Cause-specific mortality of patients with severe chronic pain referred to a multidisciplinary pain clinic: a cohort register-linkage study
Authors: Vaegter Henrik Bjarke , Støten Martine , Silseth Siv Laine , Erlangsen Annette , Handberg Gitte , Sondergaard Stine , Stenager Elsebeth 
Published: September 2018

Abstract

Background and aims
Almost 20% of the adult population suffers from chronic pain. Chronic pain may be linked to an elevated mortality; however, results from previous studies are inconsistent. Some studies find similar mortality levels in chronic pain patients and pain-free controls while other studies show elevated mortality levels among chronic pain patients, primarily with respect to cancer, diseases of the circulatory and respiratory systems, and suicide. These conflicting results are potentially due to different population samples and different operational definitions of chronic pain. Further research on overall and cause-specific mortality in patients with severe chronic pain is needed to inform clinical practice. The objective of this register-linkage study was to investigate whether patients with severe chronic pain referred to multidisciplinary pain treatment have higher cause-specific mortality rates than the general population.

Methods
In this register-linkage cohort study, data from 6,142 chronic pain patients (female: n=3,941, male: n=2,201, mean age: 48.2±14.2; range: 16–97 years) attending an interdisciplinary Pain Center in Odense, Denmark from 2005 to 2014 were linked to the Danish Register of Causes of Death. Age and gender standardized mortality ratios (SMRs) with their 95% confidence intervals (CI) were calculated and compared with those of the general population. Data from the general population was extracted from the Danish Register of Causes of Death, and Causes of death were classified according to national Classification of Disease (ICD-10).

Results
In all, 276 deaths (women: n=152, men: n=124) were observed among the chronic pain patients, and a six-fold higher overall mortality rate was found [SMR: 6.2 (95% CI: 5.5–7.0)] compared with the general population. Elevated cause-specific mortality rates were noted for chronic patients with respect to cancer and neoplasms [4.7 (95% CI: 3.7–5.9)], diseases of the circulatory system [5.7 (95% CI: 4.3–7.3)], diseases of the respiratory system [8.7 (95% CI: 6.2–11.9)], and suicide [7.3 (95% CI: 2.7–15.9)].

Conclusions
The overall mortality rate of patients with severe chronic pain in this study was six-fold higher than the rate of the general population in this region. This was reflected in select specific causes of death (cancer and neoplasms, diseases of the circulatory system, diseases of the respiratory system, and suicide). The results are in agreement with previous studies and emphasize the need to understand which factors causally affect this increased mortality allowing for targeted interventions in similar chronic pain populations.

Implications
Potential reasons for the excess mortality should be adequately addressed by future studies in order to better target this in the management of these patients. The chronic pain population included in this study may have several comorbidities contributing to the increased mortality. To better address these aspects, complete medical profiles are needed in future studies. In addition, implementation of management strategies towards potential risk factors such as poor diet, low levels of physical activity, smoking, and high BMI as well as sleep deprivation and morphine use previously shown associated with having pain may reduce the excess mortality ratio.