Frequency of narcolepsy symptoms and other sleep disorders in narcoleptic patients and their first-degree relatives.

Narcolepsy is a rare neurological sleep disorder affecting around 0.05% of the general population. Genetic factors are known to have an important role in narcolepsy. However, because of its very low prevalence, it is difficult to have groups of comparison between first-degree relatives and general population subjects in order to identify a specific spectrum of disorders in these families. Consequently, from 157 Italian patients with narcolepsy, 263 first-degree relatives were recruited, two refused to participate. These family members were compared with a matched group of 1071 subjects selected from a sample of 3970 subjects representative of the general population of Italy (46 million inhabitants). Finally, 68 spouses of narcoleptic patients were used to assess for possible role of environmental factors. All subjects were interviewed by telephone using the Sleep-EVAL system. Nineteen cases of narcolepsy were discovered among the first-degree relatives of 17 probands (10.8%). Compared with the general population subjects, the relative risk of narcolepsy among female first-degree relatives was of 54.4 and of 105.1 among male first-degree relatives. First-degree relatives were also at higher risk for idiopatic hypersomnia (OR: 23.0), obstructive sleep apnea syndrome (OR: 6.8), adjustment sleep disorder (OR: 4.0), insufficient sleep syndrome (OR: 7.0), circadian rhythm disorders (OR: 2.5), REM behavior disorder (OR: 4.4), and sleep talking (OR: 2.0). The vulnerability to sleep disorders is very high in first-degree relatives and the link with different expressivity and severity of hypersomnia can be confirmed.