Motor unit integrity in multifocal motor neuropathy: A systematic evaluation with CMAP scans

Abstract

Introduction/Aims: Progressive axonal loss in multifocal motor neuropathy (MMN) is often assessed with nerve conduction studies (NCS), by recording maximum compound muscle action potentials (CMAPs). However, reinnervation maintains the CMAP amplitude until a significant portion of the motor unit (MU) pool is lost. Therefore, we performed more informative CMAP scans to study MU characteristics in a large cohort of patients with MMN. Methods: We derived the maximum CMAP amplitude (CMAPmax), an MU number estimate (MUNE), and the largest MU amplitude stimulus current required to elicit 5%, 50%, and 95% of CMAPmax (S5, S50, S95) and relative ranges ([S95 − S5] × 100 / S50) from the scans. These metrics were compared with clinical, laboratory, and NCS results. Results: Forty MMN patients and 24 healthy controls were included in the study. CMAPmax and MUNE were reduced in MMN patients (both P