Orthostatic Tremor Is Evoked by Muscle Load Without the Need for Orthostatic Position.

backgroundPrimary orthostatic tremor (POT) is a rare movement disorder characterized by a high-frequency tremor and a considerable feeling of unsteadiness. People with POT are significantly affected in their daily activities and have reduced quality of life. The tremor occurs in standing position and dominates in the lower extremities. However, whether an orthostatic position is essential for evoking the tremor has not been fully clarified.

objectivesTo investigate how POT is responding to muscle load in non-orthostatic positions.

methodsIn this work, we measured the tremor in POT patients using a smartphone running a tremor analysis application attached to the proximal fibula in supine, seated and standing position, as well as when they were subjected to muscle load in a leg press machine in a seated position.

resultsWe demonstrate that the tremor can be elicited by muscle load while the patient is sitting, compatible with a weight-bearing isometric tremor, without the need to be in standing position. Furthermore, the weight of the load modulates the amplitude of the elicited tremor, but not its frequency.

conclusionThese findings suggest that POT is a weight-bearing hyperkinetic disorder and challenge the conventional assumption that the POT patient must be in standing position for the tremor to fully manifest. The results potentially have implications for understanding the mechanisms underlying POT, and can be of importance in future experimental studies, for example MRI, when standing position is not an available option.