Muscle is electrically inexcitable in acute quadriplegic myopathy

McFarlane and Rosenthal initially reported a case of acute myopathy following the coadministration of steroids and pancuronium in a 24-year-old asthmatic patient. ^[1] There have since been numerous papers describing an acute quadriparesis following the administration of nondepolarizing neuromuscular blocking agents and high-dose steroids, and, less commonly, with either class of agent alone. ^[2-8] Nerve conduction studies in patients with acute quadriplegic myopathy reveal a dramatic decrease in compound muscle action potential (CMAP) amplitude in the setting of normal or near normal sensory potentials. ^[3,5] EMG demonstrates small-amplitude, short-duration polyphasic motor units which recruited in a myopathic fashion. ^[3,9]

The absence of significant neuropathy ^[3,5] or persistent decrement following repetitive nerve stimulation ^[3,5-7,10] suggests that the decrease in CMAP amplitude associated with acute quadriplegic myopathy is caused by an abnormality in muscle. This abnormality could simply be secondary to loss of muscle, because in some patients there is a dramatic increase in CK, and muscle biopsy reveals widespread necrosis. ^[5,11,12] In most patients, however, there is a marked decrease in CMAP amplitude with only a minor increase in CK, and muscle biopsy does not reveal necrosis. ^[3,5] We postulated that the decrease in CMAP amplitude in these patients may be secondary to loss of muscle membrane excitability. To test this hypothesis, we directly stimulated muscle in three patients with acute quadriplegic myopathy to determine whether the muscle was electrically excitable. We also directly stimulated denervated muscle in three patients who had not received corticosteroids to determine the threshold of denervated muscle for direct excitation.

Methods.

Direct muscle stimulation was performed in paralyzed or extremely weak muscles. EMG was performed on each muscle before direct stimulation to determine the amount of spontaneous activity present and whether voluntary motor units could be recruited in the muscle. For stimulation, a stainless steel subdermal electrode …