Connecting insomnia, sleep apnoea and depression

EDITORIAL Connecting insomnia, sleep apnoea and depression Key words: depression, insomnia, sleep apnoea. Insomnia and obstructive sleep apnoea (OSA) are the two most common sleep disorders. Insomnia is a known risk factor for incidence and severity of depres- sion, recurrence of depressive episodes and even sui- cide. 1 Several studies have also shown that OSA can contribute to the development of depression. 2 Insom- nia and OSA often co-occur, but previous research has not addressed the interactions, for example, whether this co-occurrence is associated with worse depression. A new study by Lang et al. 3 attempts to address the insomnia/depression/OSA interactions. The investiga- tors assessed OSA using home unattended polysomno- graphy, insomnia using self-reported symptoms and depression using validated screening questionnaires. They examined depression across patients who pre- sented with insomnia alone, OSA alone or both condi- tions, compared to those with neither insomnia nor OSA. They found that, in general, depression scale scores were lowest among those without either condi- tion and with OSA, higher among those with insomnia and highest among those with both conditions. Thus, although OSA itself was not independently associated with depression scores in this study, when insomnia and OSA co-occurred, depression scores were higher than those with insomnia alone. This study had limitations. First, the insomnia symp- toms were not assessed using standard research diag- nostic criteria or standard clinical measures used in the insomnia literature. Second, all of the depression screening questionnaires include items that assess sleep disturbance (which may overlap with insomnia symptoms), which may have inﬂated the strength of observed relationships between insomnia and depres- sion scores. Third, patients were excluded if they were taking medication for anxiety or depression; as insom- nia is often co-morbid with psychiatric disorders, it is not clear whether many patients with insomnia were systematically excluded from the study. Still, this study makes an important contribution to the literature in that it extends the existing evidence linking insomnia to depression by showing that when insomnia is accompanied by OSA, depression may be worse. As estimates of the prevalence of insomnia among OSA patients are as high as 67%, 4 this ﬁnding suggests that many OSA patients may be at risk of depression by virtue of the co-morbid insomnia symptoms. There are several plausible reasons why OSA and insomnia may interact to worsen depression symptoms. One speculative mechanism is via arousal threshold. A core feature of insomnia is physiological and/or corti- cal hyperarousal, which likely results in a decreased © 2017 Asian Paciﬁc Society of Respirology arousal threshold. This decreased arousal threshold in insomnia can lead to increased difﬁculty initiating and maintaining sleep and worse insomnia symptoms. Low arousal threshold has also been shown to contribute to OSA severity through the increased fragmentation of sleep and destabilization of ventilatory control. 5 A low arousal threshold would not allow the accumulation of respiratory stimuli which activate upper airway dilator muscles and thus stabilize breathing. 6 Moreover, recur- rent arousals from sleep could lead to ventilatory over- shoots, unstable CO 2 levels and resulting apnoeas. 7 Recurrent apnoeas could result in both worse insomnia and worse OSA symptoms. Reduced arousal threshold may result in more fragmented and reduced quality of sleep, potentially leading to increased fatigue, increased difﬁculty concentrating and impaired regulation of emotion. More directly, cortical hyperarousal during sleep has been associated with worse depression symp- toms. Depression is also associated with increased rapid eye movement (REM) sleep, a stage in which OSA is typically worst. Taken together, the hyperarousal associ- ated with insomnia may interact with arousal threshold changes in OSA to produce worse experiences of depression (Fig. 1). A second potential link among insomnia, OSA and depression is through worsening daytime symptoms. The presence of daytime sequelae such as fatigue, irri- tability and cognitive symptoms (e.g. difﬁculty concen- trating) are part of the diagnostic criteria for insomnia disorder. These daytime symptoms are often what prompt patients to seek treatment. Daytime sleepiness, irritability, fatigue and cognitive dysfunction are also well-characterized consequences of OSA. Many studies have shown that untreated (or undertreated) OSA is associated with a wide range of daytime symptoms, potentially caused by both sleep fragmentation and intermittent hypoxia. Many of these daytime symptoms known to be related to both insomnia and OSA also reﬂect symptoms of depression. Speciﬁcally, fatigue, difﬁculty concentrating and irritability are all symptoms of depression as well. As OSA may exacerbate these symptoms as daytime consequences of sleep disrup- tion, they may also overlap as increasing depression symptoms. Interestingly, evidence suggests that day- time symptoms of insomnia (not the sleep symptoms such as difﬁculty initiating and/or maintaining sleep) correlate most highly with depression. 8 Thus, OSA may worsen daytime symptoms of insomnia, which may drive an increased contribution of insomnia to depression. In addition to these possible ways in which OSA, insomnia and depression may be linked, it is also worth noting that treatment of one of these conditions may impact the others. For example, although cogni- tive behavioural therapy for depression only minimally Respirology (2017) doi: 10.1111/resp.13090