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Obstructive sleep apnea/hypopnea syndrome (OSAHS) is one of the most important medical conditions identified in the last 50 years. It is a major cause of morbidity, a significant cause of mortality, and the most common medical cause of daytime sleepiness. Central sleep apnea is a rare clinical problem. Other sleep disorders are discussed in Chap. 27.

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Definition

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OSAHS is defined as the coexistence of unexplained excessive daytime sleepiness with at least five obstructed breathing events (apnea or hypopnea) per hour of sleep (Table 265-1). This event threshold may have to be increased in the elderly. Apneas are defined in adults as breathing pauses lasting 10 s and hypopneas as events ≥10 s in which there is continued breathing but ventilation is reduced by at least 50% from the previous baseline during sleep. As a syndrome, OSAHS is the association of a clinical picture with specific abnormalities on testing; asymptomatic individuals with abnormal breathing during sleep should not be labeled as having OSAHS.

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Table 265-1 Clinical Indicators in the Sleepy Patient 
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Mechanism of Obstruction

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Apneas and hypopneas are caused by the airway being sucked closed on inspiration during sleep. This occurs as the upper-airway dilating muscles—like all striated muscles—relax during sleep. In patients with OSAHS, the dilating muscles fail to oppose negative pressure within the airway during inspiration. The primary defect is not in the upper-airway muscles, which function normally in OSAHS patients when awake. These patients have narrow upper airways already during wakefulness, but when they are awake, their airway dilating muscles have increased activity, which ensures airway patency. However, during sleep, muscle tone falls and the airway narrows; snoring may commence before the airway occludes, and apnea results. Apneas and hypopneas terminate when the subject arouses, i.e., wakens briefly, from sleep. This arousal is sometimes too subtle to be seen on the electroencephalogram but may be detected by cardiac acceleration, blood pressure elevation, or increase in sympathetic tone. The arousal results in return of upper-airway dilating muscle tone, and thus airway patency is resumed.

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Factors predisposing to OSAHS by narrowing the pharynx include obesity—in Western populations around 50% of OSAHS patients have a body mass index (BMI) >30 kg/m2—and shortening of the mandible and/or maxilla. This change in jaw shape may be subtle and can be familial. Hypothyroidism and acromegaly predispose to OSAHS by narrowing the upper airway with ...

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