Strengths and limitations
To the best of the authors’ knowledge, this is the largest study to date that has evaluated the prevalence of 38 comorbidities of COPD in a nationally representative primary care sample; data on a third of the Scottish population were analysed. The data analysis was cross-sectional hence conclusions about causality cannot be made. Although this could be considered a potential limitation, this is inherent in any cross-sectional analysis and evaluation of cause was not this study’s aim. Furthermore, the presence of a chronic illness may lead to increased diagnosis of other conditions due to closer monitoring.21
Certain comorbidities were not specifically included in the analysis, such as lung cancer or pulmonary hypertension, as this study involved a secondary data analysis and these conditions had been excluded from the primary data analysis. This could be considered a limitation because these conditions have previously been associated with COPD and evaluation of their prevalence may have been of interest.8
Moreover, the authors’ acknowledge that certain conditions, such as viral hepatitis, were associated with smaller sample sizes (42 patients with COPD had viral hepatitis) compared with more common conditions such as diabetes (6315 patients with COPD had diabetes) and consequently the authors accept that they cannot be confident in the apparent strength of association between COPD and viral hepatitis with such a small sample.
Smoking data were not included in the data analysis, and so it was not possible to make comparisons on levels of smoking in people who did, and did not, have COPD.
Comparison with existing literature
This study’s COPD prevalence of 4.1% was similar to the 3.4% found by a Canadian study22 but higher than 1.4% in a study from England.23 Within the group of people with COPD, 86.0% had ≥1 comorbidities, slightly higher than the prevalence range of 76–82% found in three recent studies.5,22,24 The types of comorbidities included in different studies varied significantly, which is likely to account, at least in part, for these differences.
In the study presented here, the largest difference between the COPD group from those without was for bronchiectasis. Prevalence of bronchiectasis in people with COPD in this study was 1.9%. A recent meta-analysis25 that included 14 observational studies found that comorbid bronchiectasis was associated with an increased risk of severe airway obstruction and mortality.
Asthma was commonly found in patients with COPD, which was expected for two reasons:
some patients will have asthma–COPD overlap syndrome,26 whereby there are chronic airway disease features that overlap both conditions. Typically, there is variable airflow obstruction, which is not fully reversible;26
some people with COPD may be incorrectly coded as having asthma, which may overstate the prevalence of asthma as a comorbidity.
Chronic sinusitis was twice as common in the COPD group than the group without COPD. This association of sinusitis with COPD has previously been recognised, because it is thought that nasal inflammation can occur in addition to airway inflammation in COPD.27
Previous studies have established that COPD is associated with hypertension and CHD,12,20 which this study confirmed. CHD is frequently underdiagnosed in people with COPD; this is important as the coexistence of CHD and COPD results in a worse prognosis than if a patient has one disease or the other.28 This study demonstrated that individuals with COPD, compared with people without, were more likely to have heart failure, peripheral vascular disease, and cerebrovascular disease consistent with higher rates of hypertension and CHD, with smoking as a common risk factor.6,29,30 Research by Rutten et al identified a prevalence of 20.5% for heart failure in people with COPD — compared with 6.4% in this study — when screened for the condition using a number of diagnostic tests including an echocardiogram. It is therefore likely that the true prevalence of heart failure in COPD is higher than found in this study, because patients with COPD are not routinely screened for heart failure.31
The prevalence of comorbid dyspepsia in patients with COPD presented here is in keeping with a longitudinal study by Benson et al,32 who noted a high prevalence of gastro-oesophageal reflux disease in patients with COPD, which was associated with an increased risk of exacerbation. The findings presented here of increased prevalence of diabetes mellitus in people who had COPD (12.2%) compared with those who did not was lower than the prevalence documented in two cross-sectional studies of 18.7%13 and 20%.12
The study presented here found higher levels of alcohol misuse in patients with COPD, compared with those without COPD, which may go some way to explaining the higher levels of chronic liver disease; however, there were also strong associations with viral hepatitis. One small Japanese study found that chronic hepatitis C virus infection was associated with an accelerated decline in lung function in patients with COPD;33 hepatitis C was also found to be more prevalent in people with COPD than the general population in Brazil (7.5% versus 1.2–2.0%).34 Notably, the code ‘viral hepatitis’ that was used in the study presented here did not differentiate between the hepatitis subtypes, so a direct comparison cannot be made on the prevalence of hepatitis C in Brazil. Further research is needed to evaluate whether there is a significant association between hepatitis C and COPD.
A previous meta-analysis identified a statistically significant association between psoriasis and COPD, with a higher risk of developing COPD with more severe psoriasis,35 confirming this study’s finding of increased risk of psoriasis in people who had COPD compared with those who did not.
This study found that, compared with people who did not have COPD, those with COPD were more likely to have one, and more than twice as likely to have two or more, mental health conditions. The biggest difference was for alcohol misuse, with the COPD group more than twice as likely to have been coded for this. There is a lack of studies investigating the prevalence of alcohol misuse in patients with COPD, but there is evidence of high rates of smoking in those who misuse alcohol.36 The prevalence of anxiety and depression in those with COPD was 11.2% and 19.1% respectively, although both may be underdiagnosed.14 In the literature (which includes heterogeneous, often selected, populations), the estimated prevalence of comorbid anxiety varies from 10–19%; for depression the figure is 10–42%.14
The exact mechanisms underpinning the diverse comorbidities associated with COPD are likely to be multifactorial and beyond the scope of this study. In short, some conditions may share common risk factors such as CHD and smoking.4 However, evidence suggests that COPD is associated with chronic systemic inflammation, independent of smoking, which, in turn, may lead to insulin resistance (contributing to metabolic syndrome and diabetes), cachexia, and a procoagulant state.29,37 COPD medication is also likely to exacerbate certain comorbidities: for example, prednisolone could contribute to diabetes, osteoporosis, and muscle dysfunction.38 Further research is required to elucidate the exact mechanisms of the associated comorbidities.
Implications for research and practice
The findings from the current study found that there were 35 physical and mental health conditions which were more common in people who had COPD than people without. Various studies have outlined some of the implications of having conditions in addition to COPD. An Italian cohort study of 569 people with COPD found that 81.2% had additional comorbidity and acute exacerbations of COPD were more common in those with a higher number of comorbidities.5 Another study evaluating comorbid mental health conditions alongside COPD outlined significant implications such as poor compliance with treatment and increased frequency of hospital admissions, as well as prolonged inpatient stay.9,14
This study demonstrates the high prevalence of COPD and that the presence of comorbidities is the rule, rather than the exception. The current healthcare model delivers fragmented care to patients with multiple comorbidities. The authors postulate that optimal management for these complex patients would involve integrating specialist and primary care services in order to provide comprehensive, holistic health care. Primary care, which is unique in terms of offering expert generalist care, is best placed to provide this integrated approach.
The high level of COPD comorbidities is pertinent as, aside from issues regarding care quality, it also has an impact on healthcare costs. One recent Danish study39 examining resource allocation in COPD care found that multimorbidity in COPD significantly increased the annual fee for service healthcare expenditure. Although it is unsurprising for comorbidities to increase healthcare costs, this highlights evidence for the added economic burden resulting from comorbidities.
Current guidelines for COPD management do not take into account comorbidities, even those that are common. The research behind the guidelines frequently excludes patients with multimorbidity, which influences the validity and generalisability of the treatments suggested for most people who have COPD. There is a real need for guidelines and health care to reflect the complexities associated with COPD, encompassing detection of the common physical and mental health comorbidities, as well as how best to manage them in combination. This study plays a vital part in determining the prevalence of comorbidities in COPD, which could contribute to the creation of these appropriate, comprehensive guidelines. If these were developed, it might be possible to reduce the number of admissions, and improve morbidity and mortality for patients with COPD, which, in turn, could have a significant economic and healthcare impact.
The study presented here has illustrated that the majority of patients with COPD have complex physical and mental health comorbidity. The authors propose that integrating primary and secondary care services would provide optimal holistic care for these patients. In addition, COPD guidelines should be based on valid, generalisable evidence and must reflect the associated comorbidities in order to provide clinicians with clear management strategies.
