Observations on essential tremor.

Essential tremor, which is also known as familial tremor or, when it appears for the first time in old age, as senile tremor, has attracted interest out of proportion to its incidence or importance as a cause of disability. Critchley, surveying the subject in 1949, was able to provide over 100 references to the literature. This interest stems from the unusual features of the condition. It is not, so far as we know, the result of any disease or injury to the nervous system, but appears in otherwise healthy people, persisting throughout their lives, and being unassociated with other signs or symptoms. One important fact is that, though sporadic cases are by no means rare, there is clearly a strong genetic determinant in the condition. Apart from this last point, hypotheses have tended to exceed facts in the voluminous literature on the subject. The present study arose during observations on the physiological tremor which accompanies muscular activity in normal people. This tremor, which affects all parts of the body (Marshall and Walsh, 1956), has a frequency of about 10 cycles per second (c/s) in the young adult. In children, however, the common frequency is 5 or 6 c/s, a change to the adult frequency occurring about puberty (Marshall, 1959). From the age of about 40 years onwards the adult frequency declines from 10 c/s to 6 c/s (Marshall, 1961). Since the frequency of physiological tremor varies with age, it is clear that frequency cannot be the sole criterion by which essential tremor may be distinguished. The present study was therefore undertaken to define more closely the features of essential tremor and to clarify its relationship to physiological tremor.