Lack of Familial Aggregation of the "Essential-Tremor-Plus" Phenotype in Familial Essential Tremor

Background: Essential tremor (ET) is a highly prevalent neurological disease that frequently runs in families. A recent and controversial proposal is to separate ET patients into two distinct groups - ET vs. ET-plus. If this were a valid construct, one would expect in familial aggregation studies to observe that ET-plus would cluster in some families yet be absent in others, rather than being randomly distributed across families. We examined whether there is evidence of familial aggregation of ET-plus. Methods: Probands (n=84 [56 ET-plus and 28 ET]) and their first- and second-degree relatives (n=182 and 48) enrolled in a genetics study. Chi-squares and generalized estimating equations (GEE) tested associations between probands' ET-plus status and the ET-plus status of their relatives. Results: Chi-squares analyses revealed that ET-plus was no more prevalent in relatives of probands diagnosed with ET-plus than in relatives of probands diagnosed with ET, p>0.05. Restricting relatives to first-degree relatives similarly did not detect a significant association (p = 0.88). GEE yielded similar results (respective p's = 0.39 and 0.81). Conclusion: The data demonstrate that ET-plus does not seem to aggregate in families. As such, they do not lend support to the notion that ET-plus is a valid biological construct.