Asthma-COPD Overlap Syndrome: What We Know and What We Don't.

Approximately one in four patients with chronic obstructive pulmonary disease (COPD) have asthmatic features consisting of wheezing, airway hyper-responsiveness or atopy. The Global initiative for Asthma/Globalinitiative for chronic Obstructive Lung Disease committee recently labelled these patients as having asthma-COPD overlap syndrome or ACOS. ACOS also encompasses patients with asthma, ≥40 years of age, who have been cigarette smokers (more than 5-10 pack years) or have had significant biomass exposure, and demonstrate persistent airflow limitation defined as a post-bronchodilator forced expiratory volume in 1 second (FEV1)/forced vital capacity of <70%. Data over the past 30 years indicate that patients with ACOS have greater burden of symptoms including dyspnea and cough and show higher risk of COPD exacerbations and hospitalizations than those with pure COPD or pure asthma. Patients with ACOS also have increased risk of rapid FEV1 decline and COPD mortality. Paradoxically, experimental evidence to support therapeutic decisions in ACOS patients is lacking because traditionally, patients with ACOS have been systematically excluded from therapeutic COPD and asthma trials to maintain homogeneity of the study population. In this study, we summarize the current understanding of ACOS, focusing on definitions, epidemiology and patient prognosis.