title: Orthostatic Tremor: A Spectrum of Fast and Slow Frequencies or Distinct Entities?
abstract: BACKGROUND
  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 <13 Hz, so-called "slow OT". The aim of this study was to characterize patients with unsteadiness upon standing found to have <13 Hz tremor discharges on neurophysiologic testing.
  
  
  METHODS
  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.
  
  
  RESULTS
  Fourteen of 28 subjects (50%) had OT discharges of <13 Hz, of whom eight had frequencies of <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 <13 Hz OT had shorter mean disease duration at time of neurophysiology testing (2.00 years in <10 Hz group, 7.96 years 10-13 Hz group, and 11.43 years >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.
  
  
  CONCLUSIONS
  Slow tremor electromyography frequencies (<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.
authors: Department of Neurology, Movement Disorders Center, Mayo Clinic, Scottsdale, AZ, USA ; Division of Neurology, Dalhousie University, Halifax, NS, Canada.
authors: Heather B
authors: HB
authors: Rigby
authors: Department of Surgery, Dalhousie University, Halifax, NS, Canada.
authors: Matthew H
authors: MH
authors: Rigby
authors: Department of Neurology, Movement Disorders Center, Mayo Clinic, Scottsdale, AZ, USA.
authors: John N
authors: JN
authors: Caviness
keywords: Orthostatic tremor; neurophysiology