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ET and PD: Are They Related?
Growing evidence suggests that essential tremor (ET) and Parkinson’s disease (PD) are pathogenically related, at least in some patient populations. This key assessment comes from an article published in Movement Disorders (February 2011).

Dr. Joseph Jankovic of the Parkinson’s Disease Center and Movement Disorders Clinic and Dr. Robert Fekete, both at Baylor College of Medicine, examined critically reviewed findings and studies about these two common movement disorders. The review covered clinical features, epidemiologic, genetic, imaging and pathological studies. They placed this information within the context of previously published data over the past five decades.


Given the evidence from their research, the doctors conclude: Although ET and PD are distinct entities, there is increasing evidence for overlap between these disorders, particularly between ET and the tremor dominant PD.

They further state that although “there is compelling evidence for the notion that some patients with ‘pure’ ET evolve into PD, the biologic nature of the association is not well understood.” At this point, the researchers are not clear “what factors predict which ET patients later develop PD and whether patients with PD are more likely to develop ET.”

Further studies are needed to understand this mixed, ET-PD phenotype. Differentiation should be made between patients with “pure” ET and an ET-PD combination not only for studies, but also when designing therapeutic trials.

ET and PD: Linked or Not?
The article by Dr. Joseph Jankovic and Dr. Robert Fekete published in Movement Disorders (see summary above) suggests that the two movement disorders are pathogenically related in some patient populations. Not necessarily.

Another viewpoint also published in Movement Disorders, titled “Essential Tremor and Parkinson’s Disease: Lack of a Link,” argues an alternate stance. This response was written by Dr. Charles H. Adler, Parkinson’s Disease and Movement Disorders Center, Mayo Clinic; Dr. Holly Shill and Dr. Thomas Beach, both from the Sun Health Research Institute.

Jankovic and Fekete examine and discuss a range of evidence that they “interpret as proving a causal relationship between these disorders.” Adler and his peers take the position that there is a clear lack of evidence that the disorders are causally related.

A major confounding factor, when trying to determine whether ET and PD are related, is the fact that currently we have limited knowledge regarding the cause of these two disorders. It is very likely that these are syndromes and not diseases with a single cause; therefore, if a link exists, it may well be in a subgroup of these individuals.

Adler et. al point out that there is no diagnostic test for either condition. They focus specifically on neuropathlogic studies for their review rather than the broader range of evidence gathered by Jankovic and Fekete.

Clinically there appears to be clear overlap between ET and PD. Postural and kinetic tremor are common in PD, whereas the presence of rest tremor is common in ET. Bradykinesia and rigidity can occur in ET and at what point patients with these findings become clinically classified as PD is unclear.

The doctors acknowledge that “there are patients with long-standing ET who go on to develop PD, but whether this is greater than the general population, as suggested in some studies, and even if it is, whether the actual cause of PD in these cases relates to ET or other factors has not been established.”

Other key points in the article state: ET subjects as a group do not have neurochemical changes that are found in PD, do not respond to medications used to treat PD, and are not hyposmic (a reduced ability to smell and to detect odors thought to be a preclinical sign of PD). Neuroimaging studies do not find changes of PD when groups are compared.

Drs. Adler, Shill, and Beach conclude that an “overwhelming amount of evidence suggests that any link between ET and PD is coincidental and not biological.”

To establish a more definitive position, the populations of patients affected must be clearly defined and criteria for inclusion and exclusion established. They propose that long-term studies with standardized clinical and biomarker assessments followed by neuropathlogic confirmation are needed.