Effects of the amplitude threshold on the separability of neuropathic and myopathic from normal EMG using parameters of the turns/amplitude analysis

Objective: In this study, the relationship between the amplitude threshold used for the determination of the turns of the electromyographic (EMG) interference pattern and the parameters of the turns/amplitude analysis was examined. It was investigated whether the discrimination of myopathic and neuropathic from normal muscles could be optimized by an appropriate amplitude threshold. Methods: The interference patterns of the tibialis anterior muscle of 15 patients with myopathies, 30 patients with neuropathies and 56 controls were recorded, using concentric needle electrodes. A computer program performed the Willison analysis, systematically varying the amplitude threshold between 10 μV and 200 μV. Results: Amplitudes as well as the number of turns per second were non-linearly related to the amplitude threshold. The reduction of the amplitude threshold to 30 μV resulted in a clearly better separation of the distributions of the number of turns of neuropathic, myopathic and normal EMG, compared to the traditional threshold value of 100 μV. The distributions of amplitude values, however, were not affected. The distance between the turns parameter distributions of neuropathic patients and controls and between the distributions of myopathic patients and controls, expressed by the Kolmogoroff–Smirnov distance, had a maximum at 30 μV. Conclusions: For the turns/amplitude analysis of the tibialis anterior muscle an amplitude threshold of 30 μV should be selected.