Although OSAS and UARS share common symptoms, in most cases the clinical manifestations are different (Stoohs 2008). The most common symptoms of SAHS patients include chronic loud snoring, excessive daytime sleepiness, personality changes, depression and deterioration of quality of life (Pichel 2004). Hypersomnolence is the principal daytime manifestation of sleep disordered breathing. Excessive sleepiness resulting from increased breathing effort and sleep disruption is the guide symptom of UARS patients (Guilleminault 1993, 2001a). Drowsiness related to general exhaustion has a negative impact on quality of life. As in OSAS, snoring is a common symptom, predominantly in males, although the absence of snoring has also been described in this syndrome, the so-called silent UARS (Kristo 2005). In recent years, several studies have demonstrated strong associations between UARS and functional somatic syndromes, such as chronic fatigue syndrome, chronic insomnia, chronic pain, irritable bowel syndrome, fibromyalgia, depression, parasomnias and posttraumatic stress disorders (Gold 2003, 2011). Due to its association with chronic somatic diseases, UARS has been postulated to activate the hypothalamic-pituitary-adrenal axis (HPA) (Gold 2010), although not all studies support the association of sleep disordered breathing with these somatic functional disorders. (Vgontzas & Fernandez-Mendoza 2011, Trakada 2007).
A number of studies compare the clinical characteristics of UARS patients to those of SAHS subjects. Patients with UARS are usually younger than those with SAHS and have a lower level of obesity. There is no major difference in terms of gender prevalence, although UARS appears to be more frequent in postmenopausal women. An overall ratio of prevalence for men to women of 3.3 has been reported in SAHS patients (Bixler 2001).
UARS patients report both onset and maintenance insomnia (Guilleminault 2002b). A state of physiologic hyperarousal in UARS patients with chronic insomnia is accepted (Gold 2008). Some authors have reported complex insomnia, which paradoxically involves nighttime insomnia and daytime sleepiness (Krakov 2001; Gold 2008). This type of insomnia has been associated with parasomnias (Guilleminault 2006a), which mainly occur in young patients together with sleepwalking and night terrors. Insomnia is more common in UARS than in OSAS.
Powers (Powers 2009) found a tendency to hypersomnia in UARS patients. These patients showed altered results on the maintenance of wakefulness test that were not correlated with the Epworth scale. This author considered non-obese premenopausal women, who often consult for chronic insomnia and parasomnias, to represent a specific attention group.
Approximately half of UARS patients present symptoms of increased vagal tone such as orthostatic hypotension and coldness of the extremities (Guilleminault 2001b). Disturbances in heart rate variability have also been reported along with a decrease in the HF component (associated with increased vagal tone). In contrast, OSAS patients have an increase in the LF/HF ratio, associated with increased activity sympathetic. (Guilleminault 2005).
SAHS is widely associated with cardiovascular risk. Long-term effects can lead to severe cardiovascular and cerebrovascular diseases. However, there is little data regarding the association of cardiovascular disease and UARS. Some studies have found an association between hypertension and UARS, with a good response to CPAP treatment (Guilleminault 1996), but this association has been put into question. Notably, this controversy sheds light on the importance of hypoxia and sympathetic activation (which are not present in UARS) in OSAS as intermediary mechanisms associated with cardiovascular events. The diagnosis of UARS is often delayed becasue of the absence of respiratory events in polysomnography. Sometimes the symptoms of UARS have been confused with other medical conditions, such as asymptomatic habitual snoring, sleep deprivation, chronic fatigue syndrome, idiopathic hypersomnia, psychiatric disorders (Lewin & Pinto 2004) and asthma (Guerrero 2001).
Some authors report a greater likelihood of traffic accidents in UARS patients (Stoohs 1994). Among drowsy drivers, UARS is associated to a higher frequency of accidents. Thus, identification of this syndrome is of great practical importance.
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