Comorbidity of narcolepsy and depressive disorders: a nationwide population-based study in Taiwan

Highlights

• The relationship between narcolepsy and depressive disorders was investigated.

• Patients with narcolepsy had more than a six-fold greater risk of comorbidity with depression than controls.

• More than 50% had been diagnosed with depression prior to narcolepsy.

• Clinicians must pay attention to the comorbidity of narcolepsy and depression.

Abstract

Objective

Narcolepsy is a chronic sleep disorder that is likely to have neuropsychiatric comorbidities. Depression is a serious mood disorder that affects individuals' daily activities and functions. The current study aimed to investigate the relationship between narcolepsy and depressive disorders.

Methods

The study consisted of patients diagnosed with narcolepsy between January 2002, and December 2011 (n = 258), and age-matched and gender-matched controls (n = 2580) from Taiwan's National Health Insurance database. Both the patients and the controls were monitored through December 31, 2011, to identify the occurrence of a depressive disorder. A multivariate logistic regression model was used to assess the narcolepsy's potential influence on the comorbidity of a depressive disorder.

Results

During the study period, 32.7%, 24.8%, and 10.9% of the narcoleptic patients were comorbid with any depressive disorder, dysthymic disorder, and major depressive disorder, respectively. When compared to the control subjects, the patients with narcolepsy were at greater risks of having any depressive disorder (aOR 6.77; 95% CI 4.90–9.37), dysthymic disorder (aOR 6.62; 95% CI 4.61–9.57), and major depressive disorder (aOR 6.83; 95% CI 4.06–11.48). Of the narcoleptic patients that were comorbid with depression, >50% had been diagnosed with depression prior to being diagnosed with narcolepsy.

Conclusions

This nationwide data study revealed that narcolepsy and depression commonly co-occurred. Since some symptoms of narcolepsy overlapped with those of depressive disorders, the findings serve as a reminder that clinicians must pay attention to the comorbidity of narcolepsy and depression.

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.