Epidemiology

Difficulties exist with identifying the prevalence and incidence of myoclonus for a number of reasons. Because myoclonus may be a symptom of another disorder, it is often recognized along with other symptoms of a complex disorder. Therefore, describing its prevalence is difficult. Epidemiologic studies of other commonly associated conditions probably underestimate the prevalence of myoclonus.

In one study (Caviness et al., 1999) examined a defined population of patients in Olmsted County, Minnesota, and found that the annual incidence of all causes of myoclonus was 1.3 per 100,000 person years and that the population prevalence was 8.6 per 100,000. Secondary myoclonus was the most common type, accounting for 72% of cases, with the majority being associated with Lance Adams syndrome, Alzheimer's disease, and Creutzfeldt-Jakob disease. Myoclonic epilepsy comprised 17% of cases, with essential myoclonus accounting for the remaining 11%.

Prevalence

Patients with Alzheimer's disease have a 40% to 50% cumulative risk of developing myoclonus over the course of their disease, and patients with Creutzfeldt-Jakob disease also have an increased risk of developing myoclonus. In corticobasal ganglionic degeneration, 50% of patients have myoclonus at some time in the course of their disease.6 In Japan, the prevalence of early-onset juvenile dentatorubralpallidoluysian atrophy is 0.2 to 0.7 per 100,000; it is lower in Western countries, occurring in less than 1% of people with SCA. Myoclonus is a common outcome occurring in people with postpolio syndrome; 63% of polio survivors report muscle twitching or jumping at sleep onset. One in 100,000 people in the general population has hemifacial spasm, which is a form of localized myoclonus that is more common in middle-aged or elderly women.

	Diagnosis