Recognizing the impact of obstructive sleep apnea in patients with asthma

Obstructive sleep apnea (OSA) and asthma are both common medical problems.^1,2 It is estimated that up to 40 million persons in the United States have OSA, while 17 million have asthma.^1-4 Moreover, OSA and asthma may coexist,⁵ further complicating airway inflammatory pathology and the management of airway disease.

In this article, we will discuss the association between OSA and asthma and the role of certain comorbidities, such as obesity, gastroesophageal reflux disease (GERD), nasal obstruction, and rhinosinusitis, that may complicate either or both diseases.

SLEEP APNEA

Central sleep apnea is characterized by intermittent apnea or hypopnea that is secondary to cessation of respiratory effort. OSA is characterized by periodic or complete upper airway obstruction during sleep, resulting in intermittent cessation of breathing that may be complete (apnea) or partial (hypopnea) despite continued respiratory effort (Table 1).^2-4 The severity of a patient's OSA is determined by the number of apneic and/or hypopneic events occurring per hour, and OSA can be classified as mild, moderate, or severe.

Both forms of sleep apnea may coexist in a given patient. It has been estimated that sleep-disordered breathing may occur in as many as 24% of middle-aged men and 9% of middle-aged women, based on an apnea-hypopnea index (AHI) of more than 5 per hour.³ Up to 4% of men and 2% of women may have the full-blown OSA syndrome, including daytime hypersomnolence and apnea. OSA is not rare in children; the incidence may be as high as 1% to 3%.⁶ Despite the severe morbidity and mortality associated with OSA, it remains undiagnosed in most adults with this syndrome.⁶

The major risk factors for OSA include obesity, male sex, and a positive family history for the disease. A body mass index (BMI) of more than 30 kg/m² is a significant risk factor.

THE OSA-ASTHMA LINK

Some of the risk factors for OSA can also contribute to asthma (Table 2). In fact, several of these risk factors may coexist in a patient who has both OSA and asthma.

Snoring and apnea are common in patients with asthma and allergic rhinitis.⁷ In some instances, upper airway resistance syndrome and OSA may be mistaken for nocturnal asthma.⁸ Sleep deprivation, upper airway edema, and systemic inflammation associated with OSA can complicate the course of asthma.⁵

Sleep and nocturnal asthma

Many physiologic changes associated with sleep can promote nocturnal worsening of asthma. For example, sleep is routinely associated with an increase in parasympathetic tone relative to sympathetic activity, which could promote increased bronchomotor tone and bronchoconstriction.

Sleep has also been associated with reductions in lung volume in patients with asthma, who typically have hyperinflation during wakefulness.⁹ This reduction in lung volume clearly contributes to the increased lower airway resistance that has been measured during sleep. Fortunately, there is evidence that ventilatory responsiveness to induced bronchoconstriction is adequately maintained during sleep, even if there is excessive sleepiness after prolonged periods of sleep deprivation.¹⁰

Finally, there has been substantial interest in the potential association between OSA and asthma. Proposed mechanisms of contribution include a neurogenically mediated increase in bronchomotor tone resulting from snoring-associated vibration of the upper airway,¹¹ OSA-associated increase in systemic inflammation, and OSA-associated increase in pulmonary blood volume.¹²

Obesity

As noted above, obesity is a risk factor for OSA. Moreover, an increased incidence of obesity has been observed in patients with asthma.^13-15

Reflux

GERD is common in patients who have OSA, obesity, or asthma. It may occur in persons who have sleep-disordered breathing resulting from apnea or in association with obesity.¹⁶ GERD can also ex-acerbate asthma and airway inflammation.¹⁷ It can cause a va- riety of aerodigestive complications, including cough and laryngitis.^18,19 Reflux of gastric acid has been shown to worsen bronchial hyperresponsiveness.^16,18,19

Upper airway disease

Allergic and structural airway disease, such as allergic rhinitis, tonsillar enlargement, and micrognathia, may be contributory factors in some patients with OSA and/or asthma. Nasal obstruction can occur in patients with allergic asthma, vasomotor rhinitis, nonallergic rhinitis eosinophilic syndrome, nasal polyposis, or structural disease (deviated nasal septum, concha bullosa). Nasal obstruction of any cause can augment airway collapse and worsen OSA.^20-22

The CORE syndrome

The coexistence of rhinitis, nasal polyposis, obesity, and GERD may contribute to the increasing frequency of the diagnosis of concurrent asthma and OSA. The acronym "CORE" describes the interactive disease states of OSA and asthma: Cough/asthma, Obesity/ OSA, Rhinosinusitis, and Esophageal reflux.²

OSA and other CORE components need to be considered in the evaluation of patients who have asthma that is refractory to therapy. The patient described below demonstrates a classic case of the CORE syndrome.

A 67-year-old man was evaluated for severe chronic asthma, rhinosinusitis, and hypertension. He complained of persistent wheezing, dyspnea, coughing, fatigue, daytime somnolence, headaches, and poorly controlled GERD. He had a 30-pack-year history of smoking but had quit smoking 4 years ago. His current medications included salmeterol, ipratropium, inhaled triamcinolone, montelukast, and oral prednisone (10 mg/d). He used nebulized ß-agonists almost every day.

Examination revealed an obese man who weighed 275 lb and had a BMI of 39.5 kg/m². He had a short, thick neck; bilateral expiratory wheezing; and peripheral edema. Pulmonary function tests revealed a forced expiratory volume in 1 second (FEV₁) of 2.09 L (69% of predicted) and a ratio of FEV₁ to forced vital capacity of 71%. Allergy skin test results were negative, and his total IgE level was normal.

Polysomnography (PSG) recorded 69 obstructive and 8 hypopneic events, and the patient's lowest oxygen saturation was 83% (Figure 1). Continuous positive airway pressure (CPAP) therapy reversed the abnormalities and improved the patient's nocturnal asthma and daytime wheezing.

Barium examination of the patient's upper GI tract indicated GERD and tertiary contractions (Figure 2). He was advised to lose weight, avoid sedatives and alcohol, and follow antireflux measures. His GERD and asthma were managed aggressively, and his symptoms improved.

Other factors

Several endocrinopathies may play a role in the OSA-asthma relationship. Specifically, hypothyroidism needs to be evaluated and treated in OSA patients. C-reactive protein and inflammatory factors, such as cytokines and systemic inflammatory responses, elicited during apnea are also important to airway inflammation and remodeling (see "The role of inflammation in the relationship between asthma and obstructive sleep apnea").^1,2,14,23-26

DIAGNOSIS AND MANAGEMENT

Components of the diagnosis and management of OSA, asthma, and potential comorbidities are shown in Table 3.

Obesity

As noted above, obesity is a predisposing factor for OSA, and the incidence of asthma is increased in obese persons.^13-15 The measurement of body fat and calculation of BMI assist in classification and diagnosis. The management of obesity involves counseling on diet and weight loss and, in some patients, pharmacologic therapy. Weight loss can reduce the severity of OSA.

Weight reduction has been widely recommended for patients with OSA, and at least 2 prospective controlled trials have indicated that losing weight by following a low-calorie diet can achieve significant improvement in sleep-disordered breathing.^27,28 Therefore, this would probably be helpful for obese patients who have OSA with concurrent asthma.

OSA

Because poorly controlled OSA may exacerbate asthma, the possibility of OSA must be considered in a patient with poorly controlled asthma. Signs of poorly controlled asthma include moderate to severe airway obstruction, persistent daytime and exercise-induced symptoms, and excessive use of ß₂-agonists. In evaluating a patient who has poorly controlled asthma, look for clues such as excessive daytime sleepiness, morning fatigue, excessive snoring, nocturnal reflux, and peripheral edema

The history and physical examination are essential components of the evaluation of OSA. Loud guttural snoring that is worse in the supine position and that is punctuated by choking sounds, followed by cessation of breathing, is virtually pathognomonic of OSA. The snoring and apneic episodes may be worse after using alcohol or sleeping pills, because these substances decrease pharyngeal muscle tone.^29,30 A collateral history should be obtained from the patient's bed partner.

Daytime fatigue and sleepiness are the most significant complaints of patients with OSA. Frequently, the patient falls asleep during sedentary activities, such as watching television. A higher incidence of automobile accidents has been described in patients with OSA. Other symptoms include nonrestorative sleep (or fatigue), early-morning headaches, neurocognitive dysfunction, nocturia, nocturnal gastroesophageal acid reflux and heartburn, and nocturnal diaphoresis. Diminished libido or impotence is not uncommon, even in younger persons.

The physical examination focuses on craniofacial and soft tissue conditions that are associated with increased upper airway resistance, such as retrognathia, deviated nasal septum, low-lying soft palate, enlarged uvula and base of the tongue, nasal polyps, and hypertrophy of the turbinate.^31,32 An increased BMI, hypertension, and a greater neck circumference (16 inches in women and 17 inches in men) are associated with an increased risk of OSA. Increased upper body obesity, which is reflected by the neck circumference, is a particularly good predictor.

Some of the most severe cases of OSA have been seen in thin, young patients who have small jaws (micrognathia) with or without overbite or underbite. Another clue, which has not received general attention, is the tendency of a patient who has a small pharyngeal space to snort when laughing; this may represent the waking equivalent of apnea.

Despite the importance of the history and physical examination, their combined predictive value is only about 50%.⁶ The most valuable test for confirming OSA has been the overnight sleep study, or PSG.³³ Standard PSG is a laboratory-based, technician-attended multimodality recording of sleep architecture by electroencephalography, electro-oculography, and electromyography; respiratory activity by nasal and oral airflow or pressure, thoraco-abdominal inductance plethysmography, and oximetry; electrocardiography; and limb movements by lower extremity electromyography. Rarely, esophageal pressure monitoring may be required if respiratory effort-related arousals or central sleep apnea is suspected.

The principal factor to note is the AHI. OSA is defined either by an AHI of 5 or more plus evidence of nonrestorative sleep, daytime sleepiness, or other daytime symptoms, or by an AHI of 15 or higher. An American Academy of Sleep Medicine expert panel recommended that mild OSA be defined by an AHI of 5 to 14, moderate by an AHI of 15 to 30, and severe by an AHI of more than 30.³⁴

Screening studies using only a few channels (breathing effort, airflow, and oxygen saturation) are often performed at home without monitoring by technicians and may be helpful for follow-up. However, these studies are not a substitute for an in-laboratory, all-night diagnostic sleep study, and they often add to cost, since they frequently lead to further evaluation in a more formal setting.

The American Sleep Disorders Association standards of practice recommends that portable studies be reserved for patients with severe symptoms when standard PSG is not readily available and for those who are unable to be studied in the laboratory. Pulse oximetry is not considered a sufficient alternative to PSG; its value may lie at the extremes of the OSA spectrum.³⁵ If clinical suspicion is high, pulse oximetry may help determine the timing of PSG when entry to a sleep center is delayed. If clinical suspicion is low, normal results effectively exclude OSA.

Management of OSA is multidisciplinary. Conservative treatment may be an option for patients with mild OSA and may involve education on weight loss, avoidance of alcohol and tranquilizers, and sleeping in the lateral position.^2-4 Many types of oral appliances have been designed for patients who have upper airway obstruction. In some patients, such mandibular advancement therapy may be effective, but it is inferior to nasal CPAP in patients who have moderate or advanced OSA.

The use of CPAP in patients with moderate-severe OSA is accompanied by improvements in daytime somnolence, quality of life, mood, and alertness.^2-4 Studies have also shown that treatment of OSA with CPAP leads to improvements in peak flow rates in patients with concomitant asthma.⁵

The actual effects of treating OSA on the outcome of asthma have been poorly studied. A study by Guilleminault and associates³⁶ demonstrated no beneficial effect of nasal CPAP on daytime asthma. In a small study by Martin and Pak,³⁷ 2 of 7 patients with poorly controlled asthma had improved oxygen saturations and FEV₁ after nasal CPAP therapy.

One problem with the use of CPAP is compliance, partly because of the associated nasal obstruction in patients who are already suffering from rhinitis and asthma. In some patients, "self-adjusting machines" may be used. In patients who are intolerant of CPAP, surgical therapies may include uvulopalatopharyngoplasty, laser-assisted uvulopalatoplasty, tongue ablation, mandibular reconstruction, and tracheostomy. These surgical procedures have a high complication rate, and their effectiveness is unclear.

Upper airway obstruction

Appropriate evaluation of nasal obstruction includes allergy testing, CT of the paranasal sinuses, and fiberoptic rhinoscopy. Aggressive treatment may include intranasal corticosteroids, leukotriene modifiers, antihistamine-decongestants, and allergy immunotherapy.

GERD

Patients with GERD may complain of burning chest pain, regurgitation, water brash, and odynophagia. In some patients, GERD may be asymptomatic.

The evaluation may include barium study, upper endoscopy or, in difficult cases (especially in patients with nonerosive reflux), a 24-hour esophageal pH study. In select patients, therapy with a proton pump inhibitor may reduce the symptoms of GERD, thus supporting the diagnosis.

Elevation of the head of the bed; avoidance of nicotine, caffeine, and other substances that may exacerbate reflux; weight loss; surgery (fundoplication); and the use of antireflux drugs, such as proton pump inhibitors, can improve control of GERD, and of asthma as well.

Airway inflammation

Pulmonary function tests, such as spirometry, are essential to the diagnosis and classification of asthma. If the patient has persistent symptoms, such as cough, shortness of breath, and mucus production, but has essentially normal spirometric results, methacholine challenge testing may be considered to establish the diagnosis of asthma.

Because inflammation is a feature of both OSA and asthma, besides monitoring pulmonary function tests, measurement of exhaled nitric oxide and sputum levels of mediators and eosinophils may be beneficial when such techniques are available. Therapy with inhaled corticosteroids, long-acting ß₂-agonists, or leukotriene modifiers may be required.¹ A newer treatment option for patients with allergic asthma is omalizumab, a recombinant, humanized monoclonal antibody to IgE.¹

Cardiovascular disease and other complications

Neuropsychiatric complications of OSA include daytime somnolence, cognitive dysfunction, and depression. Cardiovascular disease is common in patients with OSA and can result from poorly treated OSA.

OSA can be complicated by hypertension, stroke, congestive heart failure, coronary or cerebrovascular atherosclerosis, cardiac arrhythmia, myocardial infarction, unstable coronary syndrome or angina pectoris, and ischemic ventricular dysfunction.^6,38-43 Left ventricular hypertrophy and diastolic dysfunction have also been described in OSA.^44,45

In addition, cardiac dysfunction can present as asthma or worsen underlying asthma. These events may lead to worsening hypoxemia and may contribute to progressive cognitive decline, leading to poor compliance with medications. Frequently asking about medication use, monitoring compliance, and asking the patient to bring all of his or her medications to the office visit will help address the problem.

Elderly patients with OSA who have wheezing or dyspnea may need to be evaluated for asthma as well as for systolic or diastolic cardiac dysfunction. Antihypertensive medication, dietary salt restriction, and the management of any underlying cardiac disease are important. When hypertension complicates OSA in a patient with asthma, the choice of antihypertensive agent must be made carefully. For example, ß-adrenergic antagonists may worsen bronchospasm, and angiotensin-converting enzyme inhibitors may cause cough.

References:

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