[Orthostatic tremor: clinical aspects, pathophysiology and therapy].

Orthostatic tremor is characterized by its isolated occurrence in leg and trunk muscles during standing with undisturbed sitting, lying and walking. In a female patient with this tremor syndrome the basic electrophysiological feature of muscle activity was a 16 Hz, highly synchronized tremor in all leg muscles and sometimes in the arm muscles. This rhythmic EMG activity however, was not restricted to stance but occurred during all kinds of muscle activation in sitting, lying or standing positions, despite only standing was accompanied by a subjective sensation of unsteadiness and falling to the ground. Mechanical tremor analysis at the patella revealed an additional 8 Hz tremor caused by alternating large and small amplitudes of the 16 Hz tremor bursts. The occurrence of the 8 Hz tremor was much more related to the feeling of unsteadiness than the 16 Hz tremor. Single motor units mostly fired at a frequency of 8 Hz, but only at the time of tremor bursts. Hence the 16 Hz-pattern may not be explained as the result of a pure motoneuronal abnormality. There were no indications for abnormal reflexes contributing to tremor genesis. A fixed time relation of the tremor bursts in different muscles has been found suggesting a common generator within the CNS for the tremor. After successfully treatment with Primidon the pattern of muscle activation was normalized during sitting and lying, however, during standing and walking the 16 Hz tremor was still present. We believe that an unknown central oscillator is causing the tremor and central structures which are involved in stance regulation have a predominant access to switch on this oscillator.