Tremor during movement correlates well with disability in people with essential tremor

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Essential tremor is the most common movement disorder, typically characterized by the presence of both postural and kinetic tremor of the hand. In recent studies, we described the effects of altering force and load conditions on tremor amplitude and power in people with essential tremor. In the same participants, we also measured tremor-related functional disability. In this article we report on the current study on correlations of measures of tremor severity with those of tremor-related functional disability. Twenty-one participants with essential tremor had tremor measured in their more tremorous hand. Power spectral and amplitude measures of tremor were calculated for each of 16 conditions: force tremor at 4 submaximal force levels, postural tremor in unloaded and 3 submaximal load conditions, and kinetic tremor in unloaded and 3 submaximal load conditions for each of concentric and eccentric contractions of the wrist extensors. Participants were rated on the hand items of the Fahn-Tolosa-Marin rating scale and timed on the unilateral hand tasks of the Test Évaluant la Performance des Membres supérieurs des Personnes Âgées. The most consistently high and significant correlations were found between kinetic tremor measures and the hand task scores and tremor-B scores (r = 0.548-0.780, P < .01). Postural tremor measures correlated with disability measures only in loaded conditions, most consistently with the hand task measures (r = 0.640-0.725, P < .01). Thus, measures of kinetic tremor and loaded postural tremor, but not unloaded postural tremor or force tremor, relate well to disability captured with dynamic tasks.