Myoclonus is involuntary, "shock-like", muscle jerking, arrhythmic more often than regular, of central nervous system (CNS) origin. This may be focal, multifocal, or generalized. Multiple irregular asynchronous myoclonic jerks may be termed polymyoclonus. Myoclonus may be characterized in several ways:
- Clinical classification (by observation, examination): Spontaneous
Action or intention: following voluntary action; may be elicited by asking patient to reach out to touch the examiner’s hand
Reflex, stimulus-sensitive: jerks produced by somatesthetic stimulation of a limb, in response to loud noises
- Anatomical/pathophysiological classification (by electrophysio- logical recordings):
Cortical Subcortical/reticular Propriospinal/segmental
- Etiological classification:
Physiological, e.g., "sleep starts" (hypnic jerks)
Essential: in the absence of any other abnormality of the CNS
Epileptic: as a manifestation of idiopathic epilepsy Symptomatic: of other neurological diseases, of which there are many, including:
Anoxic brain injury (Lance-Adams syndrome) Vascular lesions
Encephalopathies: especially of metabolic origin (hepatic, renal), but also toxic, viral, paraneoplastic, mitochondrial
Degenerations: basal ganglia, spinocerebellar
Malabsorption syndromes (coeliac disease, Whipple’s disease)
Storage disorders, for example, Lafora body disease, Tay-Sachs disease, sialidosis
Dementias: Alzheimer’s disease (usually late), prion disease (usually early in sporadic Creutzfeldt-Jakob disease)
The clinical differential diagnosis of myoclonus includes chorea, tic, tremor (especially with rhythmic myoclonus), and certain peripheral nerve disorders (fasciculation, myokymia).
Periodic limb movement disorder or periodic leg movements of sleep, frequently found in association with restless legs syndrome, is sometimes called "nocturnal myoclonus."
Brief lapses of muscle contraction with loss of posture are in some ways the converse of myoclonus and have in the past been labeled "negative myoclonus", although the term asterixis is now preferred.
Drugs useful in the treatment of myoclonus include clonazepam, sodium valproate, primidone, and piracetam. These may need to be given in combination to suppress severe action myoclonus.
Barker R. Myoclonus. Advances in Clinical Neuroscience & Rehabilitation 2003; 3(5): 20,22
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