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Why don’t all injuries hurt? Five theories

 •  • 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.

Ever had a cut you didn’t even notice? But then the next cut, which seems pretty much the same, throbs for three days? What’s the 🤬 difference?

Or a hangnail that just suddenly starts to hurt, and from that moment on it’s like you can feel individual molecules banging into it? What changed from one moment to the next?

Or have you ever torn your knee cartilage and had no idea? Science says “probably!” Not that you would know without a home MRI machine…

Puzzlingly painless knee injuries

The “meniscus” of the knee is an error-prone little cartilage widget in your knee. An anatomical shim that seems to tear as easily as damp origami — enough to support a whole sub-speciality of orthopedic surgery.

Sometimes those meniscal lesions hurt like the dickens, and sometimes they don’t. Zanetti et al: “Horizontal or oblique meniscal tears are frequently encountered in both asymptomatic and symptomatic knees.” Other types of mensical lesions are almost always symptomatic, for the record. But these two kinds are quite inconsistent.

This is not exactly fresh news (that’s a 2003 study). It was highlighted by a colleague on Twitter “recently” (posts often stew in my drafts for quite a while before I publish), and it got me thinking about how there are lots of studies showing generally lousy correlation between imaging results of lesions that “should” hurt but often don’t. It’s always fascinating data.

But what is the x factor? What makes a lesion hurt in some people, while so many others literally don’t even know they have a problem? What’s the difference?! Whaaaaat?!

I am tired of not knowing this.

How injuries hide from awareness: five hypotheses

Here are some ideas, just some spitballing. (I am going to lean on the term nociception quite a bit, so quick review: nociception is the conversion of potentially noxious stimuli into nerve impulses which may or may not lead to pain.)

  • The dumb luck of position and timing. Sometimes a lesion just doesn’t cause any significant nociception… or it hasn’t yet, anyway. Maybe most asymptomatic people are in trouble six months later. Has anyone checked that? Not that I’m aware of. Change over time seems like the biggest missing data point in this puzzle.
  • People are different from each other. Some people probably just get less nociception out of the same lesion… because, on average, they are less prone to inflammation and peripheral sensitization, which is in turn a function of everything from genetics to insomnia. Systemic biological traits almost certainly account for why some injuries are painless for some people.
  • People are different from themselves. You are not the animal you were yesterday, or the animal you’ll be tomorrow. The only difference between a lesion that’s painless on Tuesday and a misery on Wednesday might be changes in biology, the constantly shifting tides. There are obvious reasons why this could happen, like sleep deprivation, an infection, or waves of hormones. For every obvious possibility, there are dozens more subtle biological factors constantly fading in and out.
  • Top-down modulation. Sometimes the brain probably just takes nociception less “seriously,” for many complex neurological, psychological, and contextual reasons. Nociception is a suggestion, and pain is weird. The brain doesn’t have to believe there’s a problem just because there’s nociception — but it is more likely to do so in some situations than others. For instance, brains almost certainly take nociception more seriously when it comes from previously traumatized anatomy. In contrast, the first lesion in an area is probably more likely to be asymptomatic.
  • Thresholds and boiling frogs. A sudden flood of nociception is probably harder for the CNS to breezily ignore than a trickle or drip of the stuff. It’s possible that lesions that develop slowly literally just don’t get noticed at first. But, when they finally do get noticed — when they cross some cryptic sensory threshold and we become aware of them — the pain experience rapidly intensifies. An analogy: you don’t notice an annoying thumping stereo neighbour noise at first, but once you hear it you cannot un-hear it, and it obnoxiously takes over your consciousness. Is it louder? No. Are you more aware of it? You bet!

Stealth injuries: the Twitter poll

The boiling frog factor is probably huge, but even sudden trauma can still fall below the brain’s threshold of concern when the conditions are just right. People can fail to notice acute lesions that would be quite painful at other times or in other people, especially when there’s adrenalin flowing.

When I polled my Twitter followers on this (“science”!), a whopping seventy percent claim to have had “minor” injuries they literally didn’t know about until much later, and 22% reported “surprisingly bad” stealth injuries: significant contusions, abrasions, lacerations, fractures, and tears.

Only 6% thought they’d never had a stealth injury.

I would be willing to bet that a lot of patients with fibromyalgia have never failed to notice an injury.

Injuries that are never stealthy

There are undoubtedly limits to our ability to not notice trauma, because some sources of nociception are just too intense to ever be stealthy. Burns are a fine example: I doubt anyone has ever had a third degree burn larger than the head of a pin that didn’t hurt like hell.

Kidney stones and pancreatic cancer and appendicitis are all probably similarly impossible to not notice, or ignore: visceral pain is in a league all its own, which seems to make sense from an evolutionary perspective. Organ trouble seems like it probably should be hard to ignore, on average.

But even some seemingly trivial injuries are amazingly painful, consistently disproportionate. The ultimate example, I think: toe stubs! Or perhaps (very similar) stepping on Lego. These “injuries” always hurt a ridiculous amount (albeit briefly), despite the fact that they are trivial in terms of tissue damage. I don’t think any of my ideas in this post explain that.