The Relationship Between Parkinson's Disease and Essential Tremor: Review of Clinical, Epidemiologic, Genetic, Neuroimaging and Neuropathological Data, and Data on the Presence of Cardinal Signs of Parkinsonism in Essential Tremor
Background: The possible relationship between essential tremor (ET) and Parkinson’s disease (PD) remains controversial since the first description of PD. However, there is increasing evidence suggesting the overlap between these two disorders. The aim of this review is to examine the relationship between PD and ET, focusing on clinical, epidemiologic, genetic, neuroimaging and neuropathological data, and the presence of cardinal symptoms of parkinsonism in ET.
Methods: We conducted a PubMed search from 1966 until November 2011 on the relationship between ET and PD and the presence of postural tremor in PD patients, and the presence of rest tremor, rigidity and slowed movements in ET patients are reviewed.
Results: Clinical series, follow-up studies of ET patients, and case-control and genetic epidemiological studies indicate that ET is associated with increased risk for PD. Some neuroimaging studies and neuropathological reports suggest association between the two diseases. ET patients have shown a high prevalence of rest tremor and at least 7 studies showed slowed movements (possibly related with cerebellar dysfunction and/or bradykinesia) in patients with ET.
Conclusions: There is reasonable epidemiological and clinical evidence to support a link between ET and PD, although it is not clear what factors could predict the risk of developing PD in ET patients (or more rarely, of developing ET in PD patients). Future multicentric and multidisciplinary studies including epidemiological, clinical, neuroimaging, genetic and neuropathological assessment would be required to understand these associations.