Characteristics of rapid eye movement sleep behavior disorder in narcolepsy

Rapid eye movement (REM) sleep behavior disorder (RBD) is characterized by dream-enacting
behavior and impaired motor inhibition during REM sleep (REM sleep without atonia, RSWA). RBD is
commonly associated with Parkinsonian disorders, but is also reported in narcolepsy. Most patients
with narcolepsy with cataplexy lack the hypocretin neurons in the lateral hypothalamus. In contrast,
RBD, RSWA, and hypocretin deficiency are rare in narcolepsy without cataplexy. Phasic motor activity
in REM and non-REM (NREM) and dream-enacting behavior (RBD) coexist with cataplexy in narcolepsy
because of hypocretin deficiency. Thus, hypocretin deficiency is linked to the two major disturbances
of REM sleep motor regulation in narcolepsy: RBD and cataplexy. Moreover, it is likely that hypocretin
deficiency independently predicts periodic limb movements in REM and NREM sleep, probably via
involvement of the dopaminergic system. This supports the hypothesis that an impaired hypocretin
system causes general instability of motor regulation during wakefulness, REM and NREM sleep in
human narcolepsy. We propose that hypocretin neurons are centrally involved in motor tone control
during wakefulness and sleep in humans and that hypocretin deficiency causes a functional defect
in the motor control involved in the development of cataplexy during wakefulness and RBD/RSWA/
phasic motor activity during REM sleep.