Essential tremor most often makes its appearance late in the second decade, but it may begin in childhood and then persist. A second peak of increased incidence occurs in adults older than 35 years of age. It is a relatively common disorder, with an estimated prevalence of 415 per 100,000 persons older than the age of 40 years (Haerer et al). As described by Elble, the tremor frequency diminishes slightly with age while its amplitude increases. The tremor practically always begins in the hands and is said to be symmetrical; in approximately 15 percent of patients, however, it appears first in the dominant hand and an emerging concept has been that it is more often asymmetric than stated in older descriptions. It is also possible, of course, that the patient does not find a mild bilateral tremor troublesome until it affects activities that are dependent on the dominant hand. However, a severe isolated arm or leg tremor, or a predominant finger tremor, should still suggest another disease (Parkinson disease or focal dystonia, as described further on).
The tremor may remain limited to the upper limbs or to a side-to-side or nodding movement of the head; tremor of the chin may be added or may occur independently. In certain cases of essential tremor, there is additional involvement of the jaw, lips, tongue, and larynx, the latter imparting a severe quaver to the voice (voice tremor). Infrequently, the tremor of the head or voice precedes that of the hands. The head tremor is also postural in nature and disappears when the head is supported. It has also been noted that the limb and head tremors tend to be muted when the patient walks, in contrast to most parkinsonian tremors. In some of our patients whose tremor remained isolated to the head for a decade or more, there has been little if any progression to the arms and almost no increase of the amplitude of movement.
The lower limbs are usually spared or only minimally affected. In the large series of familial tremor cases by Bain and colleagues, solitary jaw or head tremor was not found but we have observed isolated head tremor, as noted. Most patients with essential tremor will have identified the amplifying effects of anxiety and the ameliorating effects of alcohol on their tremor. We have also observed the tremor to become greatly exaggerated during emergence from anesthesia in a few patients.
Electromyographic studies reveal that the tremor is generated by more or less rhythmic and almost simultaneous bursts of activity in pairs of agonist and antagonist muscles (Fig. 4-7B). Less often, especially in the tremors at the lower range of frequency, the activity in agonist and antagonist muscles alternates (“alternate beat tremor”), a feature more characteristic of Parkinson disease, which the tremor then superficially resembles (see below). Tremor of either pattern may be disabling, but the less common, slower, alternate-beat tremor tends to be of higher amplitude, is more of a handicap, and is usually more resistant to treatment.
Pathophysiology To date, only a few cases of essential tremor have been examined postmortem, and these have disclosed no consistent lesion to which the tremor could indisputably be attributed (Herskovits and Blackwood; Cerosimo and Koller). A singular case of a 90-year-old woman studied by Louis and colleagues demonstrated more extensive cerebellar cortical and dentate nucleus cell loss and reactive changes than had been previously reported.