Cataplexy: 'tonic immobility' rather than 'REM-sleep atonia'?

Cataplexy, a sudden loss of muscle tone in response to strong emotions, is the most specific symptom of narcolepsy. It is currently thought to be due to disturbed rapid eye movement (REM) sleep regulation, and portrayed as REM sleep atonia occurring at the wrong time. However, there are several arguments against including cataplexy in the 'state boundary control' hypothesis. It does not explain why cataplexy is triggered by emotions, and recent studies in narcoleptic dogs showed that REM sleep regulatory mechanisms were in fact intact in these animals.

We review the literature on the REM sleep dissociation theory, discuss the merits and demerits of the theory, and propose an alternative hypothesis explaining cataplexy.

Cataplexy may represent an atavism (recurrence of an ancestral characteristic) of tonic immobility. Tonic immobility (TI) denotes a condition in which an animal is rendered immobile when faced with danger. Arguments in favor of the TI hypotheses are that it explains the emotional triggering. Furthermore, centers regulating narcolepsy and TI are both located in the lateral hypothalamic area. Finally, several drugs known for their ameliorating effect on cataplexy reduce the frequency and duration of TI in animals.

Cataplexy may be due to a mechanism different from the other clinical symptoms of narcolepsy.