Counting motor units in chronic motor neuropathies.

The degree of motor unit loss can not be accurately quantified in chronic motor neuropathies with routine electrodiagnostic testing or with pathologic examination. We used motor unit number estimation (MUNE), which is a unique electrophysiologic method that can estimate the number of surviving motor units innervating a muscle, to study axonal loss in spinal muscular atrophy (SMA) and Charcot-Marie-Tooth (CMT) neuropathies. MUNE is based on the ratio of the maximal compound muscle action potential (CMAP) to the average surface-recorded motor unit potential (S-MUP). The hypothenar muscle group was studied in infant and older subjects with SMA, and the hypothenar and biceps-brachialis muscle groups were studied in adult CMT1A and CMT2 subjects. The multiple point stimulation MUNE technique was used in SMA subjects and the spike triggered averaging MUNE technique was used in CMT subjects. In SMA, motor unit loss was profound in types 1 and 2 subjects and more moderate in type 3 subjects. In CMT, motor unit loss was prominent in distal muscles in both CMT1A and 2 subjects, and present in proximal muscles in CMT2 subjects. MUNE is efficient in assessing the degree of motor unit loss in chronic motor neuropathies. SMA is considered to be a proximal muscle disorder, but loss was marked in distal muscles in all SMA types. In CMT1A, the demyelinating form, motor unit loss was marked in distal muscles, consistent with the idea that axonal loss and not slow conduction velocity is the important pathologic condition. The pattern of proximal motor unit loss differed between CMT1A and 2, suggesting differences in underlying axonal pathology.