CMAP scan discontinuities: Automated detection and relation to motor unit loss

•CMAP scan discontinuities are analyzed automatically and expressed quantitatively by means of an index.•This index, D50, is related to decreases in motor unit (MU) number and increases in MU size.•D50 is potentially useful for follow-up of neurogenic disorders.

ObjectiveTo evaluate an automated method that extracts motor unit (MU) information from the CMAP scan, a high-detail stimulus–response curve recorded with surface EMG. Discontinuities in the CMAP scan are hypothesized to result from MU loss and reinnervation.MethodsWe introduce the parameter D50 to quantify CMAP scan discontinuities. D50 was compared with a previously developed manual score in 253 CMAP scans and with a simultaneously obtained motor unit number estimate (MUNE) in 173 CMAP scans. The effect of MU loss on D50 was determined with a simulation model.ResultsWe found a high agreement (sensitivity = 86.8%, specificity = 96.6%) between D50 and the manual score. D50 and MUNE were significantly correlated below 80 MUs (r = 0.65, n = 68, p < 0.001), but not when MUNE was larger than 120 MUs (r = 0.23, n = 59, p = 0.08).ConclusionsDiscontinuities in the CMAP scan as expressed by a decreased D50 are related to significant MU loss. The determination of D50 is objective, quantitative, and less time-consuming than both manual scoring and many existing MUNE methods.SignificanceD50 is potentially useful to monitor neurogenic disorders and moderate to severe MU loss.