Impaired post-tetanic potentiation of muscle twitch in myasthenia gravis

Objective: The aim of this study was to evaluate post-tetanic potentiation of muscle twitch in myasthenia gravis (MG). Methods: Post-tetanic potentiation was evaluated by recording the compound muscle action potential (CMAP) of abductor pollicis brevis and movement-related potential (MRP) of the thumb using an accelerometer after tetanic stimulation of the median nerve at the wrist. After baseline recording, tetanic stimulation was delivered to the median nerve at a frequency of 10 Hz for 10 s. The CMAP and MRP were successively recorded at baseline and at 5, 10, 30, 60, 90 and 120 s after tetanic stimulation. The chronological changes of CMAPs and MRPs were recorded bilaterally in 11 patients with MG, 9 patients with myopathies (disease controls), and 25 healthy control subjects. Results: Maximal acceleration of MRP was significantly elevated during 10 s after tetanic stimulation without any CMAP changes in all groups. However, statistical analysis detected a significant decrease in post-tetanic potentiation of maximal acceleration of MRP in MG patients only compared to healthy controls, but not in myopathy patients, which may imply impairment of excitation-contraction coupling in MG. Conclusions: Post-tetanic potentiation of muscle twitch is significantly diminished in MG, suggesting impaired excitation-contraction coupling. Significance: Measurement of post-tetanic potentiation using an accelerometer is a simple and sensitive method to detect impairment of excitation-contraction coupling in MG. (C) 2015 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.