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Motor Unit Discharge Behavior in Parkinson’s Tremor: A Method for Analysis

Paired discharge refers to a pattern of alternating short and long interspike intervals in which the atypically short ISI durations cause excessive summation of MU twitch force and tremor in people with Parkinson’s disease (PD). In recent observations of PD related tremor, the paired MUDB varied considerably in its severity or waxes and wanes during static isometric force recordings (72-90 s). PURPOSE: We sought to develop a methodological approach for analysis of paired MUDB in tremor. METHODS: Serial correlation plots were used to assess clustering of ISI-short-long MUDB from 38 highly variable MU recordings. In MATLAB, the “kmeans” algorithm was used to fit data within 2-5 clusters and the silhouette mean provided a measure of how well-defined clusters were. Means and standard deviations of short and long ISIs within clusters were the dependent variables of interest. RESULTS: Using this method, 25 MUs were grouped into 3 clusters representing short, long and apparently normal ISIs. Inspection of cluster plots revealed that the 4th and 5th clusters were small sets of ISIs representing momentary de-recruitment or perhaps missed observations in the recording and do not provide meaningful information about the paired MUDB. CONCLUSIONS: ISI distributional statistics that assume bimodal behavior (ISI-short, ISI-long) fail to isolate ISIs of paired discharge from those that might represent non-tremor related discharge. This method allowed increased precision in the analysis of MUDB when paired discharge was inconsistent or less severe. Supported by NIH/NIGMS Grant 8 P20 GM103446-13.