LAUSR

Introduction Loss of motor units (MUs) is the primary disease process in degenerative motor neuron diseases like amyotrophic lateral sclerosis (ALS). Reinnervation can compensate for MU loss. The net result is a reduced number of MUs, and increased MU size. The assessment of the number and size of MUs will be essential to study disease progression, and to study the efficacy of treatment. Methods The motor unit number index (MUNIX) reflects the number of MUs. It is measured in three steps. First the compound muscle action potential (CMAP) is recorded using standard nerve conduction techniques. Next, the surface EMG interference pattern (SIP) is recorded when the subject exerts various levels of isometric force of contraction ranging from slight to maximum. Finally a mathematical model analyzes the relationship between the area and power of CAMP and SIP to compute MUNIX. Dividing MUNIX into the CMAP amplitude gives Motor unit size index (MUSIX). The “Megascore” is obtained by adding MUNIX from many different muscles. Results MUNIX is relatively easy to perform and requires less than 5 min per muscle. It has moderate reproducibility. The reference limits have been derived for many muscles (abductor pollicis brevis, abductor digiti minimi, biceps, tibialis anterior, etc.). In ALS patients MUNIX and its megascore decreased with disease progression. The rate of decline was faster than the ALS functional rating score (ALSFRS). Reduced MUNIX and increased MUSIX was found in muscles with no clinical weakness. Conclusion MUNIX is a promising tool to study progressive motor neuron disease.