22. Hemoglobin concentration changes induced by transcranial magnetic stimulation (TMS) over the primary motor area

Abstract

The aim of this study was to elucidate the correlation between excitation–contraction coupling time (ECCT) of the masseter and the bite force in myasthenia gravis (MG). Compound muscle action potentials (CMAPs) of the masseter and movement-related potentials (MRPs) using an accelerometer placed on the chin were simultaneously recorded after trigeminal nerve stimulation by a monopolar needle electrode inserted into the mandibular notch. The ECCT was defined as the difference in onset latencies between CMAP and MRP. The repetitive nerve stimulation test (RNS) was also performed in the same masseter before the ECCT recording. The bite force was measured using a pressure-sensitive sheet immediately after the ECCT recording. Successive examinations of RNS/ECCT and the bite force measurements were performed in 18 patients with MG. We calculated percent difference of the bite force to estimate the inter-trial differences of the bite force in each subject. The ECCT was significantly correlated with the percent difference of the bite force (p = 0.02). On the other hand, the percent difference of the bite force did not show any significant relationship with either the decrement in RNS or CMAP amplitude of the masseter. Our results suggest that dysfunction of E–C coupling may contribute to the muscle weakness in MG.