Essential tremor frequency decreases with time

Objective: To quantify the extent to which tremor frequency changes with time in patients with essential tremor. Background: Tremor frequency tends to be lower in older patients. The author's previous study of 18 patients with essential tremor produced evidence that tremor frequency decreases slowly over a period of 4 to 8 years. A decrement in frequency will increase tremor amplitude because there is less attenuation of lower-frequency tremor by the low-pass filtering properties of muscle and limb mechanics. Methods: Nineteen women and 25 men with essential tremor and no other neurologic conditions were followed for 4 years. Accelerometry and surface electromyography (EMG) were used to measure hand tremor and motor unit entrainment in the extensor carpi radialis brevis every 2 years. Tremor frequency was computed from the spectral peak in the rectified filtered EMG spectrum under the condition of 300-gram loading. Results: The patients' mean +/- SD age was 68.0 +/- 9.95 years. The mean tremor frequency at baseline was 5.79 +/- 1.32 Hz. The mean decrement in tremor frequency over 4 years was 0.332 Hz (95% CI = 0.141 to 0.523) and was 0.270 Hz (95% CI = 0.122 to 0.418) when a 61-year-old outlier patient was excluded. Tremor frequency and patient age were linearly related: frequency = -0.061(age) + 9.94 (r = 0.459; p < 0.002). Conclusions: The frequency of essential tremor decreases by <similar to>0.06 to 0.08 Hz/year. This decrement in frequency is consistent with the linear relationship between age and tremor frequency.