By Melissa Healy
9:00 AM EDT, September 18, 2013
It's infuriating to chronic pain sufferers to be told their pain is "all in the head." But a new study offers strong evidence that even before a person experiences an injury, the likelihood that the resulting pain will become chronic can be predicted by examining the brain's "white matter" -- the bundles of fatty fibers that carry electrical impulses between the brain's hemispheres and among its dense network of cells and structures.
The new research, published Tuesday in the journal Pain, suggests that variations in brain structure could help identify people who, once injured, are more vulnerable to becoming chronic pain sufferers. But it will take further research to identify whether and which treatments could discourage short-term pain from settling in for a long stay.
The study was conducted by researchers at Northwestern University's Feinberg School of Medicine, led by Ali R. Mansour and A. Vania Apkarian, both pioneers in defining the link between pain and emotion.
Research has shown that there are clear differences that distinguish the brains of those with chronic pain from those without such pain. Chronic pain sufferers consistently show reduced volume in the brain's gray matter, the cortical structures key to perception, movement, memory and reasoning. Researchers have also shown that compared to healthy patients, the brains of chronic pain sufferers are wired differently, in ways that suggest that physical sensations and emotional responses are bound more tightly together.
But are those brain differences a response to chronic pain -- the brain's response to the experience of months or years of physical misery? Or do those differences predate chronic pain -- nudging what for another patient would be a short-term experience of discomfort into a lifelong ordeal? The authors of the current study, working under a federal government initiative aimed at consolidating research on pain, devised a series of experiments designed to clarify which came first.
They did so by recruiting 46 subjects who had experienced a first episode of back pain that had already lasted four to 16 weeks, and performing regular brain scans on those subjects for a year. Focusing largely on the bundles of axons that carry nerve impulses across the brain, they found that within two months of recruiting patients, discernible differences in the structure and integrity of that "white matter" could be used to distinguish subjects whose pain persisted from those whose pain was beginning to resolve.
By the 12-month mark, the structural differences in white matter allowed researchers to distinguish -- without error -- subjects whose pain had disappeared from those whose pain was persistent. Compared to subjects whose pain resolved, subjects whose pain would become chronic also showed differences in the density of connections that lashed their nucleus accumbens -- a central structure in the brain rewards, motivation, pleasure and reinforcement learning circuit -- together with their medial prefrontal cortex, a switchboard for decision-making, emotional response and long-term memory.
The authors made further comparisons between the original 46 subjects and two new groups: healthy recruits and people with a established history of chronic pain. Those comparisons showed that, from the earliest scans, the brains of subjects who would go on to become chronic pain sufferers had structural abnormalities that made them look much more like the chronic pain veterans than like healthy controls or the subjects with back pain that went away.
And throughout the study period, the white matter and brain connections of subjects with back pain that went away looked much more like those of healthy control subjects than they did like the brains of subjects whose pain became chronic.
The brain's white matter normally deteriorate with age, and the Northwestern researchers made a shocking calculation to show the difference that separated subjects with and without chronic pain: Compared to healthy controls or those whose pain subsided, the white matter in the subjects whose pain went on to become chronic "exhibits 30 to 50 years of additional aging."
If this research holds up, future patients may want to know what the stakes are if they get hurt, and make their recreational and career choices accordingly. So, too, might their prospective employers -- including the armed forces, the National Football League, or a trucking company tired of paying disability to drivers retiring with chronic back pain.
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