Obstructive sleep apnea syndrome (OSA) is characterized by airway narrowing during sleep that leads to nighttime sleep disruption and associated daytime symptoms. The recognition that OSA is a common and treatable disorder with serious complications has been the most important factor in the growth of sleep medicine over the past 20 years.
Pathogenesis and Pathophysiology. Narrowing and occlusion of the upper airway during sleep is the basis for obstructive sleep apnea. Obstruction usually occurs in the pharynx, which lacks supporting cartilage and bone, and is therefore more collapsible than other portions of the upper airway. Upper airway size is determined by the anatomy of the bones and soft tissues of the neck and face, by the pressures acting on the pharyngeal tissues, and by their compliance. During wakefulness, contraction of pharyngeal muscles occurs just before the onset of inspiration and increases the outward pressure on the pharyngeal wall by pulling the soft palate, tongue, mandible, and hyoid bone forward. This outward pressure balances the inward pressure produced by negative pressure developed within the thorax and helps maintain airway patency.y Activity of upper airway dilator muscle activity is affected by arterial blood gas concentration; by proprioceptive input to respiratory neurons from the jaw, mouth, and thorax; and by receptors sensitive to changes in upper airway transmural pressure. For example, increased negative upper airway pressure leads to increased activity of the genioglossus, which pulls the tongue forward and tends to open the airway.
Pharyngeal muscles relax during sleep, which leads to narrowing of the pharyngeal airway and increased resistance to airflow. The relaxation alone may be sufficient to close the airway in some persons. In others, inspiratory muscle activity increases to compensate for the increased airway resistance, and the increased negative pressure generated by the extra effort leads to airway collapse. Ultimately, the airway opens only after an arousal or brief awakening leads to increased activity of pharyngeal dilators. The patient then takes a few deep breaths and returns to sleep, whereupon the cycle repeats itself. Pharyngeal narrowing without complete occlusion may cause decreased ventilation. These episodes, referred to as hypopneas, lead to arousals, sleep fragmentation, daytime sleepiness, and hypoxemia, and thus have the same functional effect as apneas.
Pharyngeal narrowing with high resistance to airflow is sometimes associated with compensatory increased respiratory effort that minimizes changes in tidal volume, ventilation, and oxygenation but still leads to arousals and sleep fragmentation. The functional effect is similar to OSAs but without the associated hypoxemia. When such episodes are the predominant type of respiratory event, the disorder is referred to as the upper airway resistance syndrome. y The body weight of patients with this disorder is often normal or near-normal, and there appears to be a higher proportion of women and children with the syndrome than occurs with OSA, perhaps because their upper airways tend to be less compliant.
Airway narrowing may be due to a variety of causes (.Table.54-7 ), including craniofacial malformations (high arched palate, a long low-placed soft palate, dental malocclusion, or retrognathia), neurological diseases with incoordination of preinspiratory activation of upper airway dilators, mouth breathing with posterior displacement of the mandible, or weakness from neuromuscular diseases. The site of airway occlusion may be at the level of the velopharynx, the oropharynx, the hypopharynx, the epiglottis, or the larynx. In some patients, the occlusion may be at more than one level or at different sites during different stages
_TABLE 54-7 -- CAUSES OF UPPER AIRWAY NARROWING_
Genetic and developmental factors that influence craniofacial development
Anatomical malformations that affect craniofacial structures such as Crouzon's syndrome, Treacher Collins syndrome, Pierre Robin syndrome, Arnold-Chiari malformation achondroplasia, rheumatoid arthritis, and Klippel-Feil syndrome
Infiltration of pharyngeal tissue associated with obesity mucopolysaccharidoses, and mucosal edema and inflammation Structural lesions such as enlarged tonsils and adenoids and pharyngeal tumors
Dyscoordinated breathing from abnormal respiratory control associated with Arnold-Chiari malformation, dysautonomias, and degenerative CNS disorders Abnormally compliant airway associated with Marfan's syndrome CNS, Central nervous system.
of sleep owing to differential activity of muscles involved in maintenance of airway patency.
The reason for the more frequent occurrence of OSA in men than in women is not known. Men tend to have longer narrower upper airways, which may predispose them to airway collapse. The airway may be more compliant in men and may close at lower pressures for a given airway diameter. Women also show a greater augmentation of genioglossus activity in response to inspiratory loading, which may indicate a less collapsible airway. Testosterone may lead to increased bulk of neck muscles and may lead to a preferential deposition of fat in upper airway structures.
In susceptible persons, obstructive sleep apnea may be precipitated or made worse by sleep deprivation, evening alcohol use, sedative medications, and the supine sleeping position. Even small doses of alcohol enhance upper airway muscle relaxation that occurs during sleep, thereby exacerbating OSA. Alcohol also blunts chemoreceptor responses to blood gases and raises the arousal threshold, which increases the duration of apneas.
Arterial oxyhemoglobin saturation (SaO2 ) falls during apneas and hypopneas at a rate of 0.1 to 1.6 percent per second. The severity of hypoxemia is a function of baseline oxygenation, lung oxygen stores, the degree of airway narrowing, and apnea duration. Drops in SaO 2 to below 70 percent are common in severe obstructive sleep apnea. Apneas during REM sleep tend to produce greater decreases in SaO 2 owing to decreased lung oxygen reserves and increased duration of apneas.
Arteriolar constriction as a result of hypoxemia, acidosis, and increased sympathetic tone contribute to increases in blood pressure during apneas that may reach 190 mm Hg systolic and 110 mm Hg diastolic. Right-to-left shifts of the cardiac interventricular septum and decreased cardiac output may occur as a result of increased negative intrathoracic pressure associated with attempts to breathe. Increased vagal tone associated with fluctuations in intrathoracic pressure, stimulation of the carotid body by hypoxemia, and increased sympathetic tone that accompanies the arousals at the end of apneas contribute to bradycardia, periods of asystole, atrioventricular block, premature ventricular contractions, and ventricular tachyarrhythmias.
The repeated arousals to resume breathing, along with reduced amounts of REM sleep and slow-wave sleep, appear to be the major causes of daytime sleepiness in patients with OSA.
Epidemiology and Risk Factors. Habitual snoring, which is more common in males than in females, occurs in 6 to 12 percent of children and 5 to 20 percent of adults. OSA has a prevalence of about 1 percent in children, 2 percent in adult women, and 4 percent in adult men. 17' , '18
Clinical Features and Associated Disorders. The usual presenting symptoms are snoring, excessive sleepiness, daytime sleep episodes, and a sense that sleep is not restful. Morning headaches, frequent nocturnal urination, and nocturnal restlessness are also common. Some patients have no complaints and present only because the bed partner has noted periods of apnea. Others are referred because of a suspicion that OSA contributes to headaches, pulmonary or systemic hypertension, cardiac arrhythmias, or impotence.
Loud snoring, sometimes exceeding 90 dB, occurs in
more than 80 percent of patients and usually begins years before the onset of sleep apnea. Some patients report that they cannot go on camping trips and must have separate hotel rooms on business trips because of their obnoxious snoring. Increasing volume of snoring, snoring that has changed in character, and loud rhythmical snoring punctuated by snorts and choking noises are suggestive of obstructive sleep apnea. Snoring is produced by vibration of the soft tissues of the upper airway. The intensity and quality vary depending on the stage of sleep, the position of the body, the rate of airflow, the anatomical structure of the individual's nose and throat, and the amount of floppy airway tissue with potential to vibrate. Although snoring is common in patients with OSA, it is not universal. Snoring will not occur even with a very narrow pharynx if floppy tissue is absent.
Restless sleep is caused by the arousals at the end of apneas, which may be accompanied by jerks, twitches, and gross body movements. Changes in body position may occur several times per hour as patients attempt to find a sleeping position compatible with airway patency. With severe apnea, patients may sleep in a chair, on the couch, sitting up on the side of the bed, or leaning against a wall. Despite the restlessness, patients usually have the impression that sleep has been continuous or interrupted only by a few trips to the bathroom. A few patients describe repeated awakenings and complain mainly of insomnia.
Tiredness, sleepiness, and a sense that sleep is not refreshing are characteristic symptoms of sleep apnea, often accompanied by impaired memory and concentration. The sleepiness often increases gradually over several years as patients gain weight. Some patients feel mentally dull or disoriented each morning. Daytime sleepiness is usually most apparent after lunch, while driving or reading, or in other boring sedentary situations. Patients often minimize difficulties with sleepiness and describe it as no worse than in colleagues or peers. With more severe apnea, patients may fall asleep on the telephone or during sexual intercourse and may have episodes of "automatic behavior" during which complex activities are performed such as driving for miles with no recollection. Although daytime sleepiness is common, its absence does not rule out OSA.
Other common symptoms include irritability, impotence or reduced libido, and dry mouth. Symptoms of gastroesophageal reflux may be present if the repeated episodes of negative intrathoracic pressure associated with apneas lead to passage of gastric contents through the lower esophageal sphincter.
In children, snoring and restless sleep are common complaints, along with poor attention, decreased school performance, enuresis, and hyperactivity. Daytime sleepiness is often less pronounced than in adults unless apnea is severe.
On physical examination, patients are often obese and have thick necks. Long narrow facies, high arched palate, retrognathia, an enlarged edematous uvula, prominent tonsils, and redundant pharyngeal tissue are other common findings. Patients with severe obstructive sleep apnea may have findings consistent with right-sided heart failure.
OSA occurs in 10 to 40 percent of obese persons and in at least 25 percent of patients with hypertension. In untreated severe OSA, the prevalence of hypertension may be as high as 50 percent. A number of other disorders contribute to the development of OSA (see Table,.5.4,-7. ).
Central sleep apnea, which is much less common than obstructive sleep apnea, is a disorder in which apneas occur but there is no associated attempt to breathe. The disorder may be idiopathic, or it may be caused by congestive heart failure or brain stem pathology. Many patients have both central sleep apnea and obstructive sleep apnea, presumably because activation of upper airway afferents during airway occlusion leads to inhibition of attempts to breathe.
Differential Diagnosis. Loud snoring with gasping or choking sounds, witnessed apneas, restless sleep, hypertension, and neck circumference greater than 16.5 inches are useful predictors of OSA, but a definitive diagnosis usually requires polysomnography. Among children, snoring or noisy breathing, daytime mouth breathing, and parental observation of apneas and struggles to breathe are suggestive symptoms. The absence of snoring and obesity does not rule out OSA in either adults or children. In patients with excessive daytime sleepiness, other diagnostic considerations include narcolepsy, idiopathic hypersomnia, periodic limb movement disorder (PLM), central sleep apnea, and the insufficient sleep syndrome. The differential diagnosis of nocturnal choking and gasping includes gastroesophageal reflux, nocturnal asthma, congestive heart failure, nocturnal panic attacks, and sleep-related laryngospasm.
Evaluation. A nocturnal polysomnogram with monitoring of sleep stage, airflow, respiratory effort, electrocardiogram, leg movements, and body position is the most commonly used tool for diagnosis. If upper airway resistance syndrome is suspected, intraesophageal pressure measurements should be obtained as part of the polysomnogram. The polysomnogram helps determine the presence and type of apnea, assess the relation of breathing disturbance to sleep stage and body position, determine the severity of hypoxemia and sleep disturbance, and identify cardiac arrhythmias. Apneas and hypopneas usually last 10 to 30 seconds and typically are followed by transient arousals. During REM sleep, apneas and hypopneas are usually longer, sometimes as long as 2 to 3 minutes, and are more likely to be associated with hypoxemia and cardiac arrhythmias. There are usually reduced amounts of stages 3 and 4 sleep and REM sleep. With mild OSA, apneas may occur only after alcohol ingestion or only when the patient is supine. With severe OSA, apneas occur continuously throughout the night and in all body positions.
In patients with high pretest probability of moderate or severe OSA, a home-monitoring study may be sufficient for diagnosis. A variety of home-monitoring devices are available; the simplest types measure only a single variable, such as oxygen saturation, whereas more complex devices may record as many channels as a typical laboratory polysomnogram. Home-monitoring studies cost less than laboratory studies, but they do not allow for visual observation, correction of artifacts or faulty sensors, or therapeutic interventions. It is also more difficult to assess mild sleep apnea. Nonetheless, it is likely that these devices will assume an increasing role for diagnosis due to the reduced cost.
Additional studies may be indicated in some patients. An MSLT helps assess the presence and severity of excessive
sleepiness. Although fiberoptic endoscopy of the upper airway, cephalometric radiographs, and magnetic resonance imaging can be used to assess airway anatomy, such studies during wakefulness are of limited value in determining the site of obstruction because airway dynamics differ in sleep and wakefulness. Arterial blood gases and pulmonary function tests are indicated if obesity-hypoventilation syndrome or other causes of hypoventilation are suspected but they are not needed for routine evaluation of suspected OSA.
Some patients described as loud snorers do not snore during sleep studies. In such cases, snoring may be intermittent and related to alcohol use, body position, or allergies. Loud snoring that cannot be verified with objective assessment is sometimes an indication of marital problems.
Management. The goal of treatment is to keep the airway open during sleep, which should lead to improved sleep, better oxygenation, and enhanced daytime alertness. Treatment for OSA depends on the severity of the disorder. There is little doubt that treatment is needed for severe OSA because it is associated with increased risks of motor vehicle accidents, stroke, myocardial infarction, cardiovascular death, and death while sleeping, if left untreated or treated ineffectively. '191 y '211 '221 '231 '211 For patients with a mild case of the disorder--those with fewer than 20 apneas plus hypopneas per hour of sleep, little or no nocturnal hypoxemia, mild or absent daytime sleepiness, and no serious cardiac arrhythmias--the health benefits of treatment are less certain, and it is unknown at present whether asymptomatic patients with mild sleep apnea need to be treated.
The introduction in the 1980s of continuous positive airway pressure (CPAP) administered via a nasal mask revolutionized the management of OSA. '251 Nasal CPAP functions as an air splint to maintain positive intraluminal pressure in the upper airway. Although nasal CPAP can be used by children as well as by adults and is effective in 80 to 90 percent of patients, the proportion who use CPAP on a regular basis is substantially less. If nasal obstruction prevents the use of nasal CPAP, treatment with decongestants, steroid inhalers, septoplasty, or other forms of nasal surgery may be required. Other factors that may prevent the use of CPAP or reduce its benefit include a poorly fitting or uncomfortable mask, sinus infections, claustrophobia, chronic mouth breathing, incomplete efficacy, and lack of motivation. The more expensive bilevel positive airway pressure (BPAP) devices provide different pressures during inspiration and expiration, and are sometimes better tolerated, particularly by patients with claustrophobia. Other problems associated with nasal CPAP include noise from the machine, air swallowing, allergic rhinitis, and discomfort from misdirection of airflow into the eyes. Barotrauma to the lungs is extremely rare.
For patients who cannot tolerate CPAP or prefer not to use it, a number of other treatments may be used. Removal of enlarged tonsils and adenoids is often successful in children and sometimes in adults. Uvulopalatopharyngoplasty (UPP) with removal of the uvula, portions of the soft palate, and redundant pharyngeal tissue eliminates snoring in more than 80 percent and produces improvement of OSA in about 50 percent of patients, but complete resolution of sleep apnea is uncommon. Laser-assisted UPP is a staged outpatient procedure that eliminates snoring in about 60 percent of patients but is probably less effective for OSA than standard UPP. Maxillofacial surgery, with advancement of the mandible, the maxilla, or both, appears to be beneficial for selected patients. Orthodontic appliances that advance the tongue or mandible can improve airway patency during sleep, reduce snoring, and decrease the frequency of respiratory events during sleep, but they rarely eliminate sleep apnea completely. Weight reduction is often helpful, but it may be difficult to achieve. Protriptyline at doses of 5 mg at bedtime may provide some benefit in mild cases. For some patients with mild OSA, avoidance of precipitating factors such as sleep deprivation, evening alcohol use, sedative medications, and the supine sleeping position is the only treatment needed. Whatever treatment is used, the outcome should be assessed with a sleep study because many patients experience more subjective than objective improvement.
Prognosis and Future Perspectives. Some patients develop increasingly severe apnea with age, but worsening of the condition may be due to weight gain. The natural history of sleep apnea in the absence of weight gain is unknown. Habitual snoring remits in as many as 35 percent.
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