Subclinical tremor in normal controls with versus without a family history of essential tremor: data from the United States and Turkey

Background and purpose: Mild action tremor is very common in the population. One fundamental question is whether this tremor is related to the neurological disease essential tremor (ET), which occurs in a much smaller segment of the population? ET is often genetic, and variable phenotypic expression is well-documented in the literature. We determined whether normal controls who report a family history of ET have greater action tremor than normal controls who do not report such a history. Methods: Controls, enrolled in two epidemiological studies (New York and Turkey), were examined in detail and action tremor was rated using a valid and reliable clinical rating scale, resulting in a total tremor score (range 0-36). Results: In New York, the total tremor score was higher in 44/406 (10.8%) controls who reported a family history of ET than in 362/406 controls with no such history (4.25 +/- 2.51 vs. 3.78 +/- 2.93, P = 0.02). Controls who reported a first-degree relative with ET had the highest total tremor scores. In Turkey, the total tremor score was higher in 7/89 (7.9%) controls with a family history than in 82/89 controls with no family history (3.43 +/- 4.54 vs. 1.13 +/- 2.54, P = 0.048). All affected relatives in Turkey were first-degree. Conclusions: These data suggest that some of the normal tremor exhibited by people in the population is likely to be subclinical, partially expressed ET and that the sphere of ET is wider than is apparent from a consideration of clinically diagnosed cases.