To evaluate the association between type 2 diabetes (T2D) and Parkinson's Disease (PD) development in a population of patients with a history of essential tremor (ET).

T2D has been proposed to be a risk factor for developing PD in the general population, with a meta-analysis of several large-scale studies demonstrating a statistically significant increased risk. Insulin resistance, mitochondrial dysfunction, and aberrant expression of amyloidogenic proteins have been proposed and shown as possible links in basic science studies. We offer to examine the association of T2D and the development of PD in a cohort of patients with prior ET.

We performed a retrospective cohort study from the Geisinger Health System in central Pennsylvania. We looked at patients with an ET diagnosis made by a neurologist, and patients were designated as “T2D” or “no T2D”. Any diagnosis of subsequent PD development was made by a neurologist. Exclusion criteria included patients with vascular, secondary, atypical, and drug-induced parkinsonism. Cox Proportional Hazard Regression Model was fit treating time, starting with the ET diagnosis until PD as the outcome or censoring follow-up time at the date of the last encounter. The proportional hazards assumption was tested and confirmed.

 A total of 3,226 patients with ET were identified, with 331 going on to develop PD. T2D presented in 1,274 patients, and 200 developed PD. Those with a history of T2D showed an adjusted hazard ratio of 1.68 (95% CI 1.18 - 2.38, p = 0.003). The analysis used stratification by age, hyperlipidemia, hypertension, stroke, and REM behavior disorder. We adjusted for gender, asthma, and family history of tremors and/or parkinsonism, and appendectomy.

Overall we found a 68% increased risk for PD development in patients with ET and T2D after adjustments. Further studies are needed to evaluate this association.