Impact of stimulus duration on motor unit thresholds and alternation in compound muscle action potential scans

Boudewijn T.H.M Sleutjes,Janna Ruisch,Thijs E Nassi,Jan R Buitenweg,Leonard J Van Schelven,Leonard H Van Den Berg,Hessel Franssen,H Stephan Goedee

doi:10.1016/j.clinph.2020.10.026

Abstract

ObjectiveTo investigate the impact of stimulus duration on motor unit (MU) thresholds and alternation within compound muscle action potential (CMAP) scans. MethodsThe stimulus duration (0.1, 0.2, 0.6, and 1.0 ms) in thenar CMAP scans and individual MUs of 14 healthy subjects was systematically varied. We quantified variability of individual MU’s thresholds by relative spread (RS), MU thresholds by stimulus currents required to elicit target CMAPs of 5% (S5), 50% (S50) and 95% (S95) of the maximum CMAP, and relative range (RR) by 100*[S95-S5]/S50. We further assessed the strength-duration time constant (SDTC). Experimental observations were subsequently simulated to quantify alternation. ResultsRS, unaffected by stimulus duration, was 1.65% averaged over all recordings. RR increased for longer stimulus duration (11.4% per ms, p < 0.001). SDTC shortened with higher target CMAPs (0.007 ms per 10% CMAP, p < 0.001). Experiments and simulations supported that this may underlie the increased RR. A short compared to long stimulus duration recruited relative more MUs at S50 (more alternation) than at the tails (less alternation). ConclusionsThe stimulus duration significantly affects MU threshold distribution and alternation within CMAP scans. SignificanceStimulation settings can be further optimized and their standardization is preferred when using CMAP scans for monitoring neuromuscular diseases.

Highlights

The individual MU potentials (MUPs) responses and the compound muscle action potential (CMAP) scan were obtained in the same subject with a 1.0 ms stimulus duration
Moderate variation of strength-duration time constant (SDTC) was seen within individual subjects across the different target CMAPs, we found an inverse relation between SDTC and increasing target CMAP values with a mean SDTC shortening of 0.007 ms per target CMAP of 10% (p = 0.001, 95% CI 0.003–0.01; Fig. 3B)
Stimulation settings of the CMAP scan require further standardization as stimulus duration can affect the electrical recruitment of motor unit (MU) illustrated by our experiments and computer simulations

Summary

Introduction

⇑ Corresponding author at: Department of Neurology, F02.230, University relevant characteristics on number of motor unit (MUs), MU sizes and motor axon excitability (Drenthen et al, 2008, 2013; Garg et al, 2017; Henderson et al, 2006, 2007, 2009; Jacobsen et al, 2017; Maathuis et al, 2013; Sleutjes et al, 2014). Previous studies have shown its potential as a sensitive bedside tool to evaluate disease severity and progression in several neuromuscular disorders, such as amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), and peripheral neuropathies The CMAP scan is further well-tolerated showing high reproducibility and is less labour intensive than various other motor unit number estimate (MUNE) methods (Bostock, 2016; Jacobsen et al, 2017; Maathuis et al, 2011)

Methods

Results

Discussion

Conclusion