Background: Among 15 monozygotic twin pairs described in the literature, only four pairs were considered to be concordant. There is no detailed report of HLA-DRB1*1501/DQB1*0602 positive monozygotic twins concordant for narcolepsy, with marked difference in the age of onset. Methods: We compared a pair of female narcoleptic twins clinically. Results: Diagnosis of narcolepsy and monozygosity of the twins were confirmed. The second-born twin demonstrated a typical course of narcolepsy, whereas the first-born twin had a very late onset of recurrent daytime sleep episodes at age 45 and cataplexy at age 50 years, which was apparently triggered by chronic emotional stresses and sleep insufficiency. Conclusions: The atypical course of narcolepsy in the first-born twin supports the multifactorial model for the development of narcolepsy. It was noted that cataplexy was preceded by sustained polyphasic sleep conditions. Our observation implies that the unaffected co-twins in discordant pairs could develop narcolepsy in stressful situations later in their lives.
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