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Editorial COPD Chronic Obstructive Pulmonary Disease and Obstructive Sleep Apnoea Walter T McNicholas Department of Respiratory and Sleep Medicine, St Vincent’s University Hospital, School of Medicine, University College Dublin, Dublin, Ireland T he overlap syndrome (OS) of chronic obstructive pulmonary disease (COPD) and obstructive sleep apnoea (OSA) is common but often missed in clinical practice. Different clinical COPD phenotypes influence the likelihood of co-existing OSA with the predominant emphysema phenotype being protective whereas the predominant chronic bronchitis phenotype promotes the development of OSA. The management of OS differs from COPD alone, particularly the use of nocturnal positive airway pressure (PAP), and patients with OS not treated with PAP have a worse prognosis, which underlines the importance of correct diagnosis. Keywords COPD, sleep apnoea, overlap syndrome, mechanisms, phenotypes, management Disclosure: Walter T McNicholas has no conflicts of interest to declare in relation to this article. This article is a short opinion piece and has not been submitted to external peer reviewers. No funding was received in the publication of this article. Authorship: All named authors meet the International Committee of Medical Journal Editors (ICMJE) criteria for authorship of this manuscript, take responsibility for the integrity of the work as a whole, and have given final approval for the version to be published. Open Access: This article is published under the Creative Commons Attribution Noncommercial License, which permits any non-commercial use, distribution, adaptation and reproduction provided the original author(s) and source are given appropriate credit. Received: 20 February 2017 Published Online: 14 April 2017 Citation: European Respiratory & Pulmonary Diseases, 2017;3(1):23–4 Corresponding Author: Walter T McNicholas, Department of Respiratory and Sleep Medicine, St Vincent’s University Hospital, Elm Park, Dublin 4, Ireland. E: walter.mcnicholas@ucd.ie For two such highly prevalent disorders, the co-existence of chronic obstructive pulmonary disease (COPD) and obstructive sleep apnoea (OSA) in the same patient gets remarkably little attention in routine clinical practice. Recent estimates indicate that up to 50% of adult males in the general population have sleep-disordered breathing based on an apnoea-hypopnoea frequency (AHI) over five events per hour, although the prevalence of a clinically significant disorder based on associated symptoms is considerably lower. 1,2 COPD is also highly prevalent, with population estimates of 10% for COPD associated with significant airflow obstruction. 3 Thus, by chance alone, COPD and sleep- disordered breathing can be predicted to occur in about 3–4% of the general adult population, with COPD and a clinically significant OSA syndrome occurring in 1–2%. 4 In considering the relationships between COPD and OSAS, several questions can be considered. For example, does the presence of either COPD or OSA increase the likelihood of the other disorder in an individual patient? Does the presence of both disorders together, termed ‘overlap syndrome’, increase the likelihood of co-morbidities? And what are the implications of overlap syndrome for patient management and prognosis? Regarding COPD predisposing to OSA, one has to consider the influence of different COPD phenotypes. In particular, the emphysema phenotype that is typically associated with lung hyperinflation and low body mass index (BMI) is likely to be protective against OSA, 5,6 whereas the chronic bronchitis phenotype that is associated with chronic productive cough, higher BMI, and predisposition to cor pulmonale is more likely to predispose towards OSA. 7 Recent evidence indicates that lung hyperinflation is associated with reduced pharyngeal collapsibility, 5 and AHI is negatively correlated with the degree of emphysema on computed tomography (CT) of the thorax in COPD patients. 6 Peripheral oedema in patients with cor pulmonale predisposes to OSA as a result of rostral fluid shift during the night associated with the supine position, contributing to narrowing of the oropharynx by fluid build-up. 8 The evidence that OSA contributes to COPD is less clear, although there is some epidemiological evidence that both COPD and asthma are more prevalent in patients with OSA, but the mechanisms of this association have not been established. 9 Concerning co-morbidities, COPD and OSA are both recognised as independent risk factors for cardiovascular co-morbidity, 10,11 and it appears reasonable to postulate that patients with overlap syndrome will be at greater risk of co-morbidity. However, there have been remarkably few reports in the literature regarding the risk of cardiovascular disease in patients with overlap syndrome. Overlap patients demonstrate more pronounced hypoxaemia during sleep than patients with either COPD or OSA alone and therefore they are more prone to develop pulmonary hypertension. 12 Both COPD and OSA alone are associated with evidence of systemic inflammation, 13,14 which is postulated as an important mechanism in the development of cardiovascular and metabolic disease. However, there have been very few studies that have assessed measures of systemic inflammation or other basic mechanisms that may contribute to cardiometabolic disease in patients with overlap syndrome. TOU CH MED ICA L MEDIA 23