Narcolepsy, Type 1

**Definition:** Type 1 narcolepsy is a disorder of excessive sleepiness due to a deficiency of hypothalamic hypocretin (orexin) signaling. In addition to daily periods of irrepressible need to sleep or daytime lapses into sleep, type 1 narcolepsy is characterised by symptoms of REM sleep dissociation, most importantly cataplexy. Cataplexy is a sudden and uncontrollable loss of muscle tone arising during wakefulness that is typically triggered by a strong emotion, such as excitement or laughter. Although cataplexy is a pathognomonic symptom of type 1 narcolepsy, it may not manifest until years following onset of the sleepiness. In such cases, a diagnosis of narcolepsy, type 1 may be made based on cerebrospinal fluid (CSF)-hypocretin levels < 110 picograms per milliliter. Episodes of sleep paralysis and hypnagogic or hypnopompic hallucinations may also be present. The disorder is not attributable to a disease of the nervous system or other medical condition.

Note: A definitive diagnosis requires daily periods of irrepressible need to sleep or daytime lapses into sleep plus either: a) cataplexy and multiple sleep latency test/polysomnography (MSLT/PSG) findings characteristic of narcolepsy; or b) demonstrated CSF hypocretin deficiency.