How familial is familial tremor?

Essentia1 tremor (ET) is commonly assumed to be partly genetic. This is due to a seemingly excess aggregation within certain families. There are many families containing more than one member with ET, and some large kindreds with multiple affected individuals. Despite this, the proportion of ET cases with affected family members is not known. Estimates vary from 17 to 100%. Additionally, none of the published studies employed control subjects. Therefore, it is unknown to what extent ET aggregates within families beyond that expected by chance, and the importance of genetic susceptibility in the etiology of ET at the population level has not been established. Additionally, whether or not some clinical subtypes of ET are more likely than others to be influenced by genetic susceptibility is unclear. Some studies suggest that early-onset ET may be familial, although increased awareness and earlier recognition of symptoms is an issue. Although many data support the view that ET susceptibility is inherited in an autosomal dominant manner, most studies are kindreds or service-based studies that might favor selection of families with apparent autosomal dominant modes of inheritance. One community-based study suggested an autosomal dominant mode of inheritance, but this has not been confirmed in studies of more heterogeneous populations. Further understanding of the extent of familial aggregation, extent of genetic heterogeneity, and mode of inheritance is essential for clinical counseling and for further research aimed at localizing and identifying susceptibility genes.