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What Determines Chronic Pain?

Research Unveils the Brain’s Role in Risk for Pain

This article was originally published in the Northwestern University Feinberg School of Medicine News Center

Chronic pain has been deemed an epidemic in the United States, affecting 100 million Americans, and scientists have long wondered why some people develop chronic pain following an injury while others do not. Now, a new study led by Northwestern Medicine scientists has found that certain properties of the brain, and not the initial injury, may determine the risk of a patient developing chronic pain.

Addressing problems at the site of injury, such as treating chronic back pain as an inflammation of the back muscles, has resulted in only limited success, according to senior study author Marwan Baliki, ’09 PhD, ’12 GME, an assistant professor of Physical Medicine and Rehabilitation at Northwestern University Feinberg School of Medicine and research scientist at Rehabilitation Institute of Chicago.

“The central processes of chronic pain have largely been ignored, so our research team set out to better understand the brain’s role,” explained Baliki.

The scientists conducted the first longitudinal brain imaging study tracking patients following an acute injury to the back. As part of the study, they followed 159 patients for three years as their pain either ceased or persisted. They found that patients who developed chronic pain had a smaller hippocampus and amygdala compared with those who recovered as well as compared to healthy subjects. The hippocampus is the primary brain region involved in memory formation and retention, while the amygdala is involved in the processing of emotions and fear. In addition to changes in size, these regions also showed differences in connections to the rest of the brain, particularly to the frontal cortex, an area involved in judgment. Together, these parts of the brain accounted for 60 percent of the range in pain persistence.

The study’s results challenge long-standing views of the science of pain, establishing that the anatomical properties of the brain determine the most risk for developing chronic pain.

“These results pave the way for the development of novel and distinct approaches for the prevention and treatment of chronic pain,” added A. Vania Apkarian, PhD, professor of Physiology and senior author of the study.