ABSTRACT
UNLABELLED: Objective To 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. METHODS: We 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. RESULTS: We 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). CONCLUSIONS: Discontinuities 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. SIGNIFICANCE: D50 is potentially useful to monitor neurogenic disorders and moderate to severe MU loss.
Subject(s)
Action Potentials/physiology , Electromyography/methods , Motor Neurons/physiology , Muscle, Skeletal/physiology , Neuromuscular Diseases/diagnosis , Computer Simulation , Humans , Muscle, Skeletal/physiopathology , Neuromuscular Diseases/physiopathology , Sensitivity and SpecificityABSTRACT
OBJECTIVE: The statistical method of motor unit number estimation (MUNE) assumes that all motor unit potentials (MUPs) have the same size. The present study aims to evaluate the consequences of this assumption as well as its implications for the validity of statistical MUNEs. METHODS: We performed statistical and multiple point stimulation (MPS) MUNE with an array of 120 electrodes on the thenar muscles of 15 healthy subjects. These recordings allow isolation and quantification of the effect of non-uniform MUP size on MUNE, because the differences in submaximal CMAP size (and, hence, in MUNE) between electrodes are due almost entirely to differences in (summed) MUP size. RESULTS: We found no correlation between statistical and MPS MUNEs. Statistical MUNEs proved very sensitive to small variations in the "bandwidth" (variance) of the response series; MUNEs from electrodes only 8mm apart could deviate by as much as 60%. This variation in bandwidth resulted from spatial (and, hence, size) differences between the contributing MUPs. CONCLUSIONS: Statistical MUNEs are very sensitive to violation of the uniform MUP-size assumption, to an extent that blurs any correlation with MPS MUNE in healthy subjects. SIGNIFICANCE: Statistical MUNE cannot be used to detect mild to moderate motor unit losses.
