Myoclonus

Epidemiology

Difficulties exist with identifying the prevalence and incidence of myoclonus for a number of reasons.1 Because myoclonus can be a symptom, often an incidental finding that occurs concurrently with other symptoms of a complex disorder, identifying the prevalence is difficult. Epidemiologic studies of other commonly associated conditions, which capture only a "point in time," likely underestimate the prevalence of myoclonus. The results of large clinical trials of patients with myoclonus are tainted by referral bias and study only an undefined population with random ascertainment. Finally, true epidemiologic studies of any cause of myoclonus have looked at a defined population. Although the numbers that result from these epidemiologic studies are not perfect, they do provide a valid estimation, though in all likelihood an underestimation, of the prevalence and incidence of myoclonus.

Caviness et al.2 retrospectively examined a defined population of patients in Olmsted County, Minnesota, and found that the annual incidence of all-cause myoclonus was 1.3 per 100,000 person years and the population prevalence was 8.6 per 100,000. Symptomatic or 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%.

In Finland, Unverricht-Lundborg syndrome, with a prevalence of 5 per 100,000 births, appears to be more common than in other parts of the world and, overall, is more common in children and adolescents than in adults.3

Prevalence of myoclonus in a defined population

Figure legend
Data obtained from Caviness JN, Alving LI, Maraganore DM, Black RA, McDonnell SK, Rocca WA. The incidence and prevalence of myoclonus in Olmsted County, Minnesota. Mayo Clin Proc 1999;74(6):565-569

Prevalence
Patients with Alzheimer's disease have a 40% to 50% cumulative risk of developing myoclonus over the course of their disease (1,150 per 100,000 person years of people with Alzheimer's disease)4, and patients with Creutzfeldt-Jakob disease also have an increased risk of developing myoclonus: 0.05 to 0.1 per 100,000 person years.5 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.7 The ethnic prevalence seems to correlate with the prevalence of intermediate-sized alleles in individual populations. Myoclonus is a common sequelae occurring with postpolio syndrome; 63% of polio survivors report muscle twitching or jumping at sleep onset.8 One in 100,000 people in the general population has hemifacial spasm, which is a form of focal myoclonus that is more common in middle-aged or elderly women.9

Alzheimer's disease 40% - 50%	Early-onset juvenile dentatorubralpallidoluysian atrophy in Japan 0.2-0.7:100,000
Postpolio syndrome 63%	Hemifacial spasm 1:100,000
Corticobasal ganglionic degeneration 50%	Creutzfeldt-Jakob disease 0.05-0.4:100,000 person years