CORTICAL TREMOR - A COMMON MANIFESTATION OF CORTICAL MYOCLONUS

Ten patients, three with postural tremor and seven with action myoclon us, had stereotyped involuntary rhythmic movements when attempting to execute a sustained isometric muscle contraction. The movements were c haracterized by rhythmic EMG bursts lasting less than 50 msec and appe aring synchronously in agonist and antagonist muscles at a rate of 9 t o 18 Hz. Backaveraging of the EEG activity related to the onset of the rhythmic EMG bursts identified a cortical potential preceding the EMG bursts in all patients. These symptoms and signs fit the description of ''cortical tremor,'' a variant of cortical reflex myoclonus. Cortic al tremor is common in patients with cortical myoclonus and may be a s ource of functional disability. In two patients in whom we studied the effects of graded levels of isometric force, force recruitment modula ted the abnormal EMG bursting frequency, amplitude, and spatial distri bution of the myoclonic jerks in the activated limb. Transcranial magn etic and electrical stimulation, but not peripheral nerve stimulation, influenced the abnormal EMG bursting pattern, implying a greater depe ndence of this rhythmic phenomenon on a central generator than on peri pheral feedback loops.