ASSESSING THE IMPACT OF ESSENTIAL TREMOR ON UPPER-LIMB FUNCTION

We compared the impact of essential tremor on the performance of three manual tasks: drawing spirals, holding a cup full of water and a joys tick-controlled tracking test. Tremor amplitude and frequency were mea sured by accelerometry during the tracking test, when holding the cup and whilst a standard posture was maintained. The inter-relationships between tremor amplitude, frequency and task impairment were then exam ined. The results showed that the amplitude and frequency of essential tremor (measured from the principal spectral peak) changed with diffe rent activities, with the mildest postural tremors changing most in fr equency (by up to 4-5 Hz). The amplitude of tremor decreased in almost every case during the tasks, relative to posture, and this decrement was greatest for the most severe tremors. We also demonstrate that for practical purposes, such as routine clinical situations and therapeut ic trials, the effect of essential tremor upon upper limb function can be usefully assessed by two simple complementary techniques: rating s pirals and measuring the volume of water split from a cup. The impairm ents in carrying out these tasks and the tracking test were highly cor related with one another and also with the amplitude and frequency of postural tremor. The concept of tremor ''suppressability'' is introduc ed: the relative percentage decrease in the amplitude of a particular tremor during the performance of a specific task compared to that reco rded whilst holding a standard posture.