Essential Tremor

Tremor Classification Based on Relative Frequency

Tremors may also be classified based on the number of repetitions of complete waveforms per second. Physiologic and enhanced physiologic tremors have a relatively high frequency, often ranging from approximately 7 to 12 cycles per second. In contrast, the rate of many pathologic tremors is about 2 to 7 Hz. Primary orthostatic tremor is a notable exception, with a 13 to 18 Hz entrainment of motor unit activity. In addition, cortical tremor, recognized as a form of rhythmic myoclonus, is characterized by irregular, postural and kinetic tremor-like jerks with a range between 7 to 18 Hz.

Cerebellar and rubral or Holmes tremors tend to be of relatively low frequency, primarily occurring below 4.5 or 5 Hz. The pure rest tremor seen in some with PD is characteristically greater than 4 Hz, although the upper frequency limit of these tremors has not been firmly established. For those parkinsonian patients with a combination of rest and postural/kinetic tremors, relative frequencies may be similar—or postural/kinetic tremors may have a frequency that is more than 1.5 Hz higher as related to rest tremor rate.

In classic ET, the rate of tremor may vary between 4 to 12 Hz, with older patients typically demonstrating tremor frequencies toward the lower range. In contrast, younger patients with mild ET tend to have tremor frequencies that extend into the 7 to 12 Hz range of physiologic tremor.