Respiratory Muscle Function in Myasthenia Gravis

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Abstract
Global respirtory muscle function and diaphragmatic strength was assessed and compared with quadriceps femoris muscle strength in 17 patients with generalized mild-to-moderate myasthenia gravis and breathlessness. Initial measurments, made 10 h after the last dose of oral anticholinesterase therapy, demonstrated reduced maximal static expiratory (52.4 .+-. 26.8% predicted) an inspiratory (54.0 .+-. 23.5% predicted) mouth pressures in 16 patients, and reduced quadriceps femoris muscle strength in all cases. Vital capacity (VC) (70.9 .+-. 19.0% predicted) was abnormal in 12 patients. Transdiaphragmatic pressure recorded during maximal sniffs (sniff Pd) was reduced in eight patients, whereas pressure recorded during bilateral phrenic nerve stimulation at 1 Hz (twitch Pdl) was reduced in only three. There was no relationship between the grade of myasthenia or the severity of dyspnea and any of the measurements of respiratory muscle strength. After the administration of edrophonium hydrochloride (Tensilon), there was a significant increase in maximal static expiratory and inspiratory mouth pressure in quadriceps muscle strength and in sniff Pdl. The small increase in VC was not significant, and twitch Pdl increased in only one patient. Phrenic nerve conduction time was normal before and after Tensilon. Two patients with severe long-standing myasthenia showed no improvement in any measurement after Tensilon. We conclude that expiratory and inspirtory muscle weakness was not uncommon in patients with myasthenia gravis. Respiratory muscle strength improved after Tensilon. Vital capacity was a less sensitive measure of respiratory muscle strength than were respiratory mouth pressures and sniff Pdl. Diaphragmatic involvement was not detected by twitch Pdl unless the weakness was severe. There was a poor correlation between respiratory and quadriceps muscles strength. Thus, patients with myasthenia gravis may have severe respiratory muscle involvement even when peripheral muscle weakness is only mild.