Discrimination of spontaneous activity in needle EMG based on the quantitative assessment of the discharge rhythm using “Random Index”

Objective: Discrimination between EMG activity such as fibrillation potentials/positive sharp waves (Fib/PSW), end plate spikes (EPS), fasciculation potentials (FP), and contaminating voluntary motor unit potentials (MUP) is mandatory for EMG diagnosis. Discharge rhythm is the key for discrimination. We devised a new parameter, Random Index (RI), which quantifies the rhythm and takes a value from 0 to 1, smaller for regular trains of discharges. This study evaluated the utility of RI as well as modified versions of the regularity indices proposed in past reports. Methods: EMG records of patients with amyotrophic lateral sclerosis were retrospectively reviewed. EPS were collected also from a healthy volunteer. The EMG activity was classified by an expert. RI and other regularity indices as well as the median instantaneous firing rate (IFRm) were calculated. Results: Analyzed sequences were 73 Fib/PSW, 27 EPS, 24 FP, and 36 MUP. The four types were clearly separated over the 2-dimensional plots of regularity indices vs. IFRm. Especially, Fib/PSW and EPS were far separated in these plots. RI achieved significantly better discrimination between Fib/PSW and MUP than other indices. Conclusion: RI is a robust tool for discriminating EMG activity. Significance: RI and other regularity indices would be useful for educational purpose.