Repetitive phrenic nerve stimulation in myasthenia gravis

Objective: In patients with MG it may be difficult to determine whether res piratory 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 hem idiaphragm with surface electrodes before and after exercising the diaphrag m for 10 and 90 seconds. The percent decrement of the negative peak (NP) ar ea between the first and the fifth or sixth diaphragmatic compound muscle a ction potential (DCMAP) was analyzed and results compared with those from 1 0 healthy individuals. Results: The mean +/- standard deviation percent cha nge of the NP area in healthy individuals was -2.1 +/- 4.2%, with a normal cutoff of greater than or equal to 11%. Twelve patients (48%) had an abnorm al decrement of DCMAP-9 had a decrement when the diaphragm was rested, 3 on ly after fatiguing of the diaphragm. The mean percent change in the 12 pati ents was -20% at rest, -18% after 10 seconds of exercise, and -23% after 90 seconds of exercise-a pattern consistent with MG. Repetitive stimulation o f the accessory nerve with recording of the trapezius CMAP (TCMAP) was abno rmal in nine patients (86%). The three patients with abnormal decrement of the DCMAP despite normal TCMAP had symptoms of dyspnea. Conclusions: Repeti tive phrenic nerve stimulation studies are a promising tool in the diagnosi s of respiratory muscle weakness in MG and should be part of electrophysiol ogic studies in patients with undiagnosed respiratory failure.