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BackgroundThere is longstanding controversy surrounding the possible link between essential tremor (ET) and Parkinson’s disease (PD). Inconsistent and unreliable diagnostic criteria may in part account for some of the difficulties in defining the relationship between these two common movement disorders.MethodsReferences for this systematic review were identified using PubMed with the search terms “essential tremor” AND “Parkinson’s disease” with articles published in English between 1960 and September 2018 included.ResultsIn this review we provide evidence that some patients diagnosed with ET have an increased risk of developing PD years or decades after onset of action tremor. There are several still unresolved questions about the link between the two disorders including lack of verifiable diagnostic criteria for the two disorders and marked overlap in phenomenology. Here we review clinical, epidemiologic, imaging, pathologic, and genetic studies that address the ET–PD relationship. Several lines of evidence support the association between ET and PD, including overlapping motor and non-motor features, relatively high prevalence of rapid eye movement sleep behavior disorder (26–43%) in ET patients, increased prevalence of PD in patients with longstanding antecedent ET, increased prevalence of ET in family members of patients with PD, and the presence of Lewy bodies in the brains of some ET patients (15–24%).DiscussionThere is a substantial body of evidence supporting the association between ET and PD within at least a subset of patients, although the nature and possible pathogenic mechanisms of the relationship are not well understood.

Despite numerous efforts to identify specific and sensitive biomarkers, the diagnosis of Parkinson's disease (PD) is still based on clinical criteria that include the presence of a combination of cardinal motor features (tremor, rigidity, bradykinesia, and postural instability), other motor features (including freezing of gait and abnormal postures), and numerous nonmotor features. In addition, the presence of atypical features may suggest an alternative diagnosis. Levodopa therapy remains the gold standard in the management of motor features of PD. New formulations of levodopa and novel delivery systems are currently being evaluated and gradually introduced in clinical practice in an attempt to prevent or treat levodopa-related motor complications. Dopamine agonists also play an important role as monotherapy in mild or adjunctive therapy in moderately advanced disease. As the disease progresses and patients develop complications from levodopa therapy, specifically motor fluctuations and dyskinesias, deep brain stimulation becomes an alternative therapeutic option. Clinical trials of experimental therapeutics are currently fueling the PD therapeutic pipeline.

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Digital health technologies can provide continuous monitoring and objective, real-world measures of Parkinson’s disease (PD), but have primarily been evaluated in small, single-site studies. In this 12-month, multicenter observational study, we evaluated whether a smartwatch and smartphone application could measure features of early PD. 82 individuals with early, untreated PD and 50 age-matched controls wore research-grade sensors, a smartwatch, and a smartphone while performing standardized assessments in the clinic. At home, participants wore the smartwatch for seven days after each clinic visit and completed motor, speech and cognitive tasks on the smartphone every other week. Features derived from the devices, particularly arm swing, the proportion of time with tremor, and finger tapping, differed significantly between individuals with early PD and age-matched controls and had variable correlation with traditional assessments. Longitudinal assessments will inform the value of these digital measures for use in future clinical trials.