Original articleHow age influences the expression of narcolepsy
Introduction
Narcolepsy is a rare neurological disorder affecting less than 0.05% of the general population [1], [2], although Japanese population surveys found higher rates [3], [4]. Earlier reports from cohort and clinical studies [5], [6] were already signaling the scarcity of this affection. This disease is characterized by daytime sleep attacks and manifestations of various REM sleep abnormalities (cataplexy, sleep paralysis, and hypnagogic hallucinations).
Unfortunately, few studies have investigated the influence of age on the characteristics of narcolepsy, perhaps because this disorder is usually diagnosed late in the life of the patient. A recent study was underlying a fact: In half of the cases, narcolepsy was recognized after the age of 40 years [7]. At the level of clinical, polysomnographic, and multiple sleep latency test (MSLT) assessments, although most of the studies did not find significant changes with age [7], [8], a recent study reported a decrease with age in the number of sleep-onset REM periods and an increase in the mean sleep latency on the MSLT [9].
Studies that investigated memory and attention in participants with narcolepsy had mixed results. Some studies that examined the level of sleepiness in relationship with cognitive performance in healthy participants found that sleepiness has an impact on memory [10], while others found that memory function remained intact [11], [12]. Some studies reported that up to half of narcoleptic patients complained of memory problems [13], [14], while other studies found that narcoleptics did not differ from normal participants on memory tasks [15], [16]. Studies assessing the performance of narcoleptics on attention tasks found, on one hand, that there was little or no impairment in the ability to sustain attention [15], [17], while, on the other hand, others demonstrated a clear deficit in vigilance and attention [18], [19]. Moreover, the cognitive impairment related to age was not clearly identified in those patients. This issue could be of importance in determining the real impact due to the disease on cognitive functions. This disabling disorder has a deep impact on psychosocial functioning of the patients [20], [21].
In fact, age and narcolepsy in its expressivity has not yet been thoroughly investigated. Consequently, this study aims to answer the following questions: (1) what are the first manifestations of the disease as reported by the patients? At what age did they appear? (2) What are the current clinical symptoms? Are they the same at different ages? (3) What are the specific sleep characteristics of narcoleptic patients by age categories? (4) What is the impact of narcolepsy on cognitive functions? Are these cognitive impairments related to age?
Section snippets
Methods
The patients were recruited from two sleep clinics. They were recorded and diagnosed in these two sleep laboratories. Blood samples also were collected for HLA typing.
General characteristics
The characteristics of the participants are presented in Table 1. The narcoleptic group included a greater proportion of men than did the control group. It also had a smaller proportion of married individuals and of retired individuals compared with the control group. The BMI of the narcoleptic group was also higher than the one of the control group, even after controlling for age and gender.
Narcolepsy-cataplexy represented 75% of the narcoleptic group. A total of 24.4% of narcoleptic
Discussion
This study included 321 participants. Among them, 157 were diagnosed with narcolepsy at one of the two participant sleep disorders clinics and constituted the narcoleptic group. The other 164 participants were the spouses of narcoleptic patients or the spouses of other patients of the sleep disorders clinics and constituted the control group.
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