Myoclonus is defined as brisk, shock-like, involuntary movements. Hiccup and nocturnal myoclonus are physiological myoclonus. Myoclonus is usually caused by abrupt, instantaneous contraction of muscles (positive myoclonus), but it can be also caused by transient interruption of the ongoing muscle contraction (negative myoclonus, asterixis). Myoclonus is usually irregular, but some myoclonus may occur at certain intervals (rhythmic or periodic myoclonus). Myoclonus is caused by excessive excitation of motor neurons in the motor cortex (cortical myoclonus, epileptic myoclonus), in the brainstem, or in the spinal cord (spinal myoclonus). Cortical myoclonus is commonly associated with convulsive seizures and is also called epileptic myoclonus. Syndromes characterized by myoclonus, generalized convulsion, and cognitive impairment are coined the progressive myoclonus epilepsies and are caused by multiple neurodegenerative diseases.
