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142 Duntley<br />

disorders has been confirmed, although the clinical significance of this relationship<br />

continues to be defined.<br />

EPIDEMIOLOGY<br />

The epidemiology of insomnia in sleep disorders is dependent on the epidemiology<br />

of the primary sleep disorder itself. One community-based population study<br />

found about 2% of women and 4% of men have OSA syndrome (4). A recent university-based<br />

study of patients with sleep-disordered breathing found that of 231<br />

patients sampled, 116 complained of clinically significant insomnia (5). Among<br />

postmenopausal women complaining of insomnia, 83% were noted to have upper<br />

airway resistance syndrome (UARS) or OSA syndrome (6). <strong>Insomnia</strong> complaints<br />

in patients with OSA syndrome, like insomnia complaints in general, may be more<br />

frequent among women than men (7). Central sleep apnea syndrome is less common<br />

than OSA, but patients appear to be less likely to report daytime hypersomnolence<br />

and more likely to complain of insomnia (8). RLS is common, with a recent<br />

population-based study revealing bedtime symptoms in 10–15% of individuals (9).<br />

Difficulty falling asleep was reported by 84.7% of patients and 86% reported frequent<br />

awakenings with difficulty falling back asleep because of symptoms (10).<br />

PLMD increases with age, being uncommon before 30 years of age, seen in 5% of<br />

individuals between 30 and 50 years of age, and in about 44% of individuals aged<br />

65 and older (11). One multicenter study found PLMD to be the primary diagnosis<br />

in 17% of patients complaining of insomnia (12). Narcolepsy has a prevalence of<br />

about 0.05% (13). Fragmented sleep is found in up to 90% of patients with narcolepsy<br />

(14) and tends to be relatively mild initially, increasing in severity over time<br />

(15). Nightmares with a frequency of once a week or greater are seen in 4% of the<br />

adult population in Austria (16). One study in France found that 18.3% of insomnia<br />

patients were diagnosed as having nightmares (17). Occasional sleep starts are a<br />

nearly universal phenomenon. Sleep starts may rarely become repetitive at sleep<br />

onset resulting in sleep-onset insomnia.<br />

ETIOLOGY<br />

The cause of insomnia varies according to the primary sleep disorder, and may<br />

result from the sleep-onset and maintenance difficulties inherent in the disorder<br />

itself or from secondary symptoms of the disorder. For instance, in OSA syndrome,<br />

the termination of the apnea is associated with an arousal or awakening that may be<br />

perceived and remembered by the patient. OSA syndrome is also associated with<br />

other symptoms, such as nocturia and dry mouth, which can contribute to the development<br />

of an insomnia complaint. In RLS, the dysesthesias and need to move the<br />

legs prevents sleep onset, and may prevent returning to sleep after awakenings in<br />

the middle of the night. In PLMD, limb movements are associated with arousals or<br />

awakenings that may lead to the perception of difficulty initiating or maintaining<br />

sleep. In narcolepsy, the neurochemical defect leads to dysregulation of sleep

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