Objective:

To determine relationships between ulnar (hypothenar) multipoint incremental motor unit number estimation (MUNE), single motor unit potential amplitude (SMUP), and compound motor unit potential amplitude (CMAP) versus measures of disease severity in ambulatory (A-SMA) vs non-ambulatory (N-SMA) adults with spinal muscular atrophy (SMA).

Background:

MUNE is an estimate of the number of functioning motor neurons innervating a muscle(s), and SMUP provides an index of motor unit size. Here, we investigated the relationship between MUNE and CMAP in A-SMA versus N-SMA.

Design/Methods:

Cross sectional analysis of electrophysiological and functional outcome data obtained in ambulatory (n=15) and non-ambulatory (n=25) untreated adults with SMA. 

Results:

Mean CMAP amplitude for A-SMA vs N-SMA were 8.74 ± 1.82 and 2.37 ± 2.3 (p<0.001). Mean MUNE for A-SMA vs N-SMA were 102.53 ± 30.44 and 34.65 ± 31.83 (p<0.001).  Mean SMUP for A-SMA vs N-SMA were 86.3 ± 18.1 and 91.93 ± 37.09 (p=0.96).  There was good correlation in N-SMA between MUNE and CMAP (r = 0.93, p<0.001), CMAP and Hammersmith rating scale (r = 0.645, p<0.001), MUNE and Hammersmith rating scale (r = 0.5405, p<0.05), U-CMAP and % Predicted FVC (r = 0.771, p<0.001), MUNE and % Predicted FVC (r = 0.7417, p=0.001). In contrast, these measures did not correlate in A-SMA.  

Conclusions:

MUNE and CMAP are useful measures in SMA showing strong correlation with severity and other measures of function.  SMUP, a measure of motor unit size providing an index of motor neuron compensation following denervation, did not demonstrate the ability to distinguish severity in adults with SMA.  These findings suggest that motor unit compensation is outstripped by motor unit loss across the spectrum of SMA and that motor unit size may an important functional determinant with more severe phenotype.