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A clinical maneuver to increase tremor differentiation between Parkinson disease and essential tremor

Sir, We agree with Louis [1] on the fallacy inherent in many diagnostic schemes for essential tremor (ET) [2], and on possible similarities between ET and Parkinson s disease (PD) [3,4]. ET might share with PD mild parkinsonian features (bradykinesia, rest tremor) [5]. While this further underscores the connection between ET and PD, it does increase diagnostic uncertainty. On the other hand, dystonic tremor is often misdiagnosed as PD or ET. [6] We have described a maneuver that elicits neurophysiological differences between ET and PD [7]. It consists of passively modifying the shoulder joint angle so as to attain two different static positions: 30 horizontal adduction anterior (ANT) and 30 horizontal abduction posterior (POST) with respect to neutral position at 0 in the horizontal plane. This maneuver causes opposite effects in PD and ET tremor amplitude in relation to shoulder position: PD tremor is reduced while ET is amplified in POST, and viceversa in ANT. A change in the level of activation of the corticospinal system with different shoulder position may be one possible factor responsible for the postural effects on tremor [8]. We think it worthwhile to explore the potential applicability of the shoulder maneuver in the clinical setting. Bradykinesia, defined as the progressive reduction in the amplitude of repetitive actions, is magnified in POST [7], because of defective or inadequately modulated cortical output in this position. [8] The shoulder maneuver could improve the differentiation of tremor by detecting meaningful differences in tremor amplitude, as measured by accelerometry and clinical rating scales. The ability to improve early identification of PD has clinical implications in terms of possibly modifying the course of the disease. References