Standard and modified statistical MUNE evaluations in spinal-bulbar muscular atrophy

Motor unit number estimation (MUNE), a technique used in amyotrophic lateral sclerosis (ALS) clinical trials to quantitatively assess motor neuron loss, should also be valuable in assessing progression in spinal bulbar muscular atrophy (SBMA), an x-linked neuronopathy. In ALS, instability of single motor units (SMUP) prompted Shefner et al.6(6) to modify the statistical MUNE method to exclude SMUPs < or = 40 microV. It is unknown if there is similar SMUP instability in the more chronic degenerative disease of SBMA. In this study the standard parameter of excluding SMUP < 10 microV was compared with the exclusion of SMUP < 40 microV in the calculation of the statistical MUNE. The mean statistical MUNE, using the standard method and the Shefner et al. method, was 60 +/- 21 to 47 +/- 23, respectively. Similar to ALS, SBMA showed an increased proportion (17%) of individual SMUPs < or = 40 microV compared to normal controls. In conclusion, excluding SMUPs < or = 40 microV from the statistical MUNE calculations is appropriate for SBMA subjects because their SMUP, characteristics are similar to ALS. Exclusion of the low-amplitude SMUPs reduces the calculated MUNE.