Masseteric repetitive nerve stimulation in the diagnosis of myasthenia gravis

Objective: (1) to develop a method for masseteric repetitive nerve stimulation (RNS) and to obtain normative data for amplitude and area decrement of the muscle (M) response. (2) To investigate myasthenia gravis (MG) patients with masseteric RNS. Masticatory muscles are frequently affected in MG, but no RNS test is available to investigate this district. Methods: Fifteen healthy subjects and 17 MG patients were examined. The masseteric nerve was stimulated by a monopolar needle (cathode), inserted between the mandibular incisure and the zygomatic arch, and a surface electrode (anode), on the contralateral cheek. Masseteric M response was recorded using surface electrodes on the muscle belly and below the mandibular angle. Stimuli were delivered at 3 Hz in trains of 9, at rest and after isometric effort. Results: Normal subjects: mean amplitude decrement was 0.3 ± 1.2% at rest, and 1.9 ± 1.3% after isometric effort. Patients: 15 patients (88%) were positive on masseteric RNS; in 3 of th...