The importance of comorbidity in patients with asthma is amply demonstrated in the study by Steppuhn and colleagues in this issue of the PCRJ.1 In an analysis of a large population sample of adults with asthma in Germany they found high levels of comorbidity of other important chronic conditions. Indeed, adults who only had asthma were in a minority, with over 60% of asthma sufferers having one or more additional comorbidities out of a list of eight conditions (diabetes mellitus, hypertension, chronic heart failure, depression, osteoarthritis, stroke, coronary heart disease, and cancer). For seven of these eight conditions (cancer being the exception), prevalence rates were higher in patients with asthma compared with the general population, even after adjusting for potential confounding variables. The majority of patients with comorbidity had more than one comorbid condition, with almost one in five having three or more additional conditions. Increasing levels of comorbidity were associated with more unscheduled health service utilisation for asthma, both in terms of hospital admissions (a greater than three-fold increase), and outpatient care (greater than two-fold increase).

This study1 confirms and builds on the small number of previous studies on asthma and comorbidity, and is generally in line with previous findings. As the authors rightly state, the phenomenon of comorbidity in asthma warrants further study in terms of underlying mechanisms, drug-drug interactions, effects on quality of life, and implications for management — including self-management. Asthma is a common condition, and thus the implications of a heavy burden of comorbidity are important in terms of population health and healthcare costs. Importantly, the study also demonstrated high levels of comorbidity in young as well as older patients, suggesting that without effective interventions such younger comorbid asthmatics may be on a trajectory towards high burden (and high cost) multimorbidity as they age. The fact that comorbidity was found to be socially patterned, being highest in those of lowest educational status, is also important with respect to health inequalities and the apparently ever widening gap between rich and poor.2

It is also important to place these findings within the growing research and policy focus on multimorbidity. Comorbidity — as in the study by Stepphun et al.1 — is defined as an index condition plus additional conditions, whereas multimorbidity — a term being increasingly used — is simply defined as the co-existence of two or more conditions within an individual without any single disease being the focus.3 This may be a more useful term in primary care given the longitudinal nature of the care provided and the fact that the importance of any one condition may wax and wane over the life-course. In a recent nationally representative study of almost 1.8 million people of all ages in Scotland (about one third of the population) we found that multimorbidity affected one in four people.4 Out of the 40 chronic conditions studied, including asthma, there was no condition in which most people had that condition only.

Thus multimorbidity is the norm, not the exception, in chronic disease. It is also the norm for most of the elderly population. Yet, as Steppuhn and colleagues also point out for asthma, a single-disease paradigm dominates healthcare policy, delivery and training. Guidelines are based on evidence from unusual people who only have a single condition; most randomised controlled trials (RCTs) exclude patients with other conditions. Therefore, the relevance of guidelines to most people with most chronic conditions is questionable. The growth of single-disease guidelines (which suggest when to start new medications but seldom when to stop) together with the rise in multimorbidity prevalence (in part due to the ageing population), is responsible for the burgeoning rise in polypharmacy.5 Not all polypharmacy is bad,6 but it does often result in poorer adherence to medication, increases the risk of serious drug-drug and drug-disease interactions, and adds to treatment burden.5,7,8

The increased risk of depression in asthma reported by Steppuhn and colleagues is well recognised in a wide-range of chronic conditions.9 The risk of depression rises as the level of multimorbidity increases.4 Once again, this is heavily socially patterned, with those living in more deprived areas having a higher risk of an associated mental health problem as the number of physical conditions increases.4 Such clinical complexity calls for a generalist approach, compounded in deprived areas by the continuing existence of the ‘inverse care law’.10

Interventions targeted specifically at patients with comorbidity and multimorbidity are desperately needed. A recent systematic review found only a handful of intervention studies worldwide that focused on multimorbid patients, with disappointing outcomes.11 Complex clinical situations (involving comorbid or multimorbid patients) almost certainly require complex interventions, and research bodies must begin to fund such studies.