New Algorithm for Computing Step Percentage of Compound Muscle Action Potential Scan in Modeling Motor Unit Number Estimate.

Introduction: Compound Muscle Action Potential Scan (CMAP) Scan is an electrophysiological method for diagnosing neuromuscular diseases with axonal loss. Significant differences between CMAPs are termed as Steps. The percentage ratio of all detected steps to the maximum CMAP, a parameter being evidence of motor unit loss and enlargement due to reinnervation is defined as Step Percentage. Materials and Methods: Motor neuron groups were created and stimulated gradually through a simulator software for CMAP Scan. CMAPs were utilized to compute the step sizes. Their cumulative sum greater than two standard deviations were taken for computing Step Percentage. The simulator data were exported for processing in a MATLABⓇ Code for these calculations and for computing regression coefficients of the model for step percentage and the number of axons by the Least Square Method. Results: The greatest step percentage value corresponded to the lowest motor unit number. A steep reduction was observed from 5 axons to 120 axons with increasing axon numbers. The step percentages converged to a steady-state value for the axon numbers between 120 and 300. Discussion and Conclusion: Step percentage values were found greater for lower axon numbers as in the case of Motor Unit loss in neurogenic diseases. They tended to decrease with increasing axon numbers approaching a steady-state value as the presence of intact Motor Units. A new algorithm was proposed to determine the lowest step size quantitatively rather than detecting by inspection as in routine clinical applications for estimating the step percentage. A mathematical model was built to demonstrate the relationship between the step percentage and the number of axons.