<unk>  is brief, involuntary twitching of a muscle or a group of muscles. It describes a medical sign (as opposed to symptom) and, generally, is not a diagnosis of a disease. The myoclonic twitches or jerks are usually caused by sudden muscle contractions; they also can result from brief lapses of contraction.  Contractions are called positive<unk> ; relaxations are called negative<unk> .  The most common time for people to encounter them is while falling asleep (hypnic jerk), but myoclonic jerks are also a sign of a number of neurological disorders.  Hiccups are also a kind of myoclonic jerk specifically affecting the diaphragm.
Myoclonic jerks may occur alone or in sequence, in a pattern or without pattern. They may occur infrequently or many times each minute. Most often,<unk>  is one of several signs in a wide variety of nervous system disorders such as multiple sclerosis, Parkinson's disease, Alzheimer's disease, Subacute sclerosing panencephalitis and Creutzfeldt-Jakob disease and some forms of epilepsy.
In almost all instances in which<unk>  is caused by CNS disease it is preceded by other symptoms; for instance, in CJD it is generally a late-stage clinical feature that appears after the patient has already started to exhibit gross neurological deficits.
Anatomically,<unk>  may originate from lesions of the cortex, subcortex or spinal cord. The presence of<unk>  above the foramen magnumeffectively excludes spinal<unk> , but further localisation relies on further investigation with electromyography (EMG) and electroencephalography (EEG).
Myoclonus
Chorea
Opisthotonus
Lenticonus
Dystonia
Myotonia
Pseudoathetosis
Hypotonia