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Myoclonus is a brief muscle jerk caused by neuronal discharges. A sudden and short-lasting interruption of ongoing voluntary muscle contraction may produce a postural pause clinically very similar to myoclonus, hence the term “negative myoclonus.” Both forms often share the same etiology, coincide in the same patients, and can even affect the same muscle group.1 A myoclonic jerk consists of a single muscle discharge but can be repetitive, giving rise to a salvo of muscle activity (Fig. 37–1). The latter is particularly frequent in action myoclonus and interferes severely with the execution of even the most simple motor tasks. For this reason, action myoclonus (both positive and negative types) may be considered among the movement disorder that produces the greatest interference with voluntary movements.

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Figure 37–1.
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EMG recording from several muscles in the right arm of a patient with action myoclonus (Ramsay Hunt syndrome). From top to bottom are deltoid, biceps, triceps, finger extensors, and finger flexors. The horizontal scale bar indicates 2 seconds. The insert on the right shows the EMG discharges in more detail. Scale bar = 100 ms. The salvo of repetitive myoclonic discharges associated with silent periods is clearly seen in the proximal arm muscles. (From Obeso et al.9 with permission.)

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When including all known etiologies, myoclonus has an average annual incidence of 1.3 cases per 100,000.2

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Myoclonus can be categorized from various points of view (Table 37–1) with no definitive and universally accepted classification. The major categories are clinical presentation, neurophysiological origin, and etiology. According with clinical presentation, myoclonus may occur spontaneously, may be triggered by external stimulation ("reflex"), or may be induced during voluntary muscle activation ("action"). The distribution may be focal, segmental, generalized, or multifocal. The timing of myoclonus can be rhythmic or irregular.

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Table 37–1. Classification of Myoclonus
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The origin of the abnormal neuronal discharge giving rise to myoclonus can be ascertained as cortical, subcortical (brainstem), or spinal. The etiological classification of myoclonus has no simple approach and will be discussed in this chapter.

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Myoclonus must be differentiated from other dyskinesias, such as tics, chorea, postural tremor, dystonia, and hemifacial spasm.3 Tics, as present in Tourette's syndrome, are frequently as brief as myoclonus. The main elements to distinguish between these two categories of muscle jerks include the following. (1) Myoclonus usually interferes notably with voluntary movement and is aggravated by action, whereas tics almost never disrupt motor acts. (2) Tics can be voluntarily suppressed and myoclonus cannot. (3) A high proportion of patients will feel a somesthetic sensation ...

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