Differential diagnosis: no other movement associated with similarly rapid EMG bursts. In addition, tics often more complex; chorea slower, often random in location.

Classification: clinical, anatomic, etiologic (Table 111.1).

Localization principles: rhythmicity almost always denotes segmental origin (brainstem or spinal cord). Generalized appearance suggests brainstem origin. Focal myoclonus cortical or subcortical.

Cortical myoclonus: usually focal, reflex-induced. Cortical origin established by back-averaging EMG jerks with EEG spikes. Often with large somatosensory evoked potentials. May be idiopathic (rhythmic form, also called familial cortical tremor) or associated with other diseases (e.g., corticobasal degeneration). Epilepsia partialis continua:
rhythmic focal jerking; within the cortical myoclonus family.