‘I have a High Pain Threshold!’ Part 1

It’s a phrase I commonly hear as a physiotherapist, and yet I still find it tricky to comment on. Reviewing the literature highlights how pain is highly individual and subjective, making it so difficult to fully understand from another person’s perspective (Eccleston 2001). As a clinician, the greater my understanding of the factors influencing pain, hopefully the better I can understand and try to reason/explain my client’s problems. Thus, the aim of part one of this blog is to explore some of the factors that influence the perception of pain. Part two will expand on this to outline some of the mechanisms that also influence altered pain thresholds. Then I will look at how this could be brought together to improve the way I treat people in pain.

Nociception

Nociception is one aspect to the perception of pain; defined as a neural process of encoding and processing noxious stimuli, through specialised receptors called nociceptors (Dubin & Patapoutain 2010). The physiology of these specialised nociceptors was discovered using single-fibre electromyography; which detects the conduction velocities in peripheral nerves, (Hagbarth 2002). Three main nociceptor fibre types were found: C fibres, Aβ, and Aδ fibres, as they produce a strong response to mechanical, thermal, and chemical stimulation. The specific stimulation patterns for the different fibre types is a huge topic of research and beyond the scope of this blog. However, the discussion points from studies investigating nociceptors often talked about the association of excitation of nociceptors and the perception of pain (Torebjörk 1974; Hagbarth 1983).

One of the first examples of nociceptor research is Van Hees & Gybels (1981) paper on C fibre activity to ‘painful’ and ‘non-painful’ stimuli. They took healthy individuals and stimulated over 100 cutaneous C fibres, using their radial nerve, with chemical (nettles and paint remover), thermal (radiation of a bulb through a lens) and mechanical (different diameter von Frey hairs) inputs. They then closely monitored the conduction velocities with electromyography and gained verbal pain responses from the participants. Van Hees and Gybels’s results showed some consistency with regards to temperature of the skin or C fibre discharge and the perception of pain. Chemical stimulation had definitive start and end correlations with similar fluctuations in C fibre excitation levels and pain levels. However, high C fibre stimulation from mechanical input did not correlate to the subject’s perception of pain; as even light touch caused high discharges without pain. The paper lacks detail within their methodology; there is no information on the measure used by participants to report pain. There is potential for bias from the researcher if the questions are suggestive, with no mention of blinding. There are no specifics on the area of the body tested, and almost no participant information is given. Van Hees and Gybels suggest disparity between nociceptor firing and pain perception, from a mechanical stimulus, but there is a lack of precision in their study.

Nociception is not only affected by the type of stimulus, but also the intensity or quality; as discussed by Wall & McMahon (1986). Their review paper highlights the importance of the quality of the stimulus on the perception of pain, i.e. the contact area or the thermal quality, and how non-nociceptive inputs influence the perception of pain. Defrin et al (2002) had a novel way of testing how the quality of the stimulus is processed, using a thermal stimulus with participants that have a degree of thermal sensation loss. Their methodology is much more rigorous with clear subject groups and a reference standard (control). They recruited 77 participants with paraplegia after suffering traumatic spinal cord injuries, except for the 8 healthy controls. They detailed the testing environment, testing intervals and the areas of the body where the testing was performed. There was no blinding, but the experiment relied on the subject’s response using a remote to record the data and thus reducing the risk of bias. Defrin et al (2002) found that participants that had no thermal sensation, half perceived noxious heat or cold stimuli as a sensation of prickling at significantly higher thresholds than the control. If there was thermal sensitivity, either to heat or cold, the pain perception matched the stimulus or was perceived as the intact stimulus if the participant could only detect one thermal modality i.e. if only heat sensation was intact, noxious cold would give the sensation of heat pain. The findings are not necessarily generalisable given the participant population; however they do suggest that our pain response is an interpretation of the stimulus/event based on the inputs received.

Non-nociceptive Information

Sensory information from our surroundings causing emotions based on the current situation may have an influence on the perception of pain. Beecher (1946) completed a landmark study on men wounded in battle during world war II. This observational study closely monitored soldiers pain response and the amount of morphine administered after suffering severe wounds sustained in battle. He discovered that out of 215 patients; 32.1% had no pain, 25.6% had ‘slight pain’ and 73% did not require further pain relief when asked. The study had 5 patient groups based on their injury, 4 of which had 50 patients each and one had 15 (penetrating wounds of the cerebellum). The characteristics of each group were similar; average age, average dose of morphine, average time since last dose, and time since wounding. All participants were recruited consecutively, as able, with clear exclusion criterion; minimising selection bias. Beecher suggested that pain is an experience, influenced by many other external factors. His explanation for the low levels of pain was based on the dangerous environment that the soldiers had been in and then left for relative safety of a hospital. Thus 10 years later, Beecher compared 150 male civilian casualties to the wounded soldiers previously observed. The two groups had similar characteristics, such as average age and type of injury sustained, yet the civilian group needed more analgesia. Only 17% of the civilian group, compared to 73%, did not require further pain relief when questioned. He believed that the intensity of pain linked to their emotions at the time.  The problem with these studies is that the environment was not the only variable that differed between the groups. The groups do not necessarily represent the general population, and there is a possibility that a soldier may be more conditioned to pain. However, there is further support that emotion may influence perception of pain in a positive and negative way.

The influence of what we see and our environment on the perception of pain is further explored by Mosely & Arntz (2007) and Selm et al (2018). These studies look closer at individual responses to pain depending on what is in our visual field. Mosely & Arntz (2007) used a -20^oC rod as a stimulus, combined with either a red or blue visual cue. They found that the red cue evoked hotter, more intense and more unpleasant responses than the blue cue. The margin in temperature perception was a big as 5.5, on a scale of 11, even though the stimulus was the same. Selm et al (2018) used magnification to manipulate pain scores, eliciting delayed onset muscle soreness (DOMS) in 20 healthy individuals. 48 hours after causing DOMS, through standardised eccentric training of quadriceps, they asked the participants to contract the sore muscle and score their pain from 0-10. found that looking through magnification at the injured thigh caused a significant increase in mean pain score compared to viewing the thigh without magnification. The difference was relatively small (2.60 to 3.05) and I would question whether an individual could distinguish a difference of 0.45 on an 11-point self-reported scale. These studies base their findings on small samples of healthy subjects not in pain, and significance may change if someone is already in pain. It does suggest that the perception of pain is influenced by what we see and changing the significance of this may result in a change in pain intensity.

Responding to the Statement

If I were to now discuss pain thresholds and individual tolerances to pain intensity, I would begin by discussing the context to which their pain affects them. Reading the literature on pain perception highlights that nociceptive and non-nociceptive information influences our pain response (Wall & McMahon,1986; Defrin et al,2002; Mosely & Arntz, 2007). Past experiences have an influence on our associations and emotions relative to a stimulus, which is completely individual, and contributes to the overall picture (Beecher, 1946). It is therefore important to thoroughly explore an individual’s context/emotions with regards to their pain. This includes the impact on their day to day life, the history of their problem, perceptions of the environment around them. When someone says they have a high pain threshold; they may mean they have a high tolerance to pain.

So far, I have looked at some of the literature around the perception of pain, but most of the studies use healthy participants who are not currently in pain. Therefore, I am keen to explore the current concepts behind more persistent pain and how this can aid our treatment.

References

Beecher, H. (1946). Pain in Men Wounded in Battle. Annals of Surgery, 123(1), pp.96-105.

Beecher, H. (1956). Relationship of Significance of Wound to Pain Experienced. Journal of the American Medical Association, 161(17), pp.1609.

Defrin, R., Ohry, A., Blumen, N. and Urca, G. (2002). Sensory determinants of thermal pain. Brain, 125(3), pp.501-510.

Dubin, A. and Patapoutian, A. (2010). Nociceptors: the sensors of the pain pathway. Journal of Clinical Investigation, 120(11), pp.3760-3772.

Eccleston, C. (2001). Role of psychology in pain management. British Journal of Anaesthesia, 87(1), pp.144-152.

Hagbarth, K. (1983). Microelectrode Exploration of Human Nerves: Physiological and Clinical Implications. Journal of the Royal Society of Medicine, 76(1), pp.7-15.

Hagbarth, K. (2002). Microelectrode recordings from human peripheral nerves

Moseley, L. and Arntz, A. (2007). The context of a noxious stimulus affects the pain it evokes. Pain, 133(1), pp.64-71. (microneurography). Muscle & Nerve, 999(S11), pp.S28-S35.

van Selm, M., Gibson, W., Travers, M., Moseley, G., Hince, D. and Wand, B. (2018). Visually induced analgesia in a deep tissue experimental pain model: A randomised crossover experiment. European Journal of Pain, 22(8), pp.1448-1456.

Torebjörk, H. and Hallin, R. (1974). Identification of afferent C units in intact human skin nerves. Brain Research, 67(3), pp.387-403.

Van Hees, J. and Gybels, J. (1981). C nociceptor activity in human nerve during painful and non-painful skin stimulation. Journal of Neurology, Neurosurgery & Psychiatry, 44(7), pp.600-607.

Wall, P. and McMahon, S. (1986). The relationship of perceived pain to afferent nerve impulses. Trends in Neurosciences, 9, pp.254-255.