P44-F Estimating number of trackable axons in excitability-testing using simulated CMAP scans

Abstract

Background Excitability-testing enables assessment of treatment medication and provides insights into the underlying pathophysiology in neurodegenerative diseases. Excitability properties are mostly obtained by tracking threshold changes for a target compound muscle action potential (CMAP), which constitutes the response of a group of axons. As such, it obtains averaged excitability changes of axons in a nerve, which may mask subtle changes in single axons. The size of this group over which is averaged is however unknown. Axon loss further varies the size of this group. This study is a first step into estimating the size and variability of this group using simulated CMAP scans. Materials and methods Using a computer model to generate simulated CMAP scans, we estimated the number of probabilistically active motor units (MUs; i.e. motor axons and their innervated muscle fibers) at target CMAPs of 20%, 40%, and 60%. Input parameters involved number of MUs, their randomly assigned thresholds and size distribution. 10,000 simulated CMAP scans were generated using 100 and 250 MUs as input. Results At 100 MUs, a median of 47, 55, and 55 MUs were probabilistically active at target CMAPs of 20%, 40% and 60%. These numbers increased approximately in ratio with 250 MUs. At a target CMAP of 40%, the threshold range to generate this target was 0.61 mA at 100 MUs and 0.64 mA at 250 MUs. Conclusion Using simulated CMAP scans, we successfully estimated the group of axons that are tracked during excitability-testing and their potential impact on threshold changes.