Cataplexy is a symptom characterized by full loss of muscle tone where the patient remains fully conscious. It usually starts to manifest itself as drooping of the muscles of the face. It is, by nature, intermittent and transient. It can be distinguished from fainting, which causes loss of consciousness, catatonia, which is accompanied by delirium and seizure, which is usually characterized by random muscle movements.
It is a common symptom of narcolepsy, and in persons without any underlying pathology, it can be triggered by strong emotional reactions, such as laughing, crying or terror.
More uncommon causes are SSRI discontinuation and some neurological syndromes.