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Biological vulnerabilities: the underestimated x-factors in chronic pain

 •  • by Paul Ingraham
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Weekly nuggets of pain science news and insight, usually 100-300 words, with the occasional longer post. The blog is the “director’s commentary” on the core content of a library of major articles and books about common painful problems and popular treatments. See the blog archives or updates for the whole site.

Biological vulnerability has been on my mind a lot lately.

Imagine the most structural, biomechanical, tissue-in-trouble problems that can cause chronic pain. Things that seem very likely to hurt: A neuroma on a nerve root. A calcified, elongate styloid process stabbing someone in the throat (Eagle’s syndrome). An entrapped cluneal nerve in the butt, being strangled by connective tissue. A substantially degenerated tendon.

All things that seem likely to hurt.

And yet none of these always causes pain. Some people feel them, some people never even know about them. In fact, there is almost no physical problem in the entire world of chronic pain that is consistently symptomatic in everyone. Why is that?

Probably because some people are more vulnerable.

Close-up photograph of hot coals.

Is a fire “caused” by the fuel? Or how dry it is? Or the match? Or the oxygen? Trick question…

Probably because of biological x-factors, non-structural problems. Biochemistry. Nutrition. Cellular business. Maybe patient X is vulnerable to that cluneal nerve impingement because of a vitamin D or magnesium deficiency. Or a medication side effect. The biological consequences of sleep deprivation, or sleep apnea (blood gases get weird with that condition). Or the chronic stress of social isolation. Or “inflammaging.” Or any one of about ten thousand other possibilities. The list of could-be’s is almost literally endless.

The so-called “problem” is just like kindling for a fire: just inert fuel by itself. That doesn’t mean it’s not a problem — you don’t really want fuel for chronic pain lying around your body just waiting for you to get vulnerable enough for it to burst into flame — but you also don’t blame fuel for a fire. Well, not just the fuel anyway.

This is speculation, of course. It would be difficult indeed to prove causation for even a single example, let alone the whole class of potential vulnerabilities. But I think it’s one plausible explanation for one of the most difficult puzzles in pain science: why physical problems have such inconsistent consequences. (The other big one, of course, is the psychosocial dimension of pain.)

That kindling metaphor

The kindling metaphor came from Greg Lehman, and was the original inspiration for this post:

So perhaps we want to view these structural changes as similar to kindling for a fire.

Kindling is not a fire. Its a precursor and before it can become a fire you need some accelerant or spark. We can view degenerative changes the same way. They aren't sufficient for pain but perhaps you need some sort of sensitizing agent to create that "spark' and the "fire" of pain.

Sometimes the accelerant is too much physical loading. Perhaps the accelerant is too much psychological load. Pain is multidimensional and the accelerant come from anywhere...but so can the solution :)

Greg has also been thinking and posting about this lately (yesterday): see “Non-specific low back pain exists. You just don't want to admit it.”

The other thing that fueled this line of thinking was…

Butt nerve entrapment

The cluneal nerves pass from the low back and sacrum into the buttocks. Aota reported on “a case of severe low back pain, which was completely treated by release of the middle cluneal nerve.” Exploratory surgery identified cluneal nerves “entrapped in adhesions.” They cut them free… and that was the ticket. The patient was decisively cured of “non-specific” back pain.

#1 marks the site of the superior cluneal nerves & where the first surgery freed nerves from “adhesions.” #2 marks the middle cluneal nerve, where the second surgery freed it from entrapment in the ligament. Click to embiggen

Seems pretty specific to me. It seems like a vivid example of how the cause of pain can sometimes be completely “mechanical” in nature. Tissue with an issue. Case closed, surely?

But the more I think about it, the more I believe that even here biological vulnerabilities potentially play a major role. Even in a case as “simple” as this. How can they not? We see wide variation in directly analgous conditions like carpal tunnel syndrome: the predicament of the nerve leads to symptoms only in some people, not others. Surely the same applies to a cluneal nerve in trouble.

I’ve now written about the implications of this interesting case in my back pain book, one of my articles about back massage, in my long rants about “structuralism” and fascia. Plus more to come! Sometimes just one tiny case study can spawn many changes to!