THEORETICAL REVIEWNeurophysiological aspects of primary insomnia: Implications for its treatment
Introduction
Insomnia has received much attention the last few decades, since it has become a growing and complex problem in our society. It has long been, and still is, an under-recognized and under-treated problem. Unfortunately, approximately 60% of the people suffering from insomnia do not talk about their sleeping disturbances with their physician, which only contributes to the fact of being an under-treated disorder.1 Insomnia is defined as the complaint of difficulty initiating or maintaining sleep, early awakening, and interrupted or non-restorative sleep. Furthermore, it must be accompanied by clinically significant impairment in daytime function, for which there is no identifiable cause such as another sleep, psychiatric or medical disorder.2 However, insomnia is not always the primary disorder, but can present itself in a context of other underlying maladies. As such, comorbid insomnia can be accompanied by a diversity of medical and psychiatric conditions, such as chronic painful physical conditions (CPPC's),3 affective disorders, 1, 4, 5 like depression and anxiety, and other psychiatric pathologies,6 including substance abuse disorder.3, 7, 8, 9
There have been many studies concerning the prevalence and epidemiology of insomnia, but the lack of systematic assessment makes it difficult to compare and review these studies in a meaningful way.9, 10 Overall, it is suggested that about 10–20% of the general population reports insomnia complaints and consequently impairment of daytime functioning. Furthermore, the prevalence seems to be higher in woman and increases with age.1, 7, 9, 11
One of the characteristics of insomnia is the discrepancy between the objective polysomnographic (PSG) sleep measures and the subjective report of sleep complaints. Insomniacs tend to overestimate these objective sleeping problems in their subjective report on their sleeping pattern. Sometimes however, there seems to be a total lack of evidence of sleep disturbances in contrast to the subjective report of the patient, in which case it is diagnosed as Paradoxical insomnia (formerly known as sleep state misperception).12 The most important characteristics that determine the clinical significance of primary insomnia are its severity, frequency, duration and daytime consequences.13, 14 The frequency and duration of the complaints are two very important factors to evaluate the severity of this sleep disorder. A frequency of at least three times a week is regarded as clinically significant, especially for research inclusion criteria.15, 16 When the sleep disturbances are present for less than 1 month, it is referred to as transient insomnia. Generally, it is triggered by situational stressors and will resolve itself after the individual has adjusted to the stressful event. Insomnia lasting between 1 and 6 months is referred to as subacute or short-term insomnia. Finally, when the sleep complaints persist for more than 6 months, it is classified as chronic insomnia.12 In addition, there should be an impairment of daytime functioning. Unfortunately, there seems to be a lack of objective evidence, and only subjective reports show a possible impairment of various aspects of daytime functioning. Generally, studies suggest that insomnia patients experience an increase in fatigue, but not in sleepiness.17, 18, 19, 20, 21 Although insomniacs complain of concentration and/or attention deficits, there seems to be no consensus on the objective existence of these cognitive impairments.18, 21, 22, 23 One hypothesis introduced by Edinger et al.24 concerns possible interference of the sleep setting itself the night before daytime testing. The results surprisingly showed that insomnia patients who slept at home prior to daytime testing generally appeared the most alert on the multiple sleep latency test (MSLT) and differed significantly from normal sleepers. Although alternative explanations are possible, these findings could suggest that the insomnia patients were too aroused to fall asleep. Furthermore, post-hoc analysis in this latter study revealed that the participants' relative ease or difficulty with nocturnal sleep onset persisted into the daytime and explained their MSLT differences. This suggests that the arousal is a 24-h phenomenon, which raises the question whether insomnia is in fact only a disorder of nighttime sleep, since there seem to be substantial daytime as well as nighttime symptoms.25
Clearly, insomnia can be understood as a disorder or a comorbidity with a high prevalence in our society. Therefore it is necessary that the understanding of developing and maintenance (predisposing, precipitating and perpetuating factors), as well as its pathophysiology are better understood.
Section snippets
Insomnia and arousal
The behavioral model of insomnia posits that trait and precipitating factors result in acute insomnia, which in turn becomes sub-acute because of the reinforcement of maladaptive coping strategies. Finally, these strategies result in conditioned arousal and chronic insomnia.26 Since, this theoretical framework has been dominantly used since the 1980s, the somatic and cognitive components of the conditioned arousal have received much attention. The somatic hyperarousal has been evidenced by
Treatment implications
Regarding the pharmacological treatment, it is recommended to limit its usage for short-term or transient insomnia, since there are some concerns about potential side-effects in the long-term. It is advised not to exceed the standard limit of 4 weeks to avoid tolerance and dependence.60, 61 Consequently, pharmacotherapy is contraindicated for the treatment of chronic insomnia. Cognitive-behavioral therapies (CBT), however, seem to be quite efficient for treatment of this latter type of
Conclusions
Insomnia is a common disorder in our society and seems to have a significant impact on both nighttime and daytime functioning. A disruption of the sleeping pattern, as well as the subjective report of fatigue, impairment of daytime functioning and a low quality of life is often reported. Several attempts have been made to create a model that would explain this disorder and could be used as a foundation for treatment. However, it appeared that no model can fully comprehend and clarify all
Acknowledgements
This review was supported by the Fonds voor Wetenschappelijk Onderzoek-Vlaanderen (Belgium), research grant FWOG.0067.05.
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