Utility of motor unit number index (MUNIX) and motor unit number estimation (MUNE) in differentiating motor neuron disease and myopathy

Abstract

Motor unit number estimation (MUNE) and motor unit number index (MUNIX) are electrophysiological testing methods to assess functional motor unit number in neuromuscular diseases. In this study, we investigated the significance of assessing functional motor unit number using MUNIX and MUNE in patients with motor neuron disease (MND) and myopathy (MYO). MUNIX and MUNE were performed using the abductor pollicis brevis muscle as the test muscle, and the results were compared between 10 MND patients, 13 MYO patients, and 10 healthy subjects (CNT). MUNIX was lower for both MND and MYO compared to CNT, indicating that the motor unit size index (MUSIX), the exponential function alpha value used to calculate MUNIX, may be used to differentiate them. Furthermore, MUNE was also lower in MND and MYO than in CNT, and since MUNIX and MUNE are lower when the CMAP amplitude is low, it is important to confirm the motor unit size. Single motor unit potential (SMUP) amplitudes in MUSIX and MUNE, which are indices of motor unit size, were higher in MND, which may help differentiate MND from MYO.