Depression and chronic pain are known to be linked, but how closely?
Often occurring together, chronic pain and major depressive disorder are prevalent conditions.
Both can cause significant distress and are known to complicate and worsen other physical diseases.
Globally, some estimate that 1 in 10 adults are diagnosed with chronic pain each year.
Despite these high numbers, the risk factors involved are still shrouded in mystery, although genetics and environmental factors are both known to play their part.
A recent study found that chronic pain has a heritability of 30 percent.
Other genetic studies, including a number of twin studies, have come to similar conclusions; genes are increasingly considered to be heavily involved in the development of chronic pain.
Depression, which is estimated to impact 15.7 million adults aged 18 or older in the United States each year, also has a genetic component. The influence of genes appears to be of the same order as chronic pain. For instance, one study estimated the heritability of major depressive disorder to be 37 percent.
Although a genetic component to both conditions seems clear, the way in which genes conspire to create chronic pain and depression is not understood. To further muddy the waters, environmental factors, such as lifestyle and sleep quality, are also known to play their part.
The exact interplay between genes and environment has always been a difficult problem to solve.
A new look at the causes of chronic pain
Researchers from the University of Edinburgh in the United Kingdom set out to investigate genetic and environmental influences on depression and chronic pain. They used data from two large nationwide studies - Generation Scotland and UK Biobank projects.
Both projects were designed to understand the genetic factors behind health conditions. For the current study, more than 100,000 participants' data were examined.
Each of the individuals' genetic backgrounds were delved into, as were their experiences of depression and pain.
The team found that partners of people with depression were more likely to experience chronic pain, but a person's genetic make-up also played its part.
Overall, heritability was found to account for 38.4 percent of the variation in chronic pain risk; also, a shared environment with spouses or partners accounted for 18.7 percent of the variation in susceptibility to chronic pain.
Additionally, there were unidentified risk factors that were shared by partners or spouses. It became clear that the risk factors for chronic pain and depression had a significant overlap; the authors conclude that "the cumulative effects of genetic risk factors for depression increased an individual's chance of having chronic pain."
A door to future treatments?
These findings, published this week in PLOS Medicine, might lead the way toward more effective treatments in the future. Chronic pain, in particular, is a common cause of disability, but very little is known about its causes.
"We hope our research will encourage people to think about the relationship between chronic pain and depression and whether physical and mental illnesses are as separate as some believe."
Prof. Andrew McIntosh, chair of Biological Psychiatry at the University of Edinburgh, U.K.
The results of the study show that both genetic factors and chronic pain in a spouse or partner increase the risk of chronic pain for the individual. The authors also conclude that chronic pain is caused by an accumulation of a number of small genetic effects and is associated with some of the same environmental and genetic risk factors that increase the risk of depression.
In short, the two conditions are deeply entwined. Genes play a large part, as does the environment. The more we learn about the interaction, the easier it will be to find effective treatments for the conditions and, perhaps, understand how to prevent them from occurring.