Motor unit number estimation, isometric strength, and electromyographic measures in amyotrophic lateral sclerosis

Pathologic progression in amyotrophic lateral sclerosis (ALS) results from motor neuron death, while the clinical expression also reflects the compensatory effects of collateral reinnervation consequent to lower motor neuron loss. In a cross-sectional study of ALS subjects, we made comparisons between motor unit number estimation (MUNE) values and several measures reflecting collateral reinnervation, including isometric strength, compound muscle action potential (CMAP) amplitude, surface motor unit action potential (S-MUAP) amplitude, fiber density (FD), macro-EMG potential amplitude, turns-to-amplitude (T/A) ratio, and amplitude and recruitment pattern of low threshold voluntary motor units in elbow flexor muscles. Before comparisons were made, test-retest reproducibility of these measures was assessed in ALS subjects, and is highest for isometric strength, and lower but similar for EMG measures. When the effects of multiple comparisons are considered, borderline significant correlations are found between MUNE values and isometric strength. Neither MUNE values nor isometric strength are significantly correlated with macro-EMG amplitude, FD, T/A ratio, or amplitude and recruitment rate of low threshold voluntary motor units. There are significant correlations of CMAP and S-MUAP with MUNE values, but these are statistical artifacts with no independent interpretation. We conclude that collateral reinnervation prevents isometric strength and EMG measures from accurately reflecting lower motor neuron death in ALS. MUNE measurements are better suited to provide insight into the true natural history of the disease process and may be clinically useful to follow progression and response in drug trials.