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 th e compensatory effects of collateral reinnervation consequent to lower motor neuron loss. In a cross-sectional study of ALS subjects, we mad e comparisons between motor unit number estimation (MUNE) values and s everal measures-reflecting collateral reinnervation, including isometr ic strength compound muscle action potential (CMAP) amplitude, surface motor unit action potential (S-MUAP) amplitude, fiber density (FD), m acro-EMG potential amplitude, turns-to-amplitude (T/A) ratio, and ampl itude and recruitment pattern of low threshold voluntary motor units i n elbow flexor muscles. Before comparisons were made, test-retest repr oducibility of these measures was assessed in ALS subjects, and is hig hest for isometric strength, and lower but similar for EMG measures. W hen the effects of multiple comparisons are considered, borderline sig nificant correlations are found between MUNE values and isometric stre ngth. Neither MUNE values nor isometric strength are significantly cor related with macro-EMG amplitude, FD, T/A ratio, or amplitude and recr uitment rate of low threshold voluntary motor units. There are signifi cant correlations of CMAP and S-MUAP with MUNE values, but these are s tatistical artifacts with no independent interpretation. We conclude t hat collateral reinnervation prevents isometric strength and EMG measu res from accurately reflecting lower motor neuron death in ALS. MUNE m easurements 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. (C) 1993 John Wiley & Sons. Inc.