Loss in brain density is related to pain duration, indicating that 1.3 cubic centimeters of gray matter (the part of the brain that processes information and memory) are lost for every year of chronic pain, said lead researcher A. Vania Apkarian, associate professor of physiology at Northwestern University Feinberg School of Medicine and a researcher at the Northwestern University Institute of Neuroscience.
The study, the first to examine brain changes in chronic pain conditions, was published in the Nov. 23 issue of The Journal of Neuroscience.
At least 25 percent of Americans suffer from back pain; in one fourth of these individuals, back pain is chronic and unremitting. Although chronic pain greatly diminishes quality of life and increases anxiety and depression, it previously had been assumed that the brain reverts to its normal state after chronic pain stops.
Apkarian and co-researchers used structural magnetic resonance imaging brain scan data and two automated analysis techniques to contrast brain images from 26 participants with chronic back pain with those from matched normal subjects.
All participants with chronic back pain had unrelenting pain for more than a year, primarily localized to the lumbosacral region, including buttocks and thighs, with or without pain radiating to the leg.
The participants were divided into neuropathic -- exhibiting pain because of sciatic nerve damage -- and non-neuropathic. Brain scans showing gray matter volume were compared.
In earlier research, Apkarian and colleagues found that back pain sustained for six months or longer is accompanied by abnormal brain chemistry, indicated by chemical changes in the area of the brain known to be important in making emotional assessments, including decision-making and for controlling social behavior. Based on these results, Apkarian's laboratory group embarked on the brain atrophy study.
It is possible that some of the observed decreased gray matter shown in this study reflects tissue shrinkage without substantial neuronal loss, suggesting that proper treatment would reverse this portion of the decreased brain gray matter, Apkarian said. The atrophy also may be attributable to more irreversible processes, such as neurodegeneration. Other research has shown that spinal cord neurons undergo apoptosis -- cell death -- in rats with neuropathic pain.
"Given that, by definition, chronic pain is a state of continuous persistent perception with associated negative affect and stress, one mechanistic explanation for the decreased gray matter is overuse atrophy caused by excitotoxic and inflammatory mechanisms," Apkarian said.
The researchers hypothesize that atrophy of brain circuitry involved in pain perception may dictate the properties of the pain state, such that as atrophy of elements of the circuitry progresses, the pain condition becomes more irreversible and less responsive to therapy.
Other researchers on the study were Yamaya Sosa, physiology; Sreepadma Sonty, neurology; Robert M. Levy, M.D., neurosurgery; R. Norman Harden, M.D., physical medicine and rehabilitation; Todd B. Parrish, radiology; and Darren R. Gitelman, M.D., neurology, radiology and the Cognitive Neurology and Alzheimer's Disease Center, at Feinberg.
This research was supported by grants from the NINDS and the NIH.