Introduction : Orthostatic tremor develops in the legs while standing up with no weakness, pain or imbalance in theleg and the tremor is characteristically not observed when walking. However there have been some confusions aboutorthostatic tremor in several aspects. For the past ten years, we have observed 4 patients with orthostatic tremor. In eachcase tests were performed to investigate the following three important areas of inquiry about orthostatic tremor. Firstly,whether this disorder is an independent diagnostic entity or a variant of essential tremor. Secondly, whether the progressof this disorder is specifically related with standing posture. Lastly, the nature of the pathophysiologic mechanismbehind the appearance of the tremor when standing after the lapse of a certain latent period and its disappearance uponthe commencement of walking.Methods : Our 4 cases of orthostatic tremor were studied clinically, electrophysiologically, and pharmacologically.Electrophysiological tests included tremor spectrum test and electromyography.Results : We observed the presence of this tremor in several other tonic postures, as well as its absence, in a verticallylifted position from all our cases. Our cases registered a variable tremor frequency between 5 and 12 Hz according tothe tremor spectrum test and EMG. Furthermore all our 4 cases demonstrated patterns of both synchronous EMG activityand alternating EMG activity at various times in homologous muscles of both legs. Orthostatic tremor was improvedsignificantly with propranolol as well as clonazepam.Conclusions : From the results of our study we drew the following conclusions. It is probable that orthostatic tremoris simply a variant of essential tremor rather than being an independent diagnostic entity and that in most cases itsdevelopment is specifically related with muscle contraction rather than merely with the act of standing. Furthermore wediscovered a clue in the previously described neural control mechanism that the nuclear bag fibers in the muscle spindlehave lag time of several seconds in their response to muscle strength and that their baseline does not reset fully in rapidlymoving muscle. This neural control mechanism could offer sufficient explanation for the phenomena of tremorappearance when standing and disappearance when walking in orthostatic tremor.
