveat.  We found that, on average (N = 206), using the first repetition only led to what appear to be inflated thresholds.  Disregarding the first 1 and using only the final 2 offered the lowest thresholds (side note: while this looks like evidence of some kind of wind-up pain, in actuality the second repetition was on average the lowest of the 3). Using all 3 offers a mid-range score that is adequately reliable and was the protocol we used for establishing prognosis in the 3rd of the 2011 papers.  That said, the ‘last 2 only’ approach also offers good reliability, but since it also requires you do 3 repetitions, there’s no real time savings there.  The caveat I mentioned is this: If the first 2 tests are consistent, in that they don’t vary by greater than the standard error reported in this paper (and calculated automatically for you on our PPT Android apps), then you should be able to get away with 2 repetitions and get the same general properties as the ‘all 3’ protocol.

2.  Is PPT responsive to change?  This one gets a ‘sort of’.  For people with neck pain, PPT at the Tibialis Anterior, while potentially valuable from a prognostic or screening perspective, does not appear to be a very useful instrument for measuring change in neck pain (perhaps not surprisingly).  When measure over the angle of the upper trapezius, it appears to be more useful, but is much better at ruling change in than ruling it out.  In other words, when discriminating between those who self-reported clinically-meaningful change over 1 week (by virtue of the GROC score) and those reporting no change, a change of about 80-85 kPa was only 50% sensitive but offer up to 92% specificity.  What that means is that many people will have improved a clinically meaningful amount but not seen a PPT increase of at least 80 kPa, but on the other hand, those who have changed at least 80 kPa are very likely to have also changed a meaningful amount.  If that doesn’t make sense, email me and I can explain it to your further.

3.  What is the association between PPT and pain catastrophizing, fear, anxiety, depression, and ‘kinesiophobia’?  My aversion to the word ‘kinesiophobia’ notwithstanding, the answer to this one is: weak, and only of real relevance for pain catastrophizing.  This surprised me, I won’t lie.  When it came to the debate regarding PPT as a true measure of nociceptive sensitivity vs. simply an indicator of psychological aversion to pain, I admit I fell on the latter side of the fence (psychological aversion).  However, at least the results that we found in our database would suggest that if negative psychological states are related to PPT, in this sample it would appear as though the association is very weak and of questionable clinical relevance.  I keep saying ‘in this sample’ because different results have been found in different populations using different QST measures.  So, had I had these results at the NeuPSIG meeting in May 2013, I would have had to state that, based on our data, we’re not looking at a strong psychological correlate here, but that it may well be that QST is providing us with a quantifiable indication of sensitivity of the nociceptive aspects of the nervous system.  This will no doubt continue to evolve with time.

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