Did you ever wonder why one person develops chronic pain following an injury, while someone else with a similar injury fully recovers and has no pain? Researchers at Northwestern University wondered so they set out to discover an answer. Their findings were published online July 1 by Nature Neuroscience.

The first longitudinal brain imaging study to track participants with a new back injury found that the chronic pain is all in their heads––quite literally.

No, they're not saying the pain is psychological. What the new Northwestern Medicine study shows for the first time is that the more two sections of the brain––related to emotional and motivational behavior––talk to each other, the more likely that chronic pain will develop. The more they communicate following the initial injury, the greater the chance a patient will develop chronic pain.

Study Design and Results

A total of 40 participants who had an episode of back pain that lasted four to 16 weeks––but with no prior history of back pain––were studied. All subjects were diagnosed with back pain by a clinician. Brain scans were conducted on each participant at study entry and for three more visits during one year.

Researchers were able to predict, with 85 percent accuracy at the beginning of the study, which participants would go on to develop chronic pain based on the level of interaction between the frontal cortex and the nucleus accumbens.

“For the first time we can explain why people who may have the exact same initial pain either go on to recover or develop chronic pain,” said A. Vania Apakarian, senior author of the paper and professor of physiology at Northwestern University Feinberg School of Medicine.

“The injury by itself is not enough to explain the ongoing pain. It has to do with the injury combined with the state of the brain. This finding is the culmination of 10 years of our research.”

The more emotionally the brain reacts to the initial injury, the more likely the pain will persist after the injury has healed. “It may be that these sections of the brain are more excited to begin with in certain individuals, or there may be genetic and environmental influences that predispose these brain regions to interact at an excitable level,” Apkarian said.

The nucleus accumbens is an important center for teaching the rest of the brain how to evaluate and react to the outside world, Apkarian noted, and this brain region may use the pain signal to teach the rest of the brain to develop chronic pain.

“Now we hope to develop new therapies for treatment based on this finding,” Apkarian added.

Chronic pain participants in the study also lost gray matter density, which is likely linked to fewer synaptic connections or neuronal and glial shrinkage, Apkarian said. Brain synapses are essential for communication between neurons.