Chronic obstructive pulmonary disease (COPD) and obstructive sleep apnea (OSA) each affect at least 10% of the general adult population and, thus, both disorders together, commonly referred to as the overlap syndrome, could be expected in at least 1% of adults by chance alone. However, there is evidence of important interactions between the disorders that influence the prevalence of the overlap, which have implications for the development of comorbidities,and also for management (McNicholas WT. Chest. 2017; 152:1318). Furthermore, sleep quality is typically poor in COPD, which has been linked to worse pulmonary function and lung hyperinflation and may contribute to daytime fatigue.
Interactions between COPD and OSA that may influence the prevalence of overlap
Previous reports have presented conflicting results regarding the likely association between COPD and OSA, which may partly reflect different definitions of OSA, patient populations, and methodologies of investigation. However, COPD represents a spectrum of clinical phenotypes ranging from the hyperinflated patient with low BMI (predominant emphysema phenotype) to the patient with higher BMI and tendency to right-sided heart failure (predominant chronic bronchitis phenotype). The predominant emphysema phenotype may predispose to a lower likelihood of OSA, and there is recent evidence that lung hyperinflation is protective against the development of OSA by lowering the critical closing pressure of the upper airway during sleep. Furthermore, the degree of emphysema and gas trapping on CT scan of the thorax correlates inversely with apnea-hypopnia index in patients with severe COPD (Krachman SL et al. Ann Am Thorac Soc. 2016;13:1129).
While there is evidence of increased mortality in patients with COPD and OSA alone, a recent report based on the Sleep Heart Health Study somewhat surprisingly found that the incremental contribution of declining lung function to mortality diminished with increasing severity of SDB measured by AHI (Putcha N et al. Am J Respir Crit Care Med. 2016;194:1007). Thus, the epidemiologic relationship of COPD and OSA and related clinical outcomes remains an important research topic comparing different clinical phenotypes.
Mechanisms of interaction in the overlap syndrome and implications for comorbidity
COPD and OSA are associated with several overlapping physiological and biological disturbances, including hypoxia and inflammation, which may contribute to cardiovascular and other comorbidities. Thus, the probability should be high that the overlap syndrome will be associated with a greater risk of comorbidity than with either disease alone. Patients with the overlap syndrome demonstrate greater degrees of oxygen desaturation predisposing to pulmonary hypertension, which is especially common in these patients.
COPD and OSA are each associated with systemic inflammation and oxidative stress, and C-reactive protein (CRP) has been identified as a measure of systemic inflammation that is commonly elevated in both disorders, although in OSA, concurrent obesity is an important confounding factor. Systemic inflammation contributes to the development of cardiovascular disease, which is a common complication of both COPD and OSA. Thus, one could expect that cardiovascular disease is particularly prevalent in patients with overlap syndrome, but there are limited data on this relationship, which represents an important research topic.
Patients with the overlap syndrome present with typical clinical features of each disorder and additional features that reflect the higher prevalence of hypoxemia, hypercapnia, and pulmonary hypertension. Thus, morning headaches reflecting hypercapnia and peripheral edema reflecting right-sided heart failure may be especially common. Screening questionnaires may be helpful in the initial evaluation of likely OSA in patients with COPD, and objective clinical data, including anthropometrics such as age, sex, and BMI, and medical history such as cardiovascular comorbidity, are especially useful in clinical prediction (McNicholas WT. Lancet Respir Med. 2016;4:683). Thus, screening for OSA in patients with COPD should not be complicated, and the widespread failure to do so may reflect a lack of awareness of the possible association by the clinician involved.