OSAS is common, underdiagnosed and eminently treatable. In developed countries, it is reported to affect between 3–7% of middle-aged men and 2–5% of women. It is diagnosed on the basis of symptoms (usually daytime sleepiness) plus objective evidence of disordered breathing during sleep. The condition is characterised by frequent obstruction of the upper airway during sleep, resulting in repetitive breathing pauses accompanied by oxygen desaturation (reduced oxygen in the blood) and arousal from sleep. The sleep disruption results in daytime sleepiness and, in the long term, it can lead to cognitive impairment and cardiovascular morbidity. The clinical presentation of, and diagnostic criteria for, sleep disordered breathing are different for adult and paediatric cases. The prevalence of OSAS is higher in certain groups, particularly in the obese, and in various medical conditions, for instance Down syndrome. Many epidemiological studies have focused simply on the prevalence of obstructive breathing pauses at night (OSA) without taking the daytime consequences into account. This has introduced a degree of confusion into the epidemiological literature and contributed to the fluidity of the terminology.
The definition of OSAS
OSAS is characterised by episodes of upper airway occlusion: these are termed apnoeas if the airway is completely occluded and hypopnoeas if the occlusion is partial. An obstructive apnoea is defined pragmatically as the cessation of airflow despite continued breathing efforts for at least 10 s. At their termination, apnoeas/hypopnoeas are often, but not always, associated with a change in the electroencephalographic (EEG) signal indicative of arousal and with a drop in blood oxygen saturation. In most instances, such brief arousals are not accompanied by complete awakening and the patient is usually unaware of them. The definition of hypopnoea is rather variable, depending on the type of equipment used to measure breathing, but the core of the definition, as adopted by the American Academy of Sleep Medicine (AASM), is a 30–50% reduction in thoraco-abdominal movement from the preceding stable baseline for at least 10 s. The current (2012) AASM guidelines add an accompanying 3% desaturation or an arousal. However, in some centres, older definitions of hypopnoea are still in use.
An AASM task force in 1999 defined the severity of OSAS on the basis of two separate components: daytime sleepiness and the degree of breathing disturbance measured by overnight monitoring. The commonly used method of assessing sleepiness is discussed further below. The severity of sleep-related obstructive breathing events is assessed using the apnoea/hypopnoea index (AHI) and is graded as mild (5–15 events per h of sleep), moderate (15–30 events per h of sleep) or severe (more than 30 events per h of sleep). Although a good general working classification, this does not take into account age- or sex-related variations. There are very few normative data on either sleepiness or AHI in the healthy population.