Abstract
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 ≤ 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. © 2007 Movement Disorder Society