The incidence of PD is suggested to be elevated among ET patients. In 2008, Minen and Louis identified 53 patients with ET who developed PD, of which 30.8% developed PD in under five years. However, no large-scale study to date has assessed the demographic features of subjects with ET who develop PD.

A total of 3226 patients with ET were included, of which 331 patients were subsequently diagnosed with PD. 39.3% of ET patients who developed PD were diagnosed in under one year from the date of ET diagnosis; these subjects were significantly older than subjects with only ET, with mean ages 68.6 (95% CI 66.8-70.4) and 61.2 (95% CI 59.0-63.3) respectively. In total, during all follow-up, 20.8% of patients with a family history of tremor or parkinsonism developed PD, compared to 9.4% without positive family history, and 11.4% of male and 9.2% of female patients with ET developed PD.

Our findings suggest that once diagnosed with ET, gender, family history of tremor or parkinsonism, and especially age 65 or older, are associated with an increased risk of subsequent PD diagnosis. Our findings indicate that new-onset action tremor in patients 65 or older often reflects early PD and should at least prompt closer follow-up for monitoring these higher risk patients.