Defining the Rhythm: Developing a New Method to Describe Tremor and Myoclonus

Background: The hallmark feature of tremor is rhythmicity, which can be quantified using power spectral density (PSD) analysis. However, tremor exhibits considerable variability, ranging from highly regular to more irregular patterns. Similarly, rhythmicity in myoclonus varies, but it typically manifests as arrhythmic jerks. Objectives: To develop PSD-based measures of movement regularity for the classification of tremor and myoclonus. Methods: Electromyography data from 153 patients were analyzed retrospectively, including orthostatic tremor (OT) (n = 36), essential tremor (ET) (n = 40), hand dystonic tremor (DT) (n = 42), and limb cortical myoclonus (n = 35). Five PSD analysis-derived variables were assessed: peak prominence, peak-to-broadband power ratio, peak frequency, peak width, and harmonics. Discriminant analysis evaluated classification accuracy across groups. Results: Peak prominence was highest in OT and higher in ET than DT or myoclonus. Peak-to-broadband power ratio showed similar trends. Peak frequency differed across groups, with myoclonus highest and OT exceeding ET and DT. Peak width was larger in myoclonus and, to a less extent, in DT compared with ET. Harmonics were greater in OT and ET compared with DT and myoclonus. Discriminant analysis correctly classified 86.3% of cases, with overlap between ET and DT. Receiver operating characteristic (ROC) curve analysis for peak prominence and width demonstrated high classification accuracy between ET and DT. Conclusions: Our findings represent a promising step toward establishing objective, PSD-based measures for the classification of tremor and myoclonus. These tools could enhance diagnostic accuracy and deepen insights into these disorders. © 2025 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.