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Long duration asymmetric postural tremor in the development of Parkinson’s disease
  1. D G Grosset1,
  2. A J Lees2
  1. 1Southern General Hospital, Glasgow, UK
  2. 2Weston Institute of Neurological Sciences and National Hospital for Neurology and Neurosurgery, Queen Square, London, UK
  1. Correspondence to:
 Donald G Grosset
 Institute of Neurological Sciences, 3rd floor, Southern General Hospital, Glasgow, UK; d.grossetclinmed.gla.ac.uk

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Long term asymmetric postural tremor is likely to predict development of Parkinson’s disease and not essential tremor

Patients presenting with essential tremor who later develop Parkinson’s disease (PD) are recalled by most practising neurologists, but there remains debate around the relationship of the two diagnoses. In this issue the paper by Chaudhuri et al(pp 115) reports long term clinical follow-up of patients with asymmetric or unilateral postural tremor, where the diagnosis evolved from essential tremor to PD. The case against coincidental dual diagnosis is well argued, but ascertainment bias limits application of the conclusions to prospective patient management. Selected patients in their series had abnormal presynaptic dopaminergic single photon emission computed tomography (SPECT) imaging confirming degenerative parkinsonism, but this was conducted only after parkinsonian features emerged. The next requirement is to image such cases earlier to determine whether dopamine deficiency is present, and whether this accurately predicts later evolution into PD. Cases identified in such a manner would expand the concept of benign tremulous PD, and could be considered a postural equivalent of monosymptomatic rest tremor. It would be beneficial to record family history (considering monogenetic parkinsonism with essential tremor features), and olfactory test results in such work. Functional dopaminergic imaging also results in revision of diagnosis from early PD largely to essential tremor,1 with normal SPECT or positron emission tomography (PET) presynaptic imaging in 4 to 14% of cases depending on the population studied and the duration of symptoms.1,2 In both settings, functional dopaminergic imaging gives insights to diagnosis in the more benign tremulous patient, akin to the increased understanding of differential diagnosis of degenerative parkinsonism in the classic brain bank studies.3

The time profile of evolving rest tremor (around 2–3 years) in the reported series of 13 cases, on a background of asymmetric postural tremor at least five times longer, raises interesting possibilities in relation to early detection of PD, and application of potential neuroprotective drugs. If asymmetric postural tremor without rest tremor is an early disease marker, does this variant of PD have a longer time course than akinetic-rigid parkinsonism or is the early clinical presentation simply because the patient notices early tremor much sooner than early bradykinesia or rigidity? Earlier testing with presynaptic dopaminergic imaging could be confounded if there is an extra striatal or non-dopaminergic component in the early phase of PD, at a time when postural tremor is the only manifestation.

In the specialist movement disorder clinic of Chaudhuri et al, less than 3% of patients evolved from essential tremor to PD. Based on the community prevalence of essential tremor at least 10 times higher than PD, the risk of a patient with an initial essential tremor diagnosis needing to be revised to PD is low. Because of the study design, we do not know whether symmetrical postural tremor sometimes evolves into PD. Until we have further data, it remains appropriate for such patients with probable essential tremor to be reassured, but asked to return should they develop worsening motor disability. Given the very long latency period of such cases before PD develops, and considering the diagnostic criteria for essential tremor,4 it seems reasonable to label such cases isolated tremor (when postural tremor is unilateral) and atypical essential tremor (when features are markedly asymmetrical), until such times as parkinsonian features emerge.

Long term asymmetric postural tremor is likely to predict development of Parkinson’s disease and not essential tremor

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