Rhythmic myoclonus may be described as a regularly recurrent muscular contraction, without opposing movements of antagonistic muscles. It is to be distinguished from nystagmus, in which slow motion of one muscle group is followed by a quick rectifying motion of an opposing muscle group, and from tremor, in which antagonistic groups of muscles contract irregularly and in opposite directions. It also differs from both of these and from all other myoclonic movements in its nearly perfect rhythmicity and its persistence during sleep. It can temporarily be suspended by voluntary contraction of the affected muscles.
It is observed most frequently in muscles of the oropharynx but is also found in muscles of the face and eyes, the larynx, the diaphragm and the intercostal muscles and more rarely in the muscles of the upper extremities.
Correlation of this clinical manifestation with definite pathologic alterations was established by Guillain, Mollaret and Bertrand,1 who
Dobson JP, Riley HA. RHYTHMIC MYOCLONUS: A Clinical Report of Six Cases. Arch NeurPsych. 1941;45(1):145–150. doi:10.1001/archneurpsyc.1941.02280130155011
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