Essential tremor frequency decreases with time

To quantify the extent to which tremor frequency changes with time in patients with essential tremor.

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.

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.

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).

The frequency of essential tremor decreases by approximately 0.06 to 0.08 Hz/year. This decrement in frequency is consistent with the linear relationship between age and tremor frequency.