Clinical reviewComorbidity of insomnia and depression
Introduction
Insomnia symptoms appear to be one of the most frequent sleep complaints in the general population with an estimated prevalence varying from 10 to nearly 60% depending in part on the use of varying definitions and data-collection methodologies.1 This estimate falls below 10% when more stringent diagnostic criteria are applied to define insomnia, including daytime consequence of insomnia; for instance, the prevalence of insomnia diagnoses according to the Diagnostic and Statistical Manual of Mental Disorders 4th edition (DSM-IV) classification has been estimated at 6%.1 Individuals reporting disturbed sleep are more likely to report emotional distress and recurrent health problems.2, 3, 4 This is not surprising since it has been shown that sleep deprivation has great impact on the daytime life of healthy subjects; alertness, attention, concentration, cognitive abilities, memory, mood and pain threshold have all been found impaired, even with a sleep deprivation of as little as 1–2 h per night.5, 6, 7
In this regard, studies have repeatedly emphasized the comorbid nature of insomnia and psychiatric disorders, especially depression but also anxiety and substance use.8, 9, 10 Clinical and epidemiological studies have shown that sleep disturbance is tightly linked to major depression. In clinical samples, about three quarters of all depressed patients complain of difficulty either in initiation or in maintaining sleep.11, 12, 13 Epidemiological studies showed a comparable prevalence of insomnia symptoms in patients with depression,14, 15 with 41% of depressed patients reporting sufficient insomnia symptoms to warrant an additional DSM-IV diagnosis of insomnia.15
Other epidemiologic studies suggest that the link between insomnia and depression is bidirectional. For instance, about 20% of patients with insomnia exhibit some depressive symptoms8, 16, 17 whereas depression and depressive symptoms have been shown to be the largest and most consistent risk factors for insomnia.4, 10 During the last decade, several studies brought evidences that insomnia could be more than a symptom of depression. For instance, clinical efficacy of antidepressant drugs18, 19 or of cognitive behavioral therapy for depression*20 is unrelated to the effects on insomnia complaints. Moreover continued insomnia has been shown either to pre-exist the onset of depression14, 21, 22, *23 or to become chronic despite successful resolution of depressive symptoms.24, 25, 26, 27
Accordingly, it has been suggested that depression and insomnia could be comorbid conditions having a clinical course different from the index diseases and requiring specific treatment procedure.*28, 29, 30 In the present paper, theoretical and methodological aspects of comorbidity studies will be first discussed with a special emphasis on the notion of causality. The sleep EEG profiles of antidepressant drugs are then discussed in relation to their use in insomnia comorbid with depression and different other treatment approaches for insomnia comorbid with depression are presented.
Section snippets
Definition and significance of comorbidity
Strictly speaking, comorbidity is defined as the occurrence of at least two distinct diseases or disorders in a same patient. The term was introduced by Feinstein31 in general medicine to denote “any distinct additional clinical entity that has coexisted or that may occur during the clinical course of a patient who has the index disease under study”. Comorbidity raises several important research questions such as: are the disorders A and B distinct or do they reflect an arbitrary division of a
Causal relationships between insomnia and depression
Knowing that one disorder co-occurs with another one well beyond chance level provides clues for identifying common risk factors, possibly causal risk factors. One disorder may directly produce the other (direct causality) or indirectly increases the risk of the other disorder (indirect causality). Direct or indirect causality implicates sequential comorbidity, i.e., that there is a temporal relationship between the two comorbid conditions. Another possibility is that insomnia and depression
Treatment approaches for insomnia comorbid with depression
Our knowledge about the nature of the comorbid relationship between insomnia and depression should influence the way treatment approaches are designed. For instance, a sequential comorbidity (insomnia precedes depression) and a possible causal relationship between the two disorders (insomnia causes depression) would indicate that the successful treatment of insomnia has a preventive effect on depression occurrence. On the other hand, concomitant comorbidity suggests common causalities and would
Antidepressant drugs for insomnia comorbid with depression
Studies that reviewed the different treatment options of insomnia comorbid with depression generally consider sedative antidepressant drugs as first-line agents.*28, 176, 177, 178 The present section discusses the sleep EEG profiles of antidepressant drugs in relation to their use in insomnia comorbid with depression.
Conclusions
The present review supports the idea that depression and insomnia could be comorbid conditions having a different clinical course than the index diseases and that require specific treatment procedure. Epidemiological research indicates that insomnia and depression are not randomly associated and that they are either causally related to each other and/or that common causalities underlie the two disorders. There are some evidences that chronic sleep disruption experienced by insomniac patients
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