Original ArticleComorbidity of narcolepsy and depressive disorders: a nationwide population-based study in Taiwan
Introduction
Narcolepsy is a lifelong disabling sleep disorder that is characterized by excessive daytime sleepiness (EDS), cataplexy, sleep paralysis, fragmented nocturnal sleep, and hypnagogic hallucination [1]. Patients suffering from narcolepsy complain of excessive daytime sleepiness nearly every day, which may develop into irresistible sleep episodes. Cataplexy, another core symptom of narcolepsy, is characterized by the sudden loss of muscle tone, which is often prompted by intense emotion or excitement. The lifelong prevalence of narcolepsy is approximate 1 in 2000 in the general population, and this condition can severely hinder daily activities [2], [3]. Furthermore, the high probability of having a comorbid condition, such as obesity, epilepsy, other sleep disorder, anxiety, and depression, significantly impacts a narcoleptic patient's daily function, and may result in delayed diagnosis and improper treatment [1], [4].
Depression is a common but serious mood disorder; symptoms affect an individual's feelings, thoughts, drive, and ability to handle daily activities and functions [5]. Therefore, identifying depression among narcoleptic patients is vital for providing adequate intervention. Vignatelli et al. found that the presence of depressive symptoms was the major independent predictor of health-related quality of life over a 5-year period in narcoleptic individuals [6]. An increased risk of suicide ideation may also be associated with excessive daytime sleepiness, particularly when comorbid with a depressive disorder [7]. However, many studies have reported a high ratio of depressive moods among narcoleptic patients, although the incidence of depressive disorders related to narcoleptic patients remains unsettled. Ruoff et al. found that narcolepsy is associated with a wide range of mental illnesses, particularly anxiety and depressive disorders [8]. Furthermore, Ohayon et al. suggested that mood and anxiety disorders occurred more frequently among the narcoleptic group than the control group in their study, with major depressive disorder and social anxiety disorder being the most common [9]. Previous studies using self-reported questionnaires, such as Medical Outcome Short Form-36 (SF-36) and Beck Depression Inventory (BDI), have revealed that 45.1–56.9% of narcoleptic patients suffer from depression [10], [11]. In contrast, two case–control studies that employed a structured psychiatric interview indicated that the prevalence of depressive disorders among narcoleptic patients was not higher than that of the healthy comparison group [12], [13]. For example, Vourdas et al. found that 16% of narcoleptic patients and 18% of the control group had one or more episodes that met the DSM-IV criteria for major depression [12]. Likewise, Fortuyn et al. reported that 7% of narcoleptic patients had a current major depressive episode with 3% of controls [13]. Therefore, it remains controversial as to whether narcolepsy patients have a higher rate of depression.
One possible reason for the discrepancy in the comorbid rate of narcolepsy and depression is the difficulty of differential diagnosis. Many somatic symptoms of narcoleptic patients (eg, sleep disturbance, fatigue, decreased attention, and weight changes) can also be considered to be depressive symptoms [14], thus posing a diagnostic dilemma when attempting to differentiate these overlapping symptoms between narcolepsy and depressive disorders in clinical settings. As a result, investigating the time sequence of the diagnoses of narcolepsy and depression may help to clarify the complexity among narcoleptic patients comorbid with a depressive disorder. It is assumed that some narcoleptic patients are diagnosed with depression prior to their narcolepsy diagnosis, while depression may be identified later in others.
Therefore, this nationwide population-based analysis was conducted to clarify the co-occurrence rate of narcolepsy and depressive disorders. An epidemiological methodology was also used to examine the time sequence between narcolepsy and depressive disorder diagnoses.
Section snippets
Data source
The Institutional Review Board at Chang Gung Memorial Hospital approved this study. Data for this study were obtained from the ambulatory claims database of the National Health Insurance Research Database of Taiwan (NHIRD-TW). Implemented in 1995, National Health Insurance (NHI) is the compulsory universal health insurance program in Taiwan. As of the end of 2000, 22.3 million people in Taiwan (>96% of the population) were enrolled in the NHI program. The study used two subsets of the NHIRD-TW,
Results
Table 1 shows the characteristics of the narcolepsy group and the control group. Compared to the control group, the narcolepsy group was more likely to have comorbidities of ADHD (8.8%), obesity (3.1%), epilepsy (8.9%), and intellectual disability (2.3%). During the monitoring period, 32.7%, 24.8%, and 10.9% of the narcoleptic patients were comorbid with any depressive, dysthymic, and major depressive disorder, respectively. In contrast, 6.3%, 4.4%, and 1.6% of the control subjects had any
Discussion
This study used a nationwide population database to demonstrate the comorbidity of narcolepsy and depression, and observed the time sequence between narcolepsy and depressive disorder diagnoses. When compared to the non-narcoleptic group, patients with narcolepsy had more than a six-fold greater risk of comorbidity with a depressive disorder, dysthymic disorder, or major depressive disorder. In line with this finding, some previous studies have also supported the high co-occurrence rate of
Acknowledgments
This study was sponsored by the Chang Gung Memorial Hospital Research Projects (CMRPG8D0581, CMRPG2G0071, CLRPG2C0023 and CGRPG2F0021). This study was based in part on data from the NHIRD-TW provided by the National Health Insurance Administration, Ministry of Health and Welfare and managed by the National Health Research Institutes (registration number: NHIRD-102-088). The interpretations and conclusions contained herein do not represent those of the National Health Insurance Administration,
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