Orthostatic Tremor: A Spectrum of Fast and Slow Frequencies or Distinct Entities?

Abstract

<strong>Background:</strong> Orthostatic tremor (OT) is defined by the presence of a high-frequency (13–18 Hz) tremor of the legs upon standing associated with a feeling of unsteadiness. However, some patients have discharge frequencies of &lt;13 Hz, so-called ‘‘slow OT’’. The aim of this study was to characterize patients with unsteadiness upon standing found to have &lt;13 Hz tremor discharges on neurophysiologic testing. <strong>Methods:</strong> A retrospective review was performed on all subjects with a diagnosis of OT who were referred to the Mayo Clinic, Scottsdale, AZ, between 1999 and 2013 for confirmation using neurophysiology. <strong>Results:</strong> Fourteen of 28 subjects (50%) had OT discharges of &lt;13 Hz, of whom eight had frequencies of &lt;10 Hz and six had frequencies of 10–13 Hz. Lower frequency discharges tended to have a broader spectral peak, greater variability in discharge duration, and lower inter-muscular coherence. Subjects with &lt;13 Hz OT had shorter mean disease duration at time of neurophysiology testing (2.00 years in &lt;10 Hz group, 7.96 years 10–13 Hz group, and 11.43 years &gt;13 Hz; p=0.002). The proportion of subjects who experienced gait unsteadiness (85.7% vs. 66.6% vs. 21.4%; p=0.016), falls (37.5% vs. 50% vs. 0%; p=0.010), and had abnormal gait on examination (71.4% vs. 66.0% vs. 14.3%; p=0.017) was greater in those with low and intermediate frequencies. <strong>Discussion:</strong> Slow tremor electromyography frequencies (&lt;13 Hz) may characterize a substantial proportion of patients labeled as OT. These subjects may have greater gait involvement and higher likelihood of falls leading to earlier presentation to subspecialty care.