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Repetitive phrenic nerve stimulation in myasthenia gravis

From Klinik Pirawarth (Dr. Zifko), Kur-und Rehabilitationsklinik, Bad Pirawarth, Austria; Myasthenia Gravis Clinic (Dr. Nicolle), Department of Clinical Neurological Sciences, London Health Sciences Centre, University of Western Ontario, London, Ontario, Canada; Department of Neurology and Ludwig Boltzmann Institute of Neurooncology (Dr. Grisold), Kaiser Franz Josef Spital, Vienna, Austria; and Neuromuscular Clinic and Clinical Neurophysiology Laboratory (Dr. Bolton), London Health Sciences Centre, University of Western Ontario, London, Ontario, Canada.

Address correspondence and reprint requests to Dr. Udo A. Zifko, Klinik Pirawarth, Kurhausstraße 100, A-2222 Bad Pirawarth, Austria.

OBJECTIVE: In patients with MG it may be difficult to determine whether respiratory insufficiency is due to a defect in neuromuscular transmission. We therefore studied the clinical value of repetitive electrical stimulation of the phrenic nerve.

METHODS: Repetitive phrenic nerve stimulation at 3 Hz was performed in 25 patients with MG. We recorded from the ipsilateral hemidiaphragm with surface electrodes before and after exercising the diaphragm for 10 and 90 seconds. The percent decrement of the negative peak (NP) area between the first and the fifth or sixth diaphragmatic compound muscle action potential (DCMAP) was analyzed and results compared with those from 10 healthy individuals.

RESULTS: The mean ± standard deviation percent change of the NP area in healthy individuals was -2.1 ± 4.2%, with a normal cutoff of 11%. Twelve patients (48%) had an abnormal decrement of DCMAP—9 had a decrement when the diaphragm was rested, 3 only after fatiguing of the diaphragm. The mean percent change in the 12 patients was -20% at rest, -18% after 10 seconds of exercise, and -23% after 90 seconds of exercise—a pattern consistent with MG. Repetitive stimulation of the accessory nerve with recording of the trapezius CMAP (TCMAP) was abnormal in nine patients (36%). The three patients with abnormal decrement of the DCMAP despite normal TCMAP had symptoms of dyspnea.

CONCLUSIONS: Repetitive phrenic nerve stimulation studies are a promising tool in the diagnosis of respiratory muscle weakness in MG and should be part of electrophysiologic studies in patients with undiagnosed respiratory failure.

Key words: Diaphragm—Phrenic nerve—Repetitive nerve stimulation—Neuromuscular transmission—Myasthenia gravis—Dyspnea—Respiratory muscle weakness.

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