Pain Science (Part 1/4): What Causes Pain?

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If you pulled a passerby off the street and asked them what causes pain, chances are you will probably be told pain results from an injury/corresponding tissue damage. If you were to ask most medical professionals (including those in the musculoskeletal world – PTs, chiros, orthopedic surgeons, etc.) you will probably be given the same answer… but is it right?

Modern healthcare has been built upon the notion that pain is a direct result of tissue damage. Interventions/examinations across healthcare professions all seek to identify and treat the physical entity that is seen to be the root of the patient’s pain. For a traditionalist chiropractor this may be a subluxed vertebra, for a surgeon it may be a labral tear, and for a physical therapist it may be muscle weakness or your posture excessively stressing a tissue.

But if this model is accurate then why aren’t outcomes more consistent or predictable?

Surgery doesn’t always work for pain relief (pain outcomes are even quite poor in some conditions)1and sham surgery has been shown to be just as effective as true surgical intervention in other conditions.2 Traditionalist chiropractors don’t treat what mainstream medicine considers the “root” of the problem… and yet many patients still get better. My own profession varies ENORMOUSLY in their treatment and interventions (McKenzie, Manual therapists, Movement-based treatment, myofascial release, etc.) and yet patients still improve under all treatment approaches.

Alright, so what the heck is going on?

I’m sure you can see where I’m going here…. Tissue damage does not equate to pain. Want proof? You got it – there is a ton of literature to show significant tissue damage can be present in the absence of pain. Most well-known is a literature review by Brinjikji in 2015 which examined the results of a spinal MRI of 3,110 asymptomatic (meaning no pain) subjects.3 Of the subjects 37% of 20-year-old subjects had some form of spinal degeneration (disc herniations, space narrowing, etc.) with the incidence increasing linearly with age to 96% by 80-years-old. These people did not have pain, showing it is totally possible (and honestly totally normal based upon the linear age-degeneration relationship) to have significant spinal “damage” without pain (so really it’s more like wrinkles for your spine than damage.) Want more proof? Researchers have found similar relationships with the cervical spine, hip labral tears, shoulder labral tears, shoulder rotator cuff tears, and knee meniscal tears.4,5,6,7,8

So are you saying tissue “damage” doesn’t matter for pain?

Nope, I’m not

There is a relationship between tissue damage and pain… but it isn’t direct or causal. In another study by our pal Brinjikji in 2015 he found individuals with spinal degenerative findings were more likely to have pain than asymptomatic controls.9 Further, Hancock in 2017 found individuals with a higher number of degenerative findings (specifically 3 or more) were more likely to have low back pain.10

The takeaway: Tissue damage does play a role in pain…. But dramatically less than once thought

Okay…. So what the heck does cause pain then?

Contemporary pain theory is embodied in the BioPsychoSocial model of pain. The model proposes pain to be an output of the brain. This is in sharp contrast to traditional models which view pain as ascending input or “pain signals” arising from some type of dysfunction (tissue, hormonal, etc.) in the periphery. Instead, the brain/nervous system interprets available information from multiple domains to determine if a threat exists (with a movement, position, body part, etc.) and generates pain in response if so. Essentially pain under this model is the body’s alarm system saying “That’s bad, don’t do that” or “something is wrong here.”

What information does the brain utilize to make this decision?

Remember how I capitalized three things in the word BioPsychoSocial? The assessment is made upon information from three broad domains: The Bio(logical), Psycho(logical) and Social.

  • The Bio domain encompasses nociceptive input (pressure, temperature, chemical) or, effectively, what traditional models have focused on like tissue damage, inflammation, posture, etc.
  • The Psycho domain encompasses the individual’s thoughts, beliefs, expectations – i.e. have I been told flexing the low back is wrong? Do I think I will injure myself by flexing the low back? Have I hurt myself before doing this motion? Do I think I will always have pain because my disc herniation? Etc.
  • The Social domain encompasses the individual’s social environment, cultural influences/expectations, family support, etc. This domain is like the red-headed step-child in that it doesn’t appear to get nearly as much attention as the other two in most circles.

However, there is no set threshold for each domain at which pain occurs. Instead, pain generation is more about the sensitivity of the individual’s nervous system

Sensitivity refers to the responsiveness of the individual system to nociceptive input. This is determined by the interaction of the above three domains. The relative contribution of each domain to an individual’s pain experience varies – some pain experiences could be primarily bio in nature, some primarily psycho, others could be an even split of all domains or some other combination.

Differences in sensitivity are why individuals respond differently to trauma. For example – If an Olympic runner sprains their ankle prior to the biggest race of their life it will likely hurt a heck of a lot more than if a completely sedentary individual does so. Why? The psycho and social factors in each case are dramatically different. The sedentary male can still do what he wants/needs to and probably won’t worry much about it. In contrast, the Olympic runner has devoted most their life to this moment, has considerable pressure to perform well and is much more likely to be hypervigilant and catastrophize about the injury. As a result, the way the nervous system interprets the relative threat of the same nociceptive input differs because of the moderating psycho and social factors. On the other hand, if two completely identical sedentary individuals injured their ankle, but one had a more severe injury the one with a worse injury would be expected to hurt worse as their responsiveness to nociceptive input are equal.

Quick Recap

  • Tissue damage does not equate to pain, though there is some relationship between the two
  • Pain is the body’s alarm system – an output of the brain responding to perceived threat
  • Biological, psychological and social factors all play a role in pain
  • Threat determination is more about how sensitive the nervous system is to input than how much absolute input there is

 

But is all pain the same? What about chronic pain? How does movement and posture fit in?

I’ll be addressing these questions in parts II-IV of this series. (Update: Click *here for part 2 on pain classification* and *here for part 3 on posture*)

Hit the Instagram (where I am most active by far) or other social media icons up top and drop me a follow for updates on future blog posts, research reviews, exercise videos and other content.

 

Thanks for reading,

~Adam

 

 

References:

  1. Chou R, Baisden J, Carragee EJ, Resnick DK, Shaffer WO, Loeser JD. Surgery for low back pain: a review of the evidence for an American Pain Society Clinical Practice Guideline. Spine. 2009;34(10):1094-109. (https://www.ncbi.nlm.nih.gov/pubmed/19363455)
  2. Beard et al. (2017). Arthroscopic subacromial decompression for subacromial shoulder pain (CSAW): a multicentre, pragmatic, parallel group, placebo-controlled, three-group, randomised surgical trial. (http://www.thelancet.com/pdfs/journals/lancet/PIIS0140-6736(17)32457-1.pdf)
  3. Brinjikji W, Luetmer PH, Comstock B, et al. Systematic literature review of imaging features of spinal degeneration in asymptomatic populations. AJNR Am J Neuroradiol. 2015;36(4):811-6. (https://www.ncbi.nlm.nih.gov/pubmed/25430861)
  4. Minagawa H, Yamamoto N, Abe H, et al. Prevalence of symptomatic and asymptomatic rotator cuff tears in the general population: From mass-screening in one village. J Orthop. 2013;10(1):8-12. (https://www.ncbi.nlm.nih.gov/pubmed/24403741)
  5. Nakashima H, Yukawa Y, Suda K, Yamagata M, Ueta T, Kato F. Abnormal findings on magnetic resonance images of the cervical spines in 1211 asymptomatic subjects. Spine. 2015;40(6):392-8. (https://www.ncbi.nlm.nih.gov/pubmed/25584950)
  6. Beattie KA, Boulos P, Pui M, et al. Abnormalities identified in the knees of asymptomatic volunteers using peripheral magnetic resonance imaging. Osteoarthr Cartil. 2005;13(3):181-6. (https://www.ncbi.nlm.nih.gov/pubmed/15727883)
  7. Schwartzberg R, Reuss BL, Burkhart BG, Butterfield M, Wu JY, McLean KW. High Prevalence of Superior Labral Tears Diagnosed by MRI in Middle-Aged Patients With Asymptomatic Shoulders. Orthopaedic Journal of Sports Medicine. 2016;4(1):2325967115623212.. (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4710128/)
  8. Register B, Pennock AT, Ho CP, Strickland CD, Lawand A, Philippon MJ. Prevalence of abnormal hip findings in asymptomatic participants: a prospective, blinded study. Am J Sports Med. 2012;40(12):2720-4. (https://www.ncbi.nlm.nih.gov/pubmed/23104610)
  9. Brinjikji W, Diehn FE, Jarvik JG, et al. MRI Findings of Disc Degeneration are More Prevalent in Adults with Low Back Pain than in Asymptomatic Controls: A Systematic Review and Meta-Analysis. AJNR Am J Neuroradiol. 2015;36(12):2394-9. (https://www.ncbi.nlm.nih.gov/pubmed/26359154)
  10. Hancock MJ, Kjaer P, Kent P, Jensen RK, Jensen TS. Is the Number of Different MRI Findings More Strongly Associated With Low Back Pain Than Single MRI Findings?. Spine. 2017;42(17):1283-1288. (https://www.ncbi.nlm.nih.gov/pubmed/28169955)

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