Motor unit number estimation of facial muscles using the M Scan-Fit method

Introduction M Scan-Fit, an automated method for motor unit number estimation (MUNE), was assessed in muscles innervated by the facial nerve. Methods Healthy volunteers were recruited. M Scans were recorded twice from nasalis and depressor anguli oris (DAO) muscles, and then fitted to a probabilistic model. Results Twenty-one subjects were evaluated; 38% were females and 62% were males, with a mean age of 34.71 years. The average number of MUs was 38.57 on both testing occasions (t <= 0.0001;P= 1.0) for the nasalis. For the DAO, results were 20.62 MUs for the first and 23.48 for the second (t= -2.12;P= .04). Pearson's interrater correlation coefficients were 0.96 (P< .0001) for nasalis and 0.87 (P <= .01) for DAO. Intraclass correlation coefficients were 0.88 (P <= .01) for nasalis and 0.39 (P= .37) for DAO. Discussion M Scan-Fit MUNE is an automated, accurate, reliable method of estimating MU number and size from facial muscles.