The overlap phenotype: the (missing) link between asthma and COPD

The definition of chronic obstructive pulmonary disease (COPD) as a preventable and treatable condition characterized by a not completely reversible chronic airflow obstruction [1] is so broad and imprecise that many different types of patients with distinct clinical characteristics, prognosis and response to treatments may fit in. These different types are now described as “clinical phenotypes” and the interest in their definition and characterisation is growing [2]. Among these phenotypes, the so-called overlap of syndromes with airflow obstruction is usually poorly considered [3]. In a first step of phenotyping, a COPD patient can potentially be classified as a predominant parenchymal destructive or predominant airflow limitation phenotype by using a few clinical, radiological and functional findings [4]. However, when a patient presents characteristics and symptoms of two or more respiratory diseases at the same time, this is described as an overlap syndrome. In particular, the asthma-COPD overlap phenotype has been described as symptoms of increased variability of airflow in association with an incompletely reversible airflow obstruction [5]. From a clinical point of view it usually corresponds to individuals diagnosed with asthma before the age of 40 who, at an older age, fulfil the criteria for COPD [6]. Recent estimations of its prevalence report that about 13-20% of subjects with COPD have the overlap phenotype [6,7], with an increasing trend in the elderly population (up to 50% in those aged over 70 years) [7]. Since they have been systematically excluded from both COPD and asthma pharmacological clinical trials as not being “pure” subjects, it is clear that we cannot really know the response to pharmacological treatment of a significant number of patients with COPD. Increased reversibility is one of the key differential aspects of individuals with the overlap asthma-COPD phenotype. Reversibility in COPD is not only possible