Pain may or may not inhibit (chronic low back) pain

In 2014 David Yarnitsky hypothesised that people may be characterised by profiles that vary from pro- to anti-nociceptive. Those with pro-nociceptive profiles would likely show a reduced capacity to enrol the body’s own pain inhibitory mechanisms, and heightened sensitivity to repetitive, painful stimuli. Those with anti-nociceptive profiles would demonstrate the opposite.

Testing conditioned pain modulation (CPM) exploits the old idea that “pain inhibits pain”. A painful “test stimulus” is applied in the presence, and absence, of a painful “conditioning stimulus”. If the test stimulus is perceived to be less painful when the conditioning stimulus is applied, pain is thought to have inhibited pain by getting the body’s own pain inhibitory mechanisms working.

When I was an undergraduate student I remember one of my lecturers describing the use of ultraviolet lamps to burn the skin on the shoulders of people with low back pain so that they had a more painful problem elsewhere in their body, in the belief that they would then feel less back pain. Now, that was over 20 years ago, but until recently little research had been done on pain inhibiting pain in people with back pain.

Together with colleagues at Curtin University, we have just published paper in the journal Manual Therapy, exploring CPM in people with chronic low back pain (CLBP) [1]. We applied pressure to the low back that led to the perception of pain, then heated up their hand, again to the point of pain (but not to the point of frying their skin!), then re-tested how much pain they perceived when we applied the same amount of pressure as before. We also did this in people who had never had any longstanding pain (71 people per group). The results were significantly different between the groups.

Seventy-three percent of people with CLBP had a facilitatory response: they perceived the pressure stimulus as more painful when their hand was heated up than when it wasn’t. This suggests that in the majority of people with CLBP their body’s own pain inhibitory mechanisms are not working very well at all. Pain did not inhibit pain, it just ramped it up even more. However, a facilitatory response was also found in 31% of the healthy participants. In some people pain did actually inhibit pain (11% of people with CLBP, compared to 36% of people with no pain). In the remainder the application of the conditioning stimulus made no real difference.

So this study may fit with Yarnitsky’s hypothesis, which goes on to suggest that a pro-nociceptive profile may be associated with chronic pain. This may also mean that those healthy participants showing a facilitatory CPM response may be pre-disposed to the development of a painful disorder, but this is currently unknown. More importantly, our results suggest that we should not go around burning the shoulders of people with CLBP with ultraviolet lamps, as it will just make them worse.

P.s. I haven’t been near an ultraviolet lamp since those undergraduate days. I wonder why?

About Martin Rabey

Martin RabeyMartin is a Specialist Musculoskeletal Physiotherapist (as awarded by the Australian College of Physiotherapists), and has just submitted his PhD at Curtin University in Perth, examining the complex interactions between factors which make low back pain persist. He moved to Sydney a year ago, and continues to research low back pain at Neuroscience Research Australia.

References

[1] Rabey M, Poon C, Wray J, Thamajaree C, East R, Slater H. (2015). Pro-nociceptive and anti-nociceptive effects of a conditioned pain modulation protocol in participants with chronic low back pain and healthy control subjects. Man Ther. 2015 Dec;20(6):763-8

Commissioning Editor: Lorimer Moseley;  Editor: Neil O’Connell.