Prevalence and features of unreported dystonia in a family study of "pure" essential tremor

Background: Essential tremor (ET) is considered to be a highly heritable disorder, yet no susceptibility genes have been identified. The search for ET genes is severely hampered by clinical and genetic heterogeneity; the existence of this heterogeneity complicates the genetic analyses. We sought to determine the prevalence and clinical features of unreported dystonia in a family study of "pure" ET.

Methods: ET probands and their reportedly affected first-and second-degree relatives were enrolled in a genetics study, the Family Study of Essential Tremor (FASET) at Columbia University Medical Center. The goal was to enroll cases with "pure" ET (i.e., ET without dystonia or other neurological problems). Each enrollee underwent a detailed neurological evaluation.

Results: There were 100 enrollees (28 probands, 72 relatives). Dystonia (primarily torticollis) occurred in 9 (32.1%) of 28 families, with 5 cases in one family, 2 cases in two families, and 1 case in six families. Those affected with dystonia included 3 (10.7%) probands and 12 (16.7%) relatives. There was a gender predilection: 14/15 (93.3%) with dystonia vs. 41/85 (48.2%) without dystonia were women (p = 0.001). Dystonia was previously undiagnosed in 14/15 (93.3%) cases.

Conclusions: Dystonia (esp. torticollis in women) was present in nearly one-third of the ET families in a genetics study, including 10.7% of ET probands. Dystonia was unreported and previously undiagnosed in nearly all of these individuals. The overarching biological issue is whether ET and dystonia should be regarded as one disease or two; this has obvious implications for the structuring of analyses in genetic studies.