Estimating motor unit numbers from a CMAP scan: Repeatability study on three muscles at 15 centres

To assess the repeatability and suitability for multicentre studies of MScanFit motor unit number estimation (MUNE), which involves modelling compound muscle action potential (CMAP) scans.

Fifteen groups in 9 countries recorded CMAP scans twice, 1-2 weeks apart in healthy subjects from abductor pollicis brevis (APB), abductor digiti minimi (ADM) and tibialis anterior (TA) muscles. The original MScanFit program (MScanFit-1) was compared with a revised version (MScanFit-2), designed to accommodate different muscles and recording conditions by setting the minimal motor unit size as a function of maximum CMAP.

Complete sets of 6 recordings were obtained from 148 subjects. CMAP amplitudes differed significantly between centres for all muscles, and the same was true for MScanFit-1 MUNE. With MScanFit-2, MUNE differed less between centres but remained significantly different for APB. Coefficients of variation between repeats were 18.0% for ADM, 16.8% for APB, and 12.1% for TA.

It is recommended for multicentre studies to use MScanFit-2 for analysis. TA provided the least variable MUNE values between subjects and the most repeatable within subjects.

MScanFit was primarily devised to model the discontinuities in CMAP scans in patients and is less suitable for healthy subjects with smooth scans.