Despite its prevalence, scientists do not know why some people develop chronic pain. A new study approaches this question from all angles, exploring the role of money and the mind.

According to the Centers for Disease Control and Prevention (CDC), chronic pain affects around 1 in 5 adults in the United States. Almost half of these people report that pain frequently limits their day-to-day activities. Scientists have linked chronic pain to opioid addiction, anxiety, depression, and reduced quality of life, among other things.

Over the years, researchers have attempted to understand chronic pain by approaching it from different angles. For instance, some have explored the sociological aspects of chronic pain, searching for societal risk factors such as lower income or employment status.

Others have looked at the role of psychological traits. For instance, according to the authors of the latest study, some older studies have concluded that catastrophizing about pain and fear of pain are both "strong predictors of chronic pain." In fact, the authors explain that many now consider psychological factors "better predictors of chronic pain disability than the actual injury itself." However, few studies attempt to mesh together psychology, brain activity, and economic factors.

Tying up loose ends

The latest study paper, which now appears in the journal PLOS Biology, set out to knit the strands together and formulate a way to develop a more holistic view of chronic pain.

Firstly, the team assessed how psychological traits linked to chronic pain interact with general personality types. Secondly, they used functional MRI (fMRI) scans to see if they could identify chronic pain-related patterns of activity. Thirdly, because researchers have already identified that socioeconomic factors play a part in chronic pain, they added these into the equation.

The scientists took data from a randomized, controlled trial that included people with chronic back pain. All participants filled out in-depth questionnaires that collected information about personality traits and socioeconomic factors.

For the study, 62 participants visited the laboratory on six occasions; on four of those occasions, they underwent a resting state fMRI scan. Another 46 participants completed the questionnaires but did not have a brain scan. Resting state fMRI scans measure brain activity when a person is not engaged in a cognitive task. These scans provide insight into the brain's functional organization.

The study authors found four clusters of characteristics, or dimensions, within the participants. Two of these dimensions, which they named Pain-trait and Emote-trait, seemed particularly influential.

Pain-trait and emote-trait

Pain-trait encompasses the traditional factors that scientists already believe increase the severity of chronic pain, such as catastrophizing about pain and fear of pain. Individuals with this dimension were more likely to report worse pain. Emote-trait, however, appeared to be protective; this dimension includes optimism, lower levels of neuroticism, and a capacity for mindfulness. According to the authors, emote-trait "seems to offset and counteract the negative emotional impact of chronic pain."

These dimensions were also reflected in resting state fMRI scans and matched what the authors refer to as "neurotraits." These neurotraits appeared to be stable, and the researchers observed them in all four brain scans. The networks that became activated as part of these neurotraits were not confined to brain areas that deal with pain. Instead, they spread to regions that scientists believe play a part in catastrophizing, for instance.

The role of income

The team found that people with higher incomes were protected against chronic pain. Those with lower incomes reported higher pain scores and increased disability. These results echo those of previous work in a similar vein.

For instance, a study that looked at the role of financial stability on people's experience of chronic pain concluded that "[e]conomic hardship was associated not only with greater exposure to daily financial worries but also with greater vulnerability to pain on days when daily financial worries were experienced."

As the authors of the new paper explain, this relationship with income might be due to other factors they did not cover in this study, such as upbringing, politics, and culture. Regardless, they conclude that the findings bolster the idea that "chronic pain experience is not only rooted in biology but also intimately embedded in society."

The authors do note several limitations of the study. For example, they excluded people with moderate or severe depression, which, they believe, might "limit the generalizability" of the conclusions.

In future projects, the researchers would like to add more depth to their questionnaires; this study did not account for a wide range of potentially important factors, such as working environment, marital status, and access to healthcare.

Also, the study included only a small number of participants, all of whom had chronic back pain. It is possible that the findings do not extend to the population at large, or to other types of chronic pain. However, these findings offer a starting point for further research; the team's approach provides a new way to investigate this poorly understood condition.