Chronic back pain may activate an unexpected brain network, study finds

For people with chronic back pain, everyday movements like bending down to tie their shoes or picking up a grocery bag can trigger painful flare-ups. Now, scientists have made a surprising discovery: These episodes may activate a brain network that's associated with our sense of self.

In a new study, the researchers found that movement-induced chronic back pain was tied to increased activity in the brain's default mode network, a group of connected brain regions that lights up during self-reflection and mind wandering. This finding was unexpected because the network typically becomes less active during acute pain, such as transient pain triggered from touching a hot stove or banging your knee on a table.

The new study, published Oct. 5 in the British Journal of Anaesthesia, suggests that chronic-pain flare-ups may involve brain processes beyond those responsible for detecting painful sensations.

How the brain responds to chronic pain

Chronic lower back pain, defined as lasting at least three months, is a leading cause of disability worldwide. It's thought that when chronic pain first begins, a network of regions in the brain called the "pain neuromatrix" becomes sensitized so that the pain persists even after the painful stimulus is gone. Although scientists know that chronic pain is associated with changes in brain activity, they don't fully understand what happens when movements trigger sudden increases in that ongoing pain.

To investigate, researchers recruited 114 adults with chronic lower back pain and used functional magnetic resonance imaging (fMRI) to examine their brain activity. fMRI measures changes in blood flow, which acts as an indirect measure of brain activity.

After an initial scan, a researcher helped lift each participant's leg until their back pain increased, held it in position for 10 seconds, and then lowered it. The researchers scanned the participants' brains immediately after this pain-inducing leg movement and compared the results with scans collected beforehand. On average, the test increased the participants' reported pain intensity by 23%.

As expected, during the leg movement, the researchers observed increased blood flow in brain regions involved in processing pain and movement. But they also detected increased activity in key regions of the default mode network, which typically becomes more active when a person is thinking about themselves or their autobiographical memories or just letting their mind wander.

Acute pain lowers activity in the default mode network, studies show, presumably because it induces an awareness of something separate from yourself.

"When we saw just the first round of analysis, we're like, 'Did we do it right?'" study co-author Morgan Gianola, a postdoctoral scholar at the University of California, San Diego, told Live Science.

"We did not expect [the default mode network] to be involved at all," said study co-author Fadel Zeidan, a neuroscientist at UC San Diego, told Live Science. "We expected it to be deactivated to the point that we had to triple-check our work to make sure that the results were legit."

The researchers also exposed participants' calves to painful heat ‪—‬ a common method of studying brain activity during acute pain. Unlike the movement-induced back pain, the heat was associated with reduced activity in part of the default mode network, suggesting a different mechanism was at play.

When pain becomes part of our internal experience

Participants who reported more severe chronic back pain had stronger connectivity between the default mode network and brain regions that process sensations from the back. This suggests that chronic pain may become intertwined with how people perceive themselves and their bodies, the study authors concluded.

"This pain flare has almost become a part of who they are from an egocentric perspective," Zeidan said.

Katherine Martucci, a neuroscientist at Duke University who wasn't involved in the study, praised the experimental design, noting that researchers typically avoid movement during brain scans because it can interfere with the clarity of the scan. This study "nicely gets around" that problem by taking scans before and after the painful motion, Martucci told Live Science in an email.

However, she noted that the observed brain activity could also reflect lingering pain or anxiety following the painful leg raise. In addition, the study couldn't determine whether the altered brain activity observed contributes to chronic pain or develops in response to it.

Nonetheless, Zeidan and colleagues think the findings could eventually inform future treatments for chronic pain. The therapies might address how people relate to their pain, rather than solely targeting the painful sensations themselves. Zeidan, who coincidentally developed chronic back pain while conducting the study, believes that people's past experiences and memories with chronic pain have influenced their brain processes to a point where the pain itself becomes part of who they are.

Zeidan's team is now investigating whether existing approaches, such as mindfulness meditation or cognitive behavioral therapy, can influence the default mode network and help relieve chronic pain. (Cognitive behavior therapy for chronic pain, or CBT-CP, is already a well-established treatment option.)

Martucci cautioned that it's too early to draw conclusions about how these results might be applied in clinical practice. But she said the current findings provide important evidence of how the brain responds to chronic pain flare-ups.

"They do provide solid data on how people with chronic low back pain show important changes in their brain responses after a condition provoking their pain," she said. "The brain imaging data validate their [the participants'] experience of pain in an important and very visible way."