Clinical characteristics of the asthma–COPD overlap syndrome – a systematic review

BACKGROUND AND OBJECTIVE: In recent years, the so-called asthma–chronic obstructive pulmonary disease (COPD) overlap syndrome (ACOS) has received much attention, not least because elderly individuals may present characteristics suggesting a diagnosis of both asthma and COPD. At present, ACOS is described clinically as persistent airflow limitation combined with features of both asthma and COPD. The aim of this paper is, therefore, to review the currently available literature focusing on symptoms and clinical characteristics of patients regarded as having ACOS. METHODS: Based on the preferred reporting items for systematic reviews and meta-analyses (PRISMA) guidelines, a systematic literature review was performed. RESULTS: A total of 11 studies met the inclusion criteria for the present review. All studies dealing with dyspnea (self-reported or assessed by the Medical Research Council dyspnea scale) reported more dyspnea among patients classified as having ACOS compared to the COPD and asthma groups. In line with this, ACOS patients have more concomitant wheezing and seem to have more cough and sputum production. Compared to COPD-only patients, the ACOS patients were found to have lower FEV(1)% predicted and FEV(1)/FVC ratio in spite of lower mean life-time tobacco exposure. Furthermore, studies have revealed that ACOS patients seem to have not only more frequent but also more severe exacerbations. Comorbidity, not least diabetes, has also been reported in a few studies, with a higher prevalence among ACOS patients. However, it should be acknowledged that only a limited number of studies have addressed the various comorbidities in patients with ACOS. CONCLUSION: The available studies indicate that ACOS patients may have more symptoms and a higher exacerbation rate than patients with asthma and COPD only, and by that, probably a higher overall respiratory-related morbidity. Similar to patients with COPD, ACOS patients seem to have a high occurrence of comorbidity, including diabetes. Further research into the ACOS, not least from well-defined prospective studies, is clearly needed.