The study, which appeared in a recent edition of the journal Sleep, also found that narcolepsy without cataplexy -- a sudden loss of muscle tone -- is an important subgroup, warranting further study.
Narcolepsy is a sleep disorder associated with excessive daytime sleepiness, involuntary daytime sleep episodes, disturbed nocturnal sleep and cataplexy (weakness with emotions such as laughter). Narcolepsy affects over 100,000 people in the United States.
The study aimed to determine the age- and sex-specific incidence rates and prevalence of narcolepsy in a United States community. It is the first rigorous community-based epidemiologic study of the disorder from North America.
The records-linkage system of the Rochester Epidemiology Project was utilized to ascertain all patients with narcolepsy seen in Olmsted County, Minn. between 1960 and 1989. Age- and sex-specific incidence rates were calculated, using census data. Prevalence of narcolepsy on Jan. 1, 1985 was calculated, a date selected in order to give sufficient time for patients to have sought medical assistance for their symptoms.
The incidence rate per 100,000 persons per year was 1.37 (1.72 for men and 1.05 for women). The incidence rate was highest in people in their 20s, followed in descending order by those in their 30s and then those below 10 years of age and those in their 40s. The prevalence on Jan. 1, 1985, was 56.3 per 100,000 persons. Approximately 36 percent of prevalence cases did not have cataplexy, a higher percentage than found in non-community-based studies.
"Narcolepsy is not a rare disorder," said Michael Silber, M.D., a Mayo Clinic neurologist and sleep disorders specialist and the study's principal author. "Narcolepsy varies in prevalence from country to country. This is the first definitive study in the United States. In a community of one million, about 500 people will have the disorder at any time, and about 14 new cases will develop every year. Sleepiness is a serious cause of social problems, and primary care physicians need to be alert to the possibility of narcolepsy, especially in patients who do not have the relatively easily recognized symptom of cataplexy."