BACKGROUND: In the investigation of patients with myasthenia gravis, repetitive supramaximal stimulation of an affected peripheral nerve is commonly performed to detect abnormal transmission at the neuromuscular junction. A study was undertaken to determine whether abnormal transmission could similarly be detected during stimulation of the phrenic nerves. METHODS: The phrenic nerves were stimulated supramaximally with surface electrodes in 13 patients with myasthenia gravis and in 16 control subjects (six control patients with diaphragmatic weakness but not with myasthenia and ten normal subjects). The amplitude of diaphragm muscle action potentials was measured with surface electrodes during phrenic nerve stimulation at frequencies of 1-5 Hz for 3-4 seconds. RESULTS: In five patients with myasthenia gravis, a significant decrement (15-43% decrease) occurred in the amplitude of diaphragm muscle action potential during stimulation at 3 Hz. When stimulation frequency was reduced to 1 Hz, diaphragm muscle action potentials returned to their original amplitude within 4-5 seconds. The decrement in the amplitude of the diaphragm muscle action potential was reduced temporarily in three of four patients after the administration of intravenous edrophonium chloride (Tensilon). There was no significant change (< 10% decrease) in the amplitude of diaphragm muscle action potentials during stimulation at increased frequencies either in the 16 control subjects or in eight of the patients with myasthenia gravis. CONCLUSION: A significant reduction in the amplitude of diaphragm muscle action potential occurred in five of 13 patients with myasthenia gravis during phrenic nerve stimulation at 3 Hz but in none of the control subjects. This may be a useful and non-invasive method for identifying patients with myasthenia gravis in whom weakness of the diaphragm is suspected.
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