Description

• Narcolepsy is a neurologic sleep disorder characterized by excessive daytime sleepiness (EDS) and may be associated with cataplexy (sudden loss of muscle control), hypnagogic hallucinations (vivid perceptual experiences while falling asleep), hypnopompic (vivid perceptual experiences while waking up from sleep), or sleep paralysis (temporary inability to move or speak that happens during transition from sleeping to awake state).
• Mainly two types identified by American Academy of Sleep Medicine:
  • Type 1 (60–70%) (formerly narcolepsy with cataplexy) and type 2 (formerly narcolepsy without cataplexy) (1)
• No cure, symptomatic management

Epidemiology

Incidence

• Affects 0.03 to 0.16% general population with incidence 1/2,000 worldwide (2)
• Bimodal distribution; age of onset peak at 15 years and again at 35 years of age.
• Predominant sex: male > female (1.6:1)
• 5% familial cases (1)
• African Americans are more likely to present without cataplexy and at a younger age.

Prevalence

• Narcolepsy type 1: 25 to 50/100,000 people
• Narcolepsy type 2: 20 to 34/100,000 people (1)

Etiology and Pathophysiology

• Primarily caused by degeneration of hypothalamic neurons that produce orexin (hypocretin). Postmortem studies show 85% loss of these neurons in type 1 and about 33% loss in type 2 (1).
• Orexin is crucial to promote wakefulness, it stimulates the RAS (reticular activating system) and inhibits rapid eye movement.
• Neurodegeneration may be caused by autoimmune process, probably stimulated by infections (such as influenza A or group A Streptococcus) or environmental factors (occurs commonly in late spring).
• Alterations in the L-PGDS (lipocalin-type prostaglandin D synthase) level also seem to be involved in causing EDS in narcolepsy (1).
• International Classification of Sleep Disorders, 3rd edition (ICSD-3) classification:
  • Narcolepsy
  • Narcolepsy type 1
  • Narcolepsy type 2
  • Narcolepsy due to medical conditions

Genetics

• Usually sporadic but increased incidence in families with positive history: 1–2% in first-degree relative of the index case (10 to 40 times the general population)
• 98% of patients with narcolepsy type 1 have human leukocyte antigen (HLA) DQB1*0602; 40–50% of patients with narcolepsy type 2 express this antigen. HLA DQB1*0602 is present in 12–30% of the general population.
• Autosomal recessive inheritance pattern
• Affects 12% of Asians and 25% of whites, and 38% of African Americans are gene carriers.

Risk Factors

• Age (peaks at 15 and 35 years), usually underdiagnosed before age of 18
• Obesity
• Head trauma
• Central nervous system infectious disease
• Anesthesia
• Psychological stress
• Positive family history
• Recent influenza A, streptococcal infection, or H1N1 vaccine (no evidence of COVID leading to narcolepsy yet) (3)

Commonly Associated Conditions

Obstructive sleep apnea (OSA) (up to 25%), obesity, anxiety