Myoclonus is a brief, jerklike contraction of a single muscle or muscle group that occurs as an isolated event or may occur in a repetitive regular or irregular manner. Myoclonus may be associated with dementias (e.g., Creutzfeldt-Jakob disease, subacute sclerosing panencephalitis, and Alzheimer disease), lipidoses (e.g., Tay-Sachs and Niemann-Pick diseases), leukodystrophies (e.g., Krabbe and Pelizaeus-Merzbacher diseases), cerebellar degenerations (e.g., Ramsay-Hunt syndrome), epilepsy syndromes (e.g., Unverricht-Lundborg disease, Lafora body disease, and neuronal ceroid lipofuscinosis), Friedreich ataxia, hypoxic and other metabolic encephalopathies (e.g., uremic and hepatic), remote effects of cancer (e.g., infantile myoclonus associated with neuroblastoma), exposure to drugs or toxins (e.g., levodopa, lead, mercury, strychnine, methylphenidate, and amphetamines), and a variety of other disorders. Negative myoclonus, or asterixis, manifests as a sudden loss of postural tone, and has been described in various metabolic or toxic encephalopathies and in certain diencephalic lesions. Segmental or spinal myoclonus is characterized by a rhythmic contraction of a group of muscles in a particular segment, such as an arm, a leg, or the abdominal muscles. Examples include palatal myoclonus, ocular myoclonus, and hiccups. Palatal myoclonus has been described in patients with lesions involving the dentato-rubro-olivary pathway (Mollaret triangle) (Sathornsumetee and Stacy in press). [Video Segment 19]
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