Statistical motor unit number estimation: Reproducibility and sources of error in patients with amyotrophic lateral sclerosis

The reliability of motor unit number estimation (MUNE) for assessment of th e long-term course of ALS is dependent on the reproducibility of the techni que. We report our results with the statistical method of MUNE on the ulnar nerve/hypothenar muscle in 16 ALS patients who were studied on 52 occasion s. On each occasion, MUNE was performed twice with one electrode placement and once with a different placement. For each MUNE, mean surface motor unit potential amplitude was determined within three different recording ranges or windows at different stimulus intensities. The MUNE results had excelle nt reproducibility with coefficients of variation of 19% and test-retest co rrelation coefficients from 0.75 to 0.86. With examination of sources for v ariability, the reproducibility of statistical MUNE is not affected by mino r variation in stimulation and recording electrode placement but may be imp roved by modifying methods for recording window selection. The high reprodu cibility of statistical MUNE supports its reliability for estimating the ra te of motor unit loss in ALS. (C) 2000 John Wiley & Sons, Inc.