Pain Depends on the Brain

Chronic pain likelihood relies on how emotional and motivational brain centers communicate

(RxWiki News) How can two people have the same injury and yet one fully recover and the other develop chronic pain? Chronic pain affects 116 million Americans each year yet the answers to these questions are not completely understood.

A recent study seeks to understand why some people recover from injuries and others go on to experience chronic pain.

The study found that chronic pain develops based on how much two different parts of the brain communicate.

These two parts of the brain, the frontal cortex and the nucleus accumbens, are the centers responsible for emotional and motivational behavior.

"Speak to your doctor about chronic pain therapies."

The brain imaging study was conducted by a team from Northwestern Medicine and led by Marwan N. Baliki. The study examined subacute back pain in 39 patients and 17 healthy controls over the course of one year.

The study participants who were chosen scored more than 40 out of 100 on a visual pain intensity scale and had pain for less than 16 weeks.

The patients were given a Magnetic Resonance Imaging (MRI) scan as soon as possible after the injury. Follow up MRI scans were made three additional times over the following year.

The control patients were also scanned four times during the course of a year.

The patients were asked to complete a short form of the McGill pain questionnaire to assess physical pain, the Positive Affect Negative Affect Score, a scale that measure positive and negative emotions in individuals, and Beck’s Depression Inventory, a questionnaire that measures depression.

The questionnaires were completed one hour before gray matter volume and density of the brain was measured with the MRI.

The results of the study showed that the more the emotional and motivational sections of the brain communicated, the more likely the patient would develop chronic pain.

Researchers could predict which patients would develop chronic pain with 85 percent accuracy based on the level of interaction between the two areas of the brain.

This suggests that the more emotionally the brain reacts, the more likely the pain will continue even long after the injury itself has healed. It is not the injury itself that causes chronic pain, it is the state of the brain combined with the injury.

The nucleus accumbens mediates punishment and reward, teaching the rest of the brain how to evaluate and react to stimulus. This study suggests that this may include how to develop chronic pain.

The researchers speculate that this could be due to genetic and environmental influences within an individual’s brain. Some people may be genetically disposed to react at a higher level.

Another result of the study was that the patients with chronic pain had a greater loss of gray matter density.

Less gray matter density often indicates neuronal shrinkage and fewer synaptic connections, a crucial element for neuron communication.

"The findings are particularly interesting because they show for the first time that changes in these
brain areas may be involved in the 'learning' that comes with sustained pain over time," says Juan Dominguez Ph.D., principal investigator of the Neuroendocrinology and Motivation Laboratory at the University of Texas in Austin.

"These findings also hold interesting implications for ways in which prolonged pain might impact mechanisms of reward and motivation," adds Dr. Dominguez. "This means chronic pain might influence our responses to rewarding stimuli by changing the brain mechanisms regulating reward."

The study was published in the July edition of Nature Neuroscience.

The research was funded by the National Institute of Neurological Disorders and Stroke and approved by the Institutional Review Board of Northwestern University.

One researcher was funded by an anonymous foundation.

The researchers report no conflicts of interest.

Review Date: 
August 18, 2012