Rhythmic movements during sleep: a physiological and pathological profile.

Rhythmic movement disorder (RMD) consists of rhythmic movements (RMs) that occur on falling asleep or during sleep, can involve any part of the body and have a reported frequency ranging from 0.5 to 2 Hz. RMs have been reported to occur in a high proportion of normal children as a self-limiting phenomenon starting and remitting within early infancy. However, there have also been descriptions of forms of RMD occurring against a background of mental retardation or persisting beyond childhood, or having onset in adulthood. So, the occurrence of RMs can be regarded as both a physiological and a pathological phenomenon. The few polysomnographic studies conducted in this field have shown that, in some forms of RMD, RMs are highly linked to arousal fluctuations. However, the mechanisms that underlie the genesis of RMs and are capable of leading to both physiological and pathological patterns of RMs are not fully understood. Here we emphasise the possibility that the central motor pattern generator, recently hypothesised to play a role in the genesis of motor phenomena during sleep in the cases of parasomnia and epileptic seizures, might account for the occurrence of RMs in both physiological and pathological conditions.