Is there a one-way street from essential tremor to Parkinson's disease? Possible biological ramifications

There is considerable evidence for an association between essential tremor (ET) and Parkinson's disease (PD), although the topic remains somewhat controversial. An important issue, not previously addressed, is what seems to be the unidirectional nature of the relationship (ET→ET + PD and not PD→PD + ET). The aims of this review are (i) to discuss the evidence for and against a unidirectional relationship and (ii) to discuss the implications of such a unidirectional relationship, if it exists, for disease mechanisms. Evidence 'for' a unidirectional relationship includes (i) abundant clinical anecdotal observation and (ii) clinical and epidemiological studies. Evidence 'against' is theoretical rather than empirical. Overall, the evidence 'for' is stronger, although additional studies are needed in order to be certain; for the time being, it might be best to leave this as an open question. The biological ramifications/extensions of such a unidirectional relationship include (i) that the association is causal (i.e. some aspect of ET pathophysiology predisposes an individual to develop PD) and (ii) that some ET cases may have a circumscribed form of Lewy body disease, and the secondary development of PD may represent a spread of those Lewy bodies in the brainstem. The presence and nature of the links between ET and PD are controversial. Further primary data (epidemiological and pathological) are needed to improve understanding of the relationship and its implications for the pathogenesis of both disorders.