Insomnia and OSA are the two most common sleep disorders and yet peer-review publications based on state-of-the-art, evidence based literature are lacking in describing the interaction and implications emerging from the inter-relationship between the two. This has been well addressed in an editorial by Krakow (33). Insomnia is generally considered an infrequent presenting night-time complaint in patients with OSA. Yet review of limited literature shows a higher percentage than generally believed. In a retrospective study by Krakow et al. (34) 50% of patients with PSG-proven SDB, reported significantly problematic insomnia symptoms. More recently, Smith et al. (35) using a rigorous methodology reported a prevalence of significant insomnia in 39% of PSG-proven patients with OSA. Women with OSA are more likely to have insomnia than are men with the same degree of OSA (36).
Looking at the reverse picture, what is the prevalence of SDB in insomnia patients? Research shows that these numbers are similarly high. Guilleminault et al. (37) documented SDB in 83% of 394 postmenopausal women complaining of chronic insomnia using PSG studies with pressure transducer and esophageal manometry. The SDB was classified mainly in the low AHI range. Similarly, Lichstein et al. (38) reported sleep apnea in 29% to 43% of a recruited sample of older individuals with insomnia, depending upon whether an AHI of 15 or 5 was used for diagnosis. Thus, irrespective of whether it is SDB in insomnia patients or insomnia in patients with SDB, there is clear indication that a comorbid relationship exists between the two. This however, remains largely unrecognized as evidenced by a lack of established guidelines requiring routine use of PSG in the diagnosis of insomnia (39).
A few researchers have gone further to evaluate the relationship between SDB, the nature of insomnia and a possible mechanism of action to explain the interaction. Chung et al. (40) reported an interesting analysis of insomnia prevalence in 150 patients with suspected SDB, of which 119 were diagnosed to have an AHI > 5 events per hour. It was noted that patients with difficulty going to sleep or returning to sleep after an awakening had significantly lower AHI than subjects with frequent awakenings and even those with no insomnia. He thus postulated that repeated apnea is not a single factor accounting for the insomnia symptoms in patients with SDB but that individual vulnerability must be considered. Krakow et al. (34) had observed similar findings of a statistically higher AHI in those without insomnia compared to those with insomnia. They suggested possibility of greater self-reported psychiatric distress and anxiety about sleep in contrast to SDB-induced respiratory compromise and sleep fragmentation. It may however be mentioned that the suggestion of increased prevalence of insomnia in patients with a lower AHI may need to be re-evaluated in the light of esophageal pressure monitoring and measurement of respiratory disturbance index (RDl) including respiratory effort-related arousals (RERAs) providing a more comprehensive picture of SDB. More recently, Chung
(41) reported similar findings in another study in which 42% of 157 sleep apnea patients had at least one problematic insomnia symptom, with a prevalence of sleep onset insomnia of 6%, sleep maintenance insomnia of 26%, and early morning awakening in 19%. Patients with sleep onset insomnia had a significantly lower AHI and arousal index. There was a significant inverse relationship between sleep onset insomnia and measures of daytime sleepiness. On the contrary, subjects with repeated awakenings had more severe sleepiness. Results were similar in patients with AHI > 5 or > 15. Chung thus postulated that OSA with sleep onset insomnia may be a state of hyperarousal. This stresses the importance of evaluating insomnia subtypes in patients with SDB as it may help with treatment decisions.
There have been studies suggesting a higher than expected prevalence of SDB in patients with post-traumatic stress disorders (PTSDs). In one study Krakow et al.
(42) reported the potential presence of SDB in 52% of female patients with PTSD, correlating positively with body mass index (BMI), increased arousal index, and PTSD severity. Improvement in insomnia and post-traumatic stress with successful treatment of SDB has also prompted the hypothesis of an arousal-based mechanism in trauma survivors and in patients with chronic insomnia (43). A greater than expected level of anxiety, depression, stress symptoms, and past history of psychiatric problems are also reported by Smith et al. (35) in patients with SDB and insomnia.
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