Objective: To clarify the long-term course of a case with primary orthostatic tremor (OT) using surface electromyography (SEMG) and stabilometry.

Background: OT is characterized by leg tremor with the distinctive frequency range of 13-18 Hz and subjective unsteadiness only on standing still but absent when sitting or lying, improved by walking or leaning. The natural history of OT has not been systematically studied, and whether OT is a progressive disorder or not remains controversial.

Method: A female case with OT in her forties was diagnosed according to the established diagnostic criteria. SEMG were performed at diagnosis and at follow-up visits up to four years after diagnosis with various conditions such as stance width. Stabilometry were also performed at diagnosis and until four years later to measure the objective unsteadiness with her eyes open and closed. The data of SEMG were analyzed using CED Spike 2 software.

Results: Diagnosis of OT was confirmed with the SEMG findings which determined the frequency of the muscle discharge to be 16 Hz. Clonazepam was started and the frequency decreased to 14 Hz. Follow-up SEMGs performed until four years after diagnosis demonstrated the frequencies of muscle discharge to be around 14 Hz, however, the frequencies were dependent on the recording conditions. Subjective unsteadiness improved after clonazepam was initiated, and stabilometric recordings also showed the improvement. After four years since diagnosis, objective unsteadiness measured by stabilometry worsened without apparent change of subjective unsteadiness or her activities of daily living.

Conclusion: The frequency of muscle discharges of our OT case has not changed during the course of four years of follow-up, but it seemed to be dependent more on the recording conditions. The stabilometric findings worsened after four years, however, subjective unsteadiness did not change. It was suggested that it would be difficult to decide whether our OT case is progressive or not from our result.
These data are also presented at the 14^th Congress of the Movement Disorder Society of Japan on July 2-4, 2020.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/long-term-course-of-a-case-with-primary-orthostatic-tremor/