Genes affecting levels of neurotransmitters - such as serotonin and norepinephrine - might affect the risk of both back pain and depression, the researchers added.

Neurotransmitters are chemicals that transmit signals from one neuron to another 'target' neuron. Thus the commonly found association between these health conditions is probably not a "true" relationship, as there are other factors influencing it, the study noted.

"Genetic factors affecting both conditions may be involved in the association between back pain and depression," said Marina Pinheiro from the University of Sydney, Australia.

For the study, the researchers analysed data from an established database (Murcia Twin Registry) of nearly 2,150 Spanish twins. Twin studies provide a unique opportunity to elucidate the association between health conditions, by eliminating the genetic and environmental factors contributing to them.

On analysis of monozygotic twins -- who are genetically identical -- the researchers found that the commonly found association between symptoms of depression and low back pain disappeared.

This suggested that the strong association found in non-identical twins resulted from the "confounding" effects of common genetic factors influencing both conditions, the researchers noted.

Previous studies have shown a "consistent relationship" between back pain and depression -- a combination that may complicate diagnosis and treatment.

The new study is the first to examine the relationship between depression and low back pain using twin data to control for genetic and familial factors, the researchers pointed out.

The study appeared in the journal PAIN.


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