Narcolepsy-like symptoms among adult twins.

The genetic architecture of narcolepsy is poorly known. Genetic and environmental components of symptoms characteristic of narcolepsy, excessive sleepiness and cataplexy were assessed in a population-based sample of middle-aged like-sexed twin pairs. Questionnaire assessment of the 11-item Ullanlinna Narcolepsy Scale (UNS), a validated screening instrument for narcolepsy [J. Sleep Res. (1994) 3, 52-59] and two subscales (sleepiness and cataplexy-like symptoms) was obtained from both twins of 3785 pairs aged 33-60 y (541 male MZ pairs, 1089 male DZ pairs, 781 female MZ and 1374 female DZ pairs) from the population-based Finnish Twin Cohort. For the UNS scores, the intraclass correlation for male MZ pairs was 0.365 and for male DZ pairs 0.072, while for female pairs the MZ correlation was 0.375 and for DZ pairs 0.155. Structural equation model fitting indicated that a model with additive and non-additive genetic effects, and idiosyncratic environmental effects best accounted for the pattern of twin resemblance in both men and women. Genetic effects accounted for 35% (in men) and for 39% (in women) of total phenotypic variance in UNS. Analysis of the subscales suggested that there may be a greater genetic component to the sleepiness subscale, while environmental components play more of a role in the development of cataplexy-like symptoms. Further investigation of the complex genetic architecture of narcolepsy and its symptoms is warranted.