Various comorbidities are often associated with asthma.1 These can influence the clinical expression of asthma, alter asthma responses to therapy and ultimately reduce a patient’s QoL.2 Of these, some of the most frequently reported are chronic rhinitis (allergic and nonallergic), chronic sinusitis and rhinosinusitis, gastro-oesophageal reflux disease (GORD), obstructive sleep apnoea, hormonal disorders, depression and anxiety disorders.1 It is not yet known how many of these comorbidities interact with asthma, particularly in cases of severe asthma. Further research is needed on the relationships between these conditions and asthma.1
Research indicates that 20–50% of people with allergic rhinitis have asthma and more than 80% of people with asthma have rhinitis.1
While a clear mechanism of interaction between asthma and rhinitis remains unclear, there is evidence that:
Almost 100% of individuals with severe asthma have been reported to have radiological sinus abnormalities. Chronic rhinosinusitis can cause asthma to be both more severe and difficult to control.2
Chronic rhinosinusitis with nasal polyps is one important phenotype associated with severe asthma comorbidity. The mechanism is disputed, however, there is evidence to suggest that nasal polyps are associated with increased production of cytokines, prolonged survival of eosinophils and increased expression of IL-5.2
Around 50–80% of asthmatics reported suffering from GORD symptoms.1
There are two main mechanisms associating GORD with asthma:
In addition, evidence suggests that asthma may also worsen GORD symptoms. An increased pressure gradient between an asthma sufferers’ abdomen and thorax can lead to the lower oesophageal sphincter to herniate into the thorax. This reduces the sphincter’s barrier function leading to increased reflux.7
The causal relationship between GORD and asthma may not necessarily be direct. Asthma and GORD are both considered to be comorbidities of obesity meaning planning effective treatment is not always straightforward.1,8
The prevalence of obesity continues to increase worldwide and is associated with an increased incidence of asthma and poor asthma control.1,9
The direction of causality is unclear; however studies have suggested that:
However, immunological pathways are not the only potential cause for obesity acting as a comorbidity of asthma. Reducing physical activity to attempt to limit asthmatic symptoms may, over time, promote weight gain.12
Obesity has also been linked with specific phenotypes of asthma including a reduced response to asthma medications.1,11
There are many other comorbidities associated with asthma and for some, the level of influence remains undetermined.
Associations are not always clear and are regularly multifactorial. Determining underlying causality of comorbidities is complex, often with immunological, genetic and environmental impacts. Future research will continue to provide us with further insights into these interactions, allowing treatment strategies to improve.
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ABPA, allergic bronchopulmonary aspergillosis; COPD, chronic obstructive pulmonary disease; GORD, gastro-oesophageal reflux disease; IL, interleukin; OSA, obstructive sleep apnoea; QoL, quality of life; Th2, T helper cell type 2.References: