The prevalence of patients concurrently suffering from both depression and low back pain is high and the combination of these conditions is associated with treatment difficulties, such as the need for engaging multi-professional teams and higher rates of health care utilization and management costs (Baumeister et al., 2012; Greenberg et al., 2003). However, the mechanisms underlying this relationship remain unclear. Understanding whether there is a causal relationship between depression and low back pain has important implications for the management of patients with these conditions.
In an attempt to disentangle the link between depression and low back pain our research group conducted a systematic review investigating depression as a risk factor for low back pain which was published in Arthritis Care and Research. In this review we only included longitudinal studies where participants were free of back pain at baseline (study entry). We found that having symptoms of depression increased the odds of people developing back pain in the future (odds ratio [OR] 1.59, 95% confidence interval [CI] 1.26 to 2.01) (Pinheiro et al., 2015b). The effect size was relatively small and the levels of adjustment employed by the studies were low. Therefore, it was not possible to conclude whether there is a true relationship between depression and low back pain or if this relationship is confounded by other factors that could influence both conditions.
Since genetic factors have been found to be associated with the development of both low back pain and depression, our research group has recently published a co-twin study in Clinical Journal of Pain (Pinheiro et al., 2017) investigating whether symptoms of depression increase the risk of developing low back pain when genetic and environmental factors are taken into account, variables not previously controlled in studies included in the systematic review published in Arthritis Care and Research.
What did we do?
We analysed data from an established database of Spanish twins, the Murcia Twin Registry. We used longitudinal data from 1,607 twins and conducted a robust analysis employing the twin design to investigate the association between these conditions, accounting for genetics and environmental factors.
Baseline data on depression-related symptoms and low back pain were collected in 2009-2011 and follow-up data in 2013. Only participants who did not report chronic low back pain at baseline were included in this study. Data analysis were conducted in two steps: 1) total sample analysis, where all twins were included, regardless of zygosity, with twins being analysed as individuals rather than pairs. This approach is similar to the one used by non-twin studies; 2) case control analysis, where only complete twin pairs, discordant for low back pain were included in the analysis. A matched within pair case-control analysis was conducted and was stratified by zygosity. By comparing the results from the two steps it is possible to understand what happens to the association between depression and low back pain when confounders are accounted for.
What did we find?
In the total sample analysis, symptoms of depression did not increase the risk of low back pain (OR 1.40, 95% CI 0.96 to 2.03), activity limiting low back pain (OR=1.09, 95%CI: 0.69-1.72) or care seeking (OR=1.21, 95%CI: 0.81-1.81). When symptoms of depression where assessed as state depression (participants symptoms at the moment of the interview), significant associations were found for activity limiting low back pain (OR=1.07, 95%CI: 1.01-1.14) and care seeking (OR=1.06, 95%CI: 1.01-1.12). When symptoms of depression were assessed as trait depression, a significant association was found only for activity limiting low back pain (OR=1.07, 95%CI: 1.01-1.14). In the case control analysis, symptoms of depression did not increase the risk of developing back pain regardless of the measures used to assess both conditions. Most analyses were not statistically significant and revealed small effect size, indicating a weak association at best.
So what do these findings tell us?
Our results showed that the temporal relationship between symptoms of depression and low back pain is weak at best. When the effects of genetic and environmental factors were accounted for in the case control analysis, no association was found, suggesting that this association is possibly confounded by genetic or environmental factors that influence both conditions. Additionally, the results varied according to the measures used to assess depression and low back pain, suggesting that the relationship is dependent on the measurement tools used to assess both conditions.
The findings from this co-twin longitudinal study were in agreement with the findings from our cross-sectional co-twin study published in Pain (Pinheiro et al., 2015a) where the association between these conditions reduced and became non-significant when the effects of genetic and environmental factors were accounted for. The study published in Pain employed the same method to analyse the data, but despite the high level of adjustment for confounders the use of cross-sectional data precluded the investigation of a temporal relationship between low back pain and symptoms of depression. This gap was addressed in this longitudinal study.
Taken together, the findings of these three previous studies published by our research group suggest that a causal association between low back pain and depression seems to be weak, at best. Additionally, our findings suggest that genetic and shared environmental factors play an important role in the association between low back pain and depression. It is plausible that common genetic and familial factors could influence the pathogenesis of both conditions. Common factors influencing both low back pain and depression, preferably modifiable factors, should be investigated in future studies so that treatment and preventative strategies could be designed for these common conditions.
About Dr Marina Pinheiro
Marina is a Lecturer in the Discipline of Physiotherapy at The University of Sydney. She was awarded her PhD in 2017. Her research interest includes understanding risk factors for musculoskeletal conditions, effectiveness of interventions for low back pain, barriers for implementation of physical activity interventions in clinical settings, and evidence-based practice including access to, quality and useability of research evidence.
About A/Prof Paulo Ferreira
Paulo is an associate professor in the Discipline of Physiotherapy at The University of Sydney, and leads the Spinal Research Group at the Faculty of Health Sciences, the University of Sydney. He has a teaching and research position and spends 40% of his time in teaching, 40% in research, and 20% in administrative roles. His research interests are in the epidemiology and management of low back pain particularly with the use of twins.
Baumeister H, Knecht A, Hutter N (2012). Direct and indirect costs in persons with chronic back pain and comorbid mental disorders-a systematic review. Journal of Psychosomatic Research, 73, 79–85.
Greenberg PE, Leong SA, Birnbaum HG, Robinson RL (2003). The economicburden of depression with painful symptoms. Journal Clinical Psychiatry, 64, 17–23.
Pinheiro, MB, Ferreira, ML, Refshauge, K, Colodro-Conde, L, Carrillo, E, Hopper, JL, Ordoñana, JR,Ferreira, PH (2015a). Genetics and the environment affect the relationship between depression and low back pain: a co-twin control study of Spanish twins. Pain, 156, 496-503.
Pinheiro, MB, Ferreira, ML, Refshauge, K, Colodro-Conde, L, Gonzalez-Javier, F, Hopper, JL, Ordonana, JR,Ferreira, PH (2017). Symptoms of Depression and Risk of Low Back Pain: A Prospective Co-Twin Study. Clinical Journal of Pain, 33, 777-785.
Pinheiro, MB, Ferreira, ML, Refshauge, K, Ordoñana, JR, Machado, GC, Prado, LR, Maher, CG,Ferreira, PH (2015b). Symptoms of Depression and Risk of New Episodes of Low Back Pain: A Systematic Review and Meta-Analysis. Arthritis Care Res (Hoboken), 67, 1591-1603.