The upper airway is a very complex structure. In SAHS patients, apneas during sleep are caused by upper airway obstruction, wich leads to progressive asphyxia and awakening. The inspiratory efforts to overcome occlusion lead to arousal, sleep fragmentation, and oxyhemoglobin desaturation. From a physiological standpoint, both UARS and OSAS present intermittent upper airway collapse. This increase in upper airway resistance occasionally accompanies airflow limitation and arousals, with little desaturation. Johnson, found that even minimal airflow limitation could produce arousals that occur before alterations in gas exchange (Johnson 2005). These episodes are of short duration, about four breaths, and present negative intrathoracic pressure increases.

Another interesting difference from OSAS is that UARS patients do not present neuropathological lesions in the upper airway (Friberg 1998; Guilleminault 2002a; Boyd 2004), which could explain why these patients tend to respond more rapidly to treatment and do not develop OSAS over the long-term. Early studies did not seem to reveal differences in sleep architecture between UARS and OSAS (Loube & Andrada 1999). However, today UARS patients are considered to have unstable sleep, characterized by a cyclic alternating pattern in nonREM sleep (Guilleminault 2005a), which predisposes to the occurrence of arousals. These findings correlate with symptoms such as tiredness and fatigue, for which these patients are often referred to sleep labs. The cyclic alternating pattern has been described in many other situations (Ferré 2006), such as fibromyalgia, chronic fatigue syndrome, and OSAS (Terzano 1996). Current research suggests that nearly 50% of fibromialgia syndrome patients experience intrusive alpha wave periods. Patients with UARS present an increase in the number of cyclic alternating patterns, with a decreased phase 1 and an increased phase A2 and A3 (Guilleminault 2005c) In recent years, polysomnography has revealed that UARS patients present nonREM-sleep instability. Alpha-delta sleep is characterized by an intrusion of alpha EEG waves into slow delta waves during deep sleep, which also occurs in insomnia and non-refreshing sleep (Guilleminault 2001a).

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