Prevalence
The prevalence of narcolepsy with cataplexy occurs with high variability, depending on the study population, and is thought to affect approximately 0.02–0.18% of the general population in Western nations.^1,5 The true prevalence of narcolepsy without cataplexy is unknown and has been reported to range from 1%6 to 36%7 of narcolepsy cases, but may be as high as 50% of all narcolepsy cases.1 Diagnostic criteria for narcolepsy with or without cataplexy are a mean sleep latency (eight minutes and (two SOREMPs on the Multiple Sleep Latency Test (MSLT),8 according to the International Classification of Sleep Disorders.¹ These criteria, however, have been observed in the general population of the Wisconsin Sleep Cohort Study9 in 5.9% of men and 1.1% of women.¹⁰ Singh and colleagues have shown that in a random sample of 333 adults who had an MSLT of (five minutes, 9.5% had two or more SOREMPs.¹¹ These data, along with others reporting the presence of SOREMPs in non-narcolepsy patients,^12,13 have led some authors to suggest that SOREMPs may be more closely related to ES than narcolepsy, and that re-evaluation of the International Classification of Sleep Disorders diagnostic criteria may be warranted.10 The age at onset of narcolepsy follows a bimodal relationship, with a first major peak at 15 years and a second smaller peak in the mid 30s.¹⁴ ES typically appears as the first symptom, with the first occurrence of cataplexy delayed by five years.¹⁴

Pathophysiology
In patients with cataplectic narcolepsy, 78–100% exhibit abnormally low circulating levels of the hypothalamic neuropeptide hypocretin-^1,15-18 perhaps due to an autoimmune process, since a strong concordance between low hypocretin-1 levels and the DQB1*0602 subtype has also been shown.^16-20 Reports of hypocretin-1 levels in patients with noncataplectic narcolepsy have shown variable results across studies,^16-18 and the HLA DQB1*0602 allele is present in only 41% of patients without cataplexy.²¹