Increased risk of essential tremor in first-degree relatives of patients with Parkinson's disease.

We conducted a historical cohort study of 981 first-degree relatives of 162 patients with Parkinson's disease (PD) and of 838 first-degree relatives of 147 controls representative of the population of Olmsted County, Minnesota. In addition, we studied 2,684 first-degree relatives of 411 patients with PD referred to the Mayo Clinic. Relatives were interviewed and screened for tremor either directly or through a proxy, and those who screened positive were examined or copies of their medical records were obtained to confirm the diagnosis of essential tremor (ET). We also obtained ET information from a medical records-linkage system (family study method). In the population-based sample, the risk of ET was significantly increased for relatives of patients with onset of PD<or=66 years (first tertile; hazard ratio [HR]=2.24; 95% confidence interval [95% CI]=1.26-3.98; P=0.006). In the referral-based sample, the risk of ET among relatives increased with younger onset of PD in patients (linear trend; P=0.001), and was higher in relatives of PD patients with the tremor-predominant or mixed form when compared with relatives of patients with the akinetic-rigid form, and in men compared with women. These findings suggest that PD and ET may share familial susceptibility factors. Copyright (c) 2007 Movement Disorder Society.