Narcolepsy is characterized by episodes of irrepressible sleep occurring during the daytime, often during monotonous activities. These episodes typically last 10–20 minutes and occur against a background of continuous sleepiness in most patients. Hypersomnia in narcolepsy is usually associated with one or more other symptoms, including cataplexy, hypnagogic hallucinations, and sleep paralysis, although these symptoms do not necessarily occur together.
Cataplexy is a sudden loss of muscle tone triggered by laughter or excitement, causing the patient to fall without losing consciousness. It must be distinguished from conditions such as epilepsy, hyperekplexia, Niemann–Pick C disease, and Coffin–Lowry syndrome. The full tetrad of narcolepsy symptoms occurs in only 10% of adult patients, with many children presenting behavior problems or learning difficulties due to sleepiness and efforts to stay awake.
Narcolepsy is a rare condition with an incidence of 0.03% in both adult and pediatric populations. Many adults with narcolepsy recall having symptoms since childhood, with about 80% of individuals experiencing onset before age 20 and up to 30% before age 15. The earliest recorded onset is in a 9-month-old child. The International Classification of Sleep Disorders (ICSD-3) recognizes two types of narcolepsy:
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Type 1 Narcolepsy: Associated with hypocretin deficiency, includes either cataplexy or reduced cerebrospinal fluid (CSF) hypocretin-1 levels. The deficiency in hypocretin-1 (orexin) peptides released from the dorsolateral hypothalamus is likely due to autoimmune destruction of hypocretin cells.
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Type 2 Narcolepsy: Characterized by normal CSF hypocretin-1 levels and polysomnographic evidence of a mean sleep latency of 8 minutes or less, plus at least two sleep onset rapid eye movement periods during multiple sleep latency tests and night polysomnography.