Myoclonus

Definition Myoclonus is defined as brief, involuntary shocklike movements. These brief jerks may be due to the contraction of one or more muscles, or occur by quick lapses of muscle contraction in active postural muscles (negative myoclonus, asterixis) [1].Using a medical records linkage system, Caviness and colleagues studied the incidence and prevalence of myoclonus in Olmstead County, Minnesota, from 1976 to 1990. They reported an average annual incidence rate of 1.3 cases per 100 000 person-years. The prevalence in 1990 was 8.6 cases per 100 000. The prevalence of myoclonus is smaller than that of other hyperkinetic movement disorders such as essential tremor, which has an estimated prevalence as high as 50.5 cases per 1000 [2].Marsden and colleagues proposed a classification system in 1982, which has been revised by several authors since. Myoclonus can be classified on the basis of clinical presentation (phenomenology), anatomy or pathophysiology, or etiology. It is useful to approach myoclonus from a combination of these classifications as it helps in deciding on treatment options. Classification Phenomenology Clinically, there is a wide expression of myoclonus. Jerks can be focal or segmental, affecting one part or region of the body; they can be multifocal, affecting different parts at different times; or generalized. If they occur repetitively in rhythmic fashion, they can appear like a tremor. Myoclonus may be present with action, or be stimulus-provoked.