Essential tremor and Parkinson's disease: Lack of a link

Background: Essential tremor (ET) is a very common disorder and proving that there is a relationship to another common movement disorder, Parkinson's disease (PD), has been debated for years. Methods: Review of the literature for links between ET and PD primarily focused on neuropathology as well as neurochemistry, epidemiology, genetics, olfactory function, and neuroimaging. Results: While there may be some evidence to suggest an increase in occurrence of PD in people who were previously diagnosed with ET, neuropathologic studies of ET with similarly assessed control subjects do not find an increase in Lewy bodies in the ET group. Studies of incidental Lewy body disease do not find an increase in ET or action tremor compared to controls. ET subjects as a group do not have neurochemical changes that are found in PD, do not respond to medications used to treat PD, are not hyposmic as is found in PD, and neuroimaging studies do not find changes of PD when groups are compared. Conclusion: The overwhelming amount of evidence suggests that any link between ET and PD is coincidental and not biological. Prospective, longitudinal cohort studies with standardized clinical and biomarker assessments followed by neuropathologic confirmation are needed.