Essential Tremor Families With Heterogeneous Motor Phenomenology: “Mixed Motor Disorder”

Objective:Essential tremor (ET) is one of the most prevalent movement disorders. Because ET is so common, individuals with other neurological disorders may also have ET. There is evidence, however, that the co-occurrence of ET with Parkinson’s disease (PD) and/or dystonia is not merely a chance co-occurrence. We have observed combinations of these three movement disorders within individuals and across individuals within families containing multiple individuals with ET. This observation has a number of implications. Our objective is to present four ET families in whom motor phenomenology was heterogeneous, and discuss the implications of this finding.Methods:ET cases and their relatives were enrolled in the Family Study of Essential Tremor (2015 – present). Phenotyping was performed by a senior movement disorders neurologist based on neurological examination.Results:We present four families, including 14 affected individuals, among whom assigned diagnoses were ET, PD, ET + PD, and ET + dystonia. In those with ET and another movement disorder, the predominant and earliest phenotype was ET.Conclusion:There are assortments of these three involuntary motor disorders, ET, dystonia and PD, both within individuals and in different individuals within ET families. This observation has mechanistic implications. Furthermore, we believe the concept of the “mixed motor disorder” should enter into and inform the clinical dialogue. In assigning diagnoses, clinicians are swayed by family history information, and they should be prepared a mix of different motor disorders to manifest within particular families.