Background: Essential tremor (ET) is a highly prevalent neurological disease that frequently runs in families. A recent and controversial proposal is to separate ET patients into two distinct groups – ET vs. ET-plus. If this were a valid construct, one would expect in familial aggregation studies to observe that ET-plus would cluster in some families yet be absent in others, rather than being randomly distributed across families. We examined whether there is evidence of familial aggregation of ET-plus.
Methods: Probands (n=84 [56 ET-plus and 28 ET]) and their first- and second-degree relatives (n=182 and 48) enrolled in a genetics study. Chi-squares and generalized estimating equations (GEE) tested associations between probands’ ET-plus status and the ET-plus status of their relatives.
Results: Chi-squares analyses revealed that ET-plus was no more prevalent in relatives of probands diagnosed with ET-plus than in relatives of probands diagnosed with ET, p>0.05. Restricting relatives to first-degree relatives similarly did not detect a significant association (p = 0.88). GEE yielded similar results (respective p’s = 0.39 and 0.81).
Conclusion: The data demonstrate that ET-plus does not seem to aggregate in families. As such, they do not lend support to the notion that ET-plus is a valid biological construct.
BackgroundFew longitudinal studies assess the progression of essential tremor (ET). One unexplored issue is whether tremor severity increases across time at a uniform rate. That is, does the observed rate of change in tremor severity within a particular patient remain constant or vary across time? This question of intra-individual differences is particularly important since it reflects a primary patient concern–will the nature of change I have seen to date be what I can expect in the future?MethodsET cases were enrolled in a prospective, longitudinal study. We selected 35 cases and assessed tremor severity via Bain and Findley ratings of Archimedes spirals assigned by a senior movement disorders neurologist. After reviewing both the change in spiral scores and the rate of change in scores, we identified five mutually exclusive patterns of severity change. We calculated the prevalence of each category using two complementary sets of classification criteria.ResultsLength of follow-up was 4.5 to 16.0 years, mean=10.2 years. Mean baseline tremor severity score was 4.6, SD=1.6. Depending upon the classification criteria used, the tremor scores of one-third to one-half of cases did not increase in a uniform fashion but were better described as demonstrating jumps and/or reversals in scores across time.ConclusionsWe document the nature of changes in ET tremor severity scores across a ten-year period via expert ratings of Archimedes spiral drawings. Such natural history data are valuable to patients and clinicians who hope to better understand and predict the likely course of ET symptoms.
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