Narcolepsy

A chronic neurological disorder that disrupts sleep–wake regulation, causing excessive daytime sleepiness and sometimes cataplexy, sudden muscle weakness triggered by emotion.

Orexin: A neuropeptide system that promotes wakefulness, arousal, and appetite regulation. Loss of orexin-producing neurons underlies most narcolepsy type 1.
Excessive daytime sleepiness: Persistent difficulty maintaining alertness during the day, often with unintended dozing. It is the central symptom shared by narcolepsy types 1 and 2.
Cerebrospinal fluid orexin measurement: A laboratory measurement of orexin-A concentration in fluid surrounding the brain and spinal cord. Very low levels can confirm orexin deficiency in narcolepsy type 1.
Idiopathic hypersomnia: A central disorder of hypersomnolence marked by persistent sleepiness without narcolepsy’s characteristic REM intrusions. It causes daytime sleepiness but typically lacks cataplexy and sleep-onset REM periods.
Cataplexy: Sudden, brief loss of muscle tone during wakefulness, usually triggered by strong emotion. It is a defining symptom of narcolepsy type 1 and reflects intrusion of REM-related paralysis.
Narcolepsy type 1: Narcolepsy characterized by cataplexy or low cerebrospinal-fluid orexin levels. This subtype is associated with orexin-neuron loss and often includes cataplexy.
Modafinil: A wake-promoting medication used to treat excessive sleepiness in several conditions. It is commonly used to reduce daytime sleepiness in narcolepsy.
Obstructive sleep apnea: A disorder in which repeated upper-airway collapse interrupts breathing during sleep. It can cause daytime sleepiness, but arises from disrupted breathing rather than narcoleptic sleep–wake regulation.
Sleep paralysis: A brief inability to move or speak while falling asleep or waking, despite awareness. Like cataplexy, it reflects REM muscle atonia appearing during wakefulness.
Narcolepsy type 2: Narcolepsy with excessive daytime sleepiness and abnormal sleep testing, without cataplexy or known orexin deficiency. It shares the sleepiness of type 1 but lacks its defining cataplexy or orexin evidence.

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