Only 10% of people with obstructive sleep apnea are diagnosed—a dismal statistic, considering the consequences.
Sumita B. Khatri, MD, MS
Co-Director, Asthma Center, Respiratory Institute, Cleveland Clinic; Associate Professor, Cleveland Clinic Lerner College of Medicine of Case Western Reserve University, Cleveland, OH
Octavian C. Ioachimescu, MD, PhD
Medical Director, Sleep Medicine Center, and Chief, Sleep Medicine Section, Atlanta VA Medical Center, Atlanta, GA; Associate Professor of Medicine, Emory University, Atlanta, GA
Address: Sumita B. Khatri, MD MS, Respiratory Institute, A90, Cleveland Clinic, 9500 Euclid Avenue, Cleveland, OH 44195; e-mail: firstname.lastname@example.org; and Octavian C. Ioachimescu, MD, PhD, Atlanta VA Clinic-Sleep Medicine Center, 250 North Arcadia Avenue, Decatur, GA 30033; e-mail: email@example.com
Chronic obstructive pulmonary disease (COPD) and obstructive sleep apnea (OSA) have synergistic detrimental effects. Their comorbid association leads to compromised gas exchange (hypoxia and hypercapnia) and higher rates of morbidity and death. As our understanding of the pathophysiologic processes of sleep evolves, the relationship between OSA and obstructive lung diseases such as COPD (“overlap syndrome”) or asthma (“alternative overlap syndrome”) has become more apparent. The pathophysiology of the combined conditions and optimal management are still being defined, but the effect on quality of life and morbidity underscore the importance of proper diagnosis and appropriately tailored management in these patients.
Many patients who have obstructive lung disease, ie, chronic obstructive pulmonary disease (COPD) or asthma, also have obstructive sleep apnea (OSA), and vice versa.
The combination of COPD and OSA was first described almost 30 years ago by Flenley, who called it “overlap syndrome.”1 At that time, he recommended that a sleep study be considered in all obese patients with COPD who snore and in those who have frequent headaches after starting oxygen therapy. In the latter group, he doubted that nocturnal oxygen was the correct treatment. He also believed that the outcomes in patients with overlap syndrome were worse than those in patients with COPD or OSA alone. These opinions remain largely valid today.
We now also recognize the combination of asthma and OSA (alternative overlap syndrome) and collectively call both combinations obstructive lung disease-obstructive sleep apnea (OLDOSA) syndrome.2 Interestingly, these relationships are likely bidirectional, with one condition aggravating or predisposing to the other.
Knowing that a patient has one of these overlap syndromes, one can initiate continuous positive airway pressure (CPAP) therapy, which can improve clinical outcomes.3–6 Therefore, when evaluating a patient with asthma or COPD, one should consider OSA using a validated questionnaire and, if the findings suggest the diagnosis, polysomnography. Conversely, it is prudent to look for comorbid obstructive lung disease in patients with OSA, as interactions between upper and lower airway dysfunction may lead to distinctly different treatment and outcomes.
Here, we briefly review asthma and COPD, explore shared risk factors for sleep-disordered breathing and obstructive lung diseases, describe potential pathophysiologic mechanisms explaining these associations, and highlight the importance of recognizing and individually treating the overlaps of OSA and COPD or asthma.
COPD AND ASTHMA ARE VERY COMMON
About 10% of the US population have COPD,7 a preventable and treatable disease mainly caused by smoking, and a leading cause of sickness and death worldwide.8,9
About 10% of the US population have COPD, and 8% have asthma
About 8% of Americans have asthma,7 which has become one of the most common chronic conditions in the Western world, affecting about 1 in 7 children and about 1 in 12 adults. The World Health Organization estimates that 235 million people suffer from asthma worldwide, and by 2025 this number is projected to rise to 400 million.10,11
The prevalence of these conditions in a particular population depends on the frequency of risk factors and associated morbidities, including OSA. These factors may allow asthma or COPD to arise earlier or have more severe manifestations.8,12
Asthma and COPD share several features. Both are inflammatory airway conditions triggered or perpetuated by allergens, viral infection, tobacco smoke, products of biomass or fossil fuel combustion, and other substances. In both diseases, airflow is “obstructed” or limited, with a low ratio of forced expiratory volume in 1 second to forced vital capacity (FEV1/FVC). Symptoms can also be similar, with dyspnea, cough, wheezing, and chest tightness being the most frequent complaints. The similarities support the theory proposed by Orie et al13 (the “Dutch hypothesis”) that asthma and COPD may actually be manifestations of the same disease.
But there are also differences. COPD is strongly linked to cigarette smoking and has at least three phenotypes:
Particularly in emphysematous COPD, smoking predisposes patients to gas-exchange abnormalities and low diffusing capacity for carbon monoxide.
In asthma, symptoms may be more episodic, the age of onset is often younger, and atopy is common, especially in allergic asthma. These episodic symptoms may correlate temporally with measurable airflow reversibility (≥ 12% and ≥ 200 mL improvement in FVC or in FEV1 after bronchodilator challenge).
However, the current taxonomy does not unequivocally divide obstructive lung diseases into asthma and COPD, and major features such as airway hyperresponsiveness, airflow reversibility, neutrophilic or CD8 lymphocytic airway inflammation, and lower concentration of nitric oxide in the exhaled air may be present in different phenotypes of both conditions (Table 1).
AIRFLOW IN OBSTRUCTIVE LUNG DISEASES AND DURING SLEEP
Normal airflow involves a complex interplay between airway resistance and elastic recoil of the entire respiratory system, including the airways, the lung parenchyma, and the chest wall (Figure 1).
In asthma and COPD, resistance to airflow is increased, predominantly in the upper airways (nasal passages, pharynx, and larynx) and in the first three or four subdivisions of the tracheobronchial tree. The problem is worse during exhalation, when elastic recoil of the lung parenchyma and chest wall also increases airway resistance, reduces airway caliber, and possibly even constricts the bronchi. This last effect may occur either due to mass loading of the bronchial smooth muscles or to large intrathoracic transmural pressure shifts that may increase extravasation of fluid in the bronchial walls, especially with higher vascular permeability in inflammatory conditions.
Furthermore, interactions between the airway and parenchyma and between the upper and lower airways, as well as radial and axial coupling of these anatomic and functional components, contribute to complex interplay between airway resistance and parenchymal-chest wall elastic energy—stretch or recoil.
The muscles of the upper and lower airway may not work together due to the loss of normal lung parenchyma (as in emphysema) or to the acute inflammation in the small airways and adjacent parenchyma (as in severe asthma exacerbations). This loss of coordination makes the upper airway more collapsible, a feature of OSA.
Additionally, obesity, gastroesophageal reflux, disease chronic rhinitis, nasal polyposis, and acute exacerbations of chronic systemic inflammation all contribute to more complex interactions between obstructive lung diseases and OSA.6
Sleep affects breathing, particularly in patients with respiratory comorbidities, and sleep-disordered breathing causes daytime symptoms and worsens quality of life.1,13–15 During sleep, respiratory centers become less sensitive to oxygen and carbon dioxide; breathing becomes more irregular, especially during rapid eye movement (REM) sleep; the chest wall moves less, so that the tidal volume and functional residual capacity are lower; sighs, yawns, and deep breaths become limited; and serum carbon dioxide concentration may rise.
OBSTRUCTIVE SLEEP APNEA
The prevalence of OSA, a form of sleep-disordered breathing characterized by limitation of inspiratory and (to a lesser degree) expiratory flow, has increased significantly in recent years, in parallel with the prevalence of its major risk factor, obesity.
OSA is generally defined as an apnea-hypopnea index of 5 or higher, ie, five or more episodes of apnea or hypopnea per hour.
OSA syndrome, ie, an apnea-hypopnea index of 5 or higher and excessive daytime sleepiness (defined by an Epworth Sleepiness Scale score > 10) was found in the initial analysis of the Wisconsin Sleep cohort in 1993 to be present in about 2% of women and 4% of men.16 A more recent longitudinal analysis showed a significant increase—for example, in people 50 to 70 years old the prevalence was up to 17.6% in men and 7.5% in women.17
Upper airway resistance syndrome, a milder form of sleep-disordered breathing, is now included under the diagnosis of OSA, as its pathophysiology is not significantly different.18
In the next section, we discuss what happens when OSA overlaps with COPD (overlap syndrome) and with asthma (“alternative overlap syndrome”)2,8 (Figure 2).
OSA AND COPD (OVERLAP SYNDROME)
Flenley1 hypothesized that patients with COPD in whom supplemental oxygen worsened hypercapnia may also have OSA and called this association overlap syndrome.
Since both COPD and OSA are prevalent conditions, overlap syndrome may also be common.
The reported prevalence of overlap syndrome varies widely, depending on the population studied and the methods used. In various studies, COPD was present in 9% to 56% of patients with OSA,19–23 and OSA was found in 5% to 85% of patients with COPD.24–27
Based on the prevalence of COPD in the general population (about 10%12) and that of sleep-disordered breathing (about 5% to 10%17), the expected prevalence of overlap syndrome in people over age 40 may be 0.5% to 1%.28 In a more inclusive estimate with “subclinical” forms of overlap syndrome—ie, OSA defined as an apnea-hypopnea index of 5 or more (about 25% of the population17) and COPD Global initiative for Chronic Obstructive Lung Disease (GOLD) stage 1 (16.8% in the National Health and Nutrition Education Survey12)—the expected prevalence of overlap is around 4%. Some studies found a higher prevalence of COPD in OSA patients than in the general population,21,29 while others did not.22,28,30 The studies differed in how they defined sleep-disordered breathing.
Larger studies are needed to better assess the true prevalence of sleep-disordered breathing in COPD. They should use more sensitive measures of airflow and standardized definitions of sleep-disordered breathing and should include patients with more severe COPD.
At near-maximal ventilatory capacity, even a mild increase in upper airway resistance increases the work of breathing
Fatigue is strongly correlated with declining lung function, low exercise tolerance, and impaired quality of life in COPD.31 Factors that contribute to fatigue include dyspnea, depression, and impaired sleep.32 Some suggest that at least half of COPD patients have sleep complaints such as insomnia, sleep disruption, or sleep fragmentation.33 Insomnia, difficulty falling asleep, and early morning awakenings are the most common complaints (30%–70% of patients) and are associated with daytime fatigue.34 Conversely, comorbid OSA can contribute to fatigue and maintenance-type insomnia (ie, difficulty staying asleep and returning to sleep).
Oxygenation abnormalities and increased work of breathing contribute to the pathophysiology of overlap syndrome. In patients with COPD, oxygenation during wakefulness is a strong predictor of gas exchange during sleep.35 Further, patients with overlap syndrome tend to have more severe hypoxia during sleep than patients with isolated COPD or OSA at rest or during exercise.36
In overlap syndrome, hypoxemia is the result of several mechanisms:
In a person who is at near-maximal ventilatory capacity, even a mild increase in upper airway resistance (as seen with snoring, upper airway resistance syndrome, or OSA) increases the work of breathing. This phenomenon can lead to early arousals even before significant oxyhemoglobin desaturation occurs.
Normally, inspiratory flow limitation is counteracted by increasing inspiratory time to maintain ventilation. Patients with COPD may not be able to do this, however, as they need more time to breathe out due to narrowing of their lower airways.40 The inability to compensate for upper airway resistance, similar to the increased work of breathing seen with exercise, may lead to early arousals and increased sleep fragmentation.
Only 10% of people with obstructive sleep apnea are diagnosed—a dismal statistic, considering the consequences.
Obstructive sleep apnea is common, poorly recognized, and, if untreated, leads to serious health consequences.
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