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

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). A literature search in PubMed (February 2025) used several combinations of keywords. Thirty-three studies (1960-2023) were identified. 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. 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.