Unpacking the burden: understanding the relationships between chronic pain and comorbidity in the general population.

We investigated the association of chronic pain with physical and mental comorbidity in the New Zealand population by measuring chronic pain status separate from comorbid conditions. Models of allostatic load provided a conceptual basis for considering multi-morbidity as accumulated comorbid load and for using both discrete conditions and cumulative measures in analyses. The nationally representative cross-sectional survey data included self-reported doctor-diagnosed chronic physical and mental health conditions, Kessler 10-item scale scores, an independent measure of chronic pain, and sociodemographic characteristics. The population prevalence of chronic pain is 16.9%, and a quarter (26%) of the population report 2 or more comorbid physical conditions statistically associated with chronic pain (unadjusted P<0.01). Results indicate that accumulated comorbid load is independently associated with chronic pain. Six physical conditions independently associated with chronic pain (adjusted odds range from 1.4 to 3.9) increase the risk of chronic pain in an additive manner, and residual accumulated load further increases risk for 2 or more conditions (adjusted odds 1.6). Anxiety/depression interacts synergistically with arthritis and neck/back disorders to increase the odds of reporting chronic pain beyond an additive model. This synergistic effect is not apparent for other conditions or for additional comorbid load. Results imply that measurement of chronic pain independent of comorbid conditions and adjustment for comorbid conditions is important for more accurate prevalence estimates and understanding relationships between conditions. Future epidemiological research might usefully incorporate independent measurement of chronic pain alongside adjustment for specific physical and mental health conditions as well as accumulated comorbid load.