Risk of incident Parkinson's disease and parkinsonism in essential tremor: a population based study

Background: A longstanding literature suggests an association between essential tremor (ET) and Parkinson's disease (PD). However, the risk of incident PD has not been quantified in cases of ET compared with controls. Objective: To estimate the risk of incident PD in a population based cohort study of 3813 older people (including ET cases and controls) in central Spain. Results: After a median of 3.3 years, 12 (5.8%) of 207 ET cases developed parkinsonism compared with 56 (1.6%) of 3606 controls (adjusted relative risk (RR) 3.47, 95% CI 1.82 to 6.59; p<0.001). Six (3.0%) of 201 ET cases developed incident PD versus 24 (0.7%) of 3574 controls (adjusted RR 4.27, 95% CI 1.72 to 10.61; p=0.002). Conclusions: Patients with ET were four times more likely than controls to develop incident PD during prospective follow-up. These data confirm and begin to quantify the link between these two diseases.