The incidence and prevalence of myoclonus in Olmsted County, Minnesota

Objective: To study the incidence and prevalence of diagnosed myoclonus in Olmsted County, Minnesota. Little is known about the frequency and distribution of myoclonus in the general population. Design: Descriptive study with case ascertainment through a records-linkage system. Patients and Methods: We used a medical records-linkage system to identify all subjects whose records contained documentation of myoclonus or of diseases known to exhibit myoclonus. The records of all potential patients were reviewed by a neurologist, and only patients with pathologic and persistent myoclonus were included. Population denominators were derived from census data. Results: The average annual incidence rate of pathologic and persistent myoclonus for 1976 through 1990 was 1.3 cases per 100,000 person-years. The rate increased with advancing age and was consistently higher in men. Symptomatic myoclonus was the most common type, followed by epileptic and essential myoclonus; dementing diseases were the most common cause of symptomatic myoclonus. The lifetime prevalence of myoclonus, as of January 1, 1990, was 8.6 cases per 100,000 population, and the prevalence increased with advancing age. Conclusion: Although our figures are probably underestimated, they are the first attempt, to our knowledge, to measure myoclonus morbidity in the general population. We found clinical features and age and sex distributions different from those previously described in clinical series.