The Association Between Essential Tremor and Parkinson's Disease: A Systematic Review of Clinical and Epidemiological Studies.

Background/Objectives: The objective is to systematically review evidence from clinical and epidemiological studies for or against an association between essential tremor (ET) and Parkinson's disease (PD). Methods: A literature search in PubMed (February 2025) used several combinations of keywords. Thirty-three studies (1960-2023) were identified. Results: The best available data are derived from a population-based study in Spain, followed by a cohort study in the US. Each of these prospective studies provided evidence that ET is a risk factor for PD, with elevated risks of ~4-5. In cross-sectional studies, in which the proportion of PD cases with ET has been reported, the weight of evidence demonstrates an association between ET and PD. In 16 (88.9%) of 18 family studies, the odds ratios or hazards ratios are elevated-i.e., there is considerable evidence that ET is over-represented in PD families and, conversely, PD is over-represented in ET families. Conclusions: A comprehensive review of published data strongly supports an association between ET and PD and, more specifically, provides evidence that ET is a risk factor for PD. Seven of nine review articles (and six of seven non-commissioned review articles) have concluded that there is an association between these two degenerative diseases. The "controversy" that surrounds the ET-PD association is more of a repeated myth than a well-informed reality. As a field, it would be more productive to finally move beyond uniformed debate and focus our efforts on attempts to elucidate the basis for the association to which the data are repeatedly pointing.