Characterization of Motor Unit Discharge Rate in Patients with Amyotrophic Lateral Sclerosis Public
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In this study, we used a custom made quadrifilar needle electrode and multichannel electromyography (EMG) software tool to decompose EMG signals and investigate the behavior of motor unit discharge rate (MUDR) of concurrently active motor units in patients with amyotrophic lateral sclerosis (ALS). Decomposition is a technique used to break down the complex EMG signal into its constituent motor units. A motor unit is a single alpha motor neuron and all the muscle fibers it innervates. ALS is a progressive degenerative disorder of both the upper and lower motor neurons. We recorded four differentially amplified EMG signals from the first dorsal interosseous (FDI) muscle of six ALS patients (four with predominant lower motor neuron pathology and two with predominant upper motor neuron pathology) and seven control subjects. Recordings were made from force contractions of 20 and 50% of maximum voluntary contraction (MVC). All control subjects were between the ages of 40 and 70 years and were examined by a practicing physiatrist for exclusion criteria including neuromuscular disorders or any medications that might affect muscle activity. We observed differences in initial firing rates and variability of active motor units between control subjects and ALS patients. Furthermore we observed differences in firing rates and variability of active motor units between ALS patients with predominant upper motor neuron pathology and ALS patients with predominant lower motor neuron pathology. Initial motor unit firing rates for control subjects were 16.22 +/- 2.06 Hz at 20% MVC and 19.79 +/- 1.66 Hz at 50% MVC. As expected, initial motor unit firing rates from patients with predominant lower motor neuron pathology were higher than those of control subjects; 18.87 +/- 4.73 Hz at 20% MVC and 24.28 +/- 5.01 Hz at 50% MVC. ALS patients with predominant upper motor neuron pathology, as expected, had initial motor unit firing rates that were lower than those observed in control subjects; 9.22 +/- 1.68 Hz at 20% MVC and 12.83 +/- 2.26 Hz at 50% MVC. Motor unit firing rate time series in ALS patients with predominant upper motor neuron pathology showed decreased variability, 0.99 +/- 0.17 Hz at 20% MVC and 1.70 +/- 0.52 Hz at 50% MVC, when compared to control subjects, 2.37 +/- 0.67 at 20% MVC and 4.20 +/- 1.00 at 50% MVC. Variability of motor unit firing rate time series in ALS patients with predominant lower motor neuron were high, 3.38 +/- 1.2 Hz at 20% MVC and 4.07 +/- 1.56 Hz at 50% MVC, compared to control subjects. At 50% MVC, motor unit substitution was observed in ALS patients with predominant upper motor neuron pathology despite the contractions lasting just a few seconds. Motor unit action potentials (MUAPs) recorded from patients were polyphasic when compared to those from control subjects, as is characteristically found in practice.
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