Bending down to tie a shoe or picking up a bag of groceries can trigger a flare in someone with chronic back pain. Researchers at UC San Diego wanted to know what that sudden spike does in the brain, and what they found surprised them.
Their new imaging study of 114 adults suggests a flare can reach into a network tied to self-reflection, personal memories, and a person’s inner world.
The team says that may help explain why chronic pain feels so personal to the people living with it, and it opens a new line of thinking about how the condition could be treated.
That is a lot of meaning to read into a brain scan, and one outside expert quickly pointed out where the evidence ends. The real story is the gap between what the scans measured and what the researchers think it means.
The Leg Lift That Set Off A Flare
To study a flare, the team first had to trigger one.
The study, published October 5 in the British Journal of Anaesthesia, involved adults with chronic low back pain, meaning pain that has lasted more than 12 weeks. Funding came in part from the National Institutes of Health, and the authors declared no conflicts of interest.
Each participant started with a baseline brain scan. Then a researcher helped lift their leg until the back pain climbed, held it there for 10 seconds, and lowered it. A second scan followed right away.
Reported pain rose by 23% on average, which is the bump the maneuver was meant to produce. The scans used arterial spin labeling, an MRI technique that tracks blood flow in the brain as an indirect measure of activity.
For comparison, everyone also got a painful but harmless heat stimulus on the calf.
Two Kinds Of Pain, Two Opposite Brain Responses
This network is usually linked to mind wandering, autobiographical memory, and a person’s sense of identity.
Researchers call it the default mode network, or DMN, a set of connected regions that stretch across the brain. Painful heat on the calf did what sudden pain typically does: DMN activity, or at least part of it, dropped, presumably because attention swung toward an outside threat.
The leg raise did the opposite, and DMN activity rose. Fadel Zeidan, the study’s corresponding author and director of UC San Diego’s Pain Health and Mindfulness Laboratory, put it this way in the university’s release: “What we show for the first time ever is that when you have a pain flare, it turns on instead. You start to identify the experience with who you are.”
Related: 8 Mind-Boggling Illusions That Prove Your Brain Is Lying to You Right Now
The Scientists Did Not See It Coming
They had predicted the opposite, so the first results sent them hunting for a mistake.
Chronic pain has been linked to sensitization in a group of regions called the pain neuromatrix, which may let pain persist after whatever started it is gone. The study’s more specific question was what happens when movement suddenly intensifies pain that’s already there.
The team had expected the DMN to dip. Postdoctoral scholar Morgan Gianola described their first look at the data: “When we saw just the first round of analysis, we’re like, ‘Did we do it right?’” Zeidan said the team triple-checked the analysis before trusting it.
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The Body Showed Up In The Data, Too
The self-reflection network was only part of what changed.
Participants with more severe chronic back pain had stronger connections between the DMN and the primary somatosensory cortex, the area that processes sensations from the back.
More intense pain during the leg raise also corresponded with greater activity in sensory and motor regions. Heart rate also climbed during the leg raise, in step with flare intensity.
Zeidan’s explanation for why pain might get tangled up with a person’s sense of self centers on everything around it: the anticipation of pain, the recovery from it, embarrassment about not being able to take part in normal activities and fear of movement.
What The Scans Cannot Settle Yet
An outside neuroscientist had praise for the design, and some pointed reservations.
Katherine Martucci, a Duke University neuroscientist who wasn’t involved in the research, said scanning before and after the leg raise avoids a problem, since movement during a scan can blur the images. On what the results offer patients, she said: “The brain imaging data validate their [the participants’] experience of pain in an important and very visible way.”
Her reservations were specific. The activity could reflect lingering pain or anxiety after the leg raise. The study also can’t show whether these brain changes help cause chronic pain or develop in response to it, and she said it’s too early to say how the findings should be used in clinical practice.
The scans measured blood flow. The link to self-reflection comes from the DMN’s association with identity, which shapes the researchers’ interpretation.
Where Treatment Could Go From Here
The findings suggest a possible new treatment target, but researchers are still gathering evidence.
Low back pain is the leading cause of disability worldwide, according to the World Health Organization, and the university says more than half a billion people experience it.
The researchers suggest that standard symptom-focused treatments, opioids among them, may miss the mark partly because movement-triggered flares involve different brain processes than ordinary acute pain. The experiment wasn’t a medication trial, so for now that explanation is a proposal.
If you live with chronic pain, does it ever feel like more than a physical sensation, and how do you describe that to others?
