The asthma - COPD overlap syndrome ( ACOS ) : hype or reality ?

Asthma and chronic obstructive pulmonary disease (COPD) are chronic diseases with high prevalence in the general population. Asthma affects 25 million Americans (18 million adults and 7 million children) (1), while about 14.2 million Americans are diagnosed with COPD and 9.8 million are estimated to have COPD that is undiagnosed (2, 3). A variable degree of airway inflammation, airway obstruction, and airway hyperresponsiveness are common pathophysiological features of both diseases (4, 5). Several studies described different inflammatory cells recruitment, mediators production, and responses to therapy in asthma and COPD. Airway obstruction is typically intermittent and reversible in asthma while it is progressive and largely poor reversible in COPD (6). Although COPD and asthma are two different and independent diseases, COPD can coexist with asthma with features of irreversible airflow limitation (7). In 1995, the guidelines on chronic obstructive pulmonary diseases by the American Thoracic Society (ATS) described eleven syndromes, including asthma, chronic bronchitis, emphysema, COPD and airflow obstruction, and an overlap at 6 of these was observed (8). More than 40% of patients with COPD report a history of asthma, and asthma is recognized as a risk factor for COPD (9). Increasing evidence suggests that the two diseases may coexist in the same individual; however, only in recent years an asthma-COPD overlap syndrome (ACOS) has been described. Epidemiological studies showed that the prevalence of overlapping diagnoses increased in older populations (10-12). The pathologic and functional overlap between asthma and COPD has been estimated to be <10% in patients younger than 50 years and >50% in patients aged 80 years or older (10). Similar results on the age distribution were found in the Italian individuals (13), in whom asthma and COPD coexist in a substantial proportion of subjects. In this study, the prevalence of asthma-COPD overlap was 1.6%, 2.1% and 4.5% in the 20-44, 45-64, 65-84 age groups, respectively. Pride et al. (14), in a study on the application of diagnostic labels for obstructive airway showed that a vast proportion of clinicians extended the diagnostic label beyond asthma to indicate the presence of other clinical features, using additional terms such as asthma with chronic bronchitis, asthma with permanent Mini-review