The asthma-COPD overlap syndrome: how is it defined and what are its clinical implications?

It is increasingly recognized that both asthma and chronic obstructive pulmonary disease (COPD) are heterogeneous diseases with a large inter-individual variability with respect to their clinical expression, disease progression, and responsiveness to the available treatments. The introduction of asthma-COPD overlap syndrome (ACOS) may lead to a better clinical characterization and improved treatment of patients with obstructive airways disease. However, it is still in its early phase and several improvements will have to be made. First, a clear definition of ACOS and preferably also its sub-phenotypes, eg, asthma-ACOS and COPD-ACOS, is urgently needed. That would also allow researchers to design clinical studies in well-defined patients. The latter is important since the interpretation of clinical studies performed so far is hampered by the use of many different definitions of ACOS. Second, future studies are needed to investigate the role of state-of-the-art techniques such as computed tomography, genetics, and genomics in the phenotyping of patients with obstructive airways disease, ie, asthma, COPD, and ACOS. Third, longitudinal studies are now needed to better define the clinical implications of ACOS with respect to the long-term outcome and treatment of ACOS and its sub-phenotypes compared to only asthma or COPD.