Clinical significance of repetitive compound muscle action potential in myasthenia gravis patients

Abstract

Repetitive compound muscle action potentials (R-CMAPs) are multiple compound muscle action potentials (CMAPs) following an initial CMAP induced by a single stimulation of a nerve. R-CMAPs indicate cholinergic neuromuscular hyperactivity, and occur in several conditions, such as congenital acetylcholinesterase (AChE) deficiency, slow-channel congenital myasthenic syndrome, and organophosphate poisoning. Although R-CMAPs may also be observed in myasthenia gravis (MG) patients with AChE inhibitor overdose, these are seldom seen in MG patients with the standard dose. The mechanism of R-CMAP induction and its clinical significance in patients with MG is also unclear. Therefore, we investigated the clinical characteristics of MG patients with R-CMAPs who were given the standard diagnostic dose of neostigmine. We retrospectively reviewed the medical records and electrodiagnostic findings of patients who underwent electrodiagnostic tests before and after the neostigmine test. Of the 71 MG patients reviewed, 24 developed R-CMAPs after receiving the standard diagnostic dose of neostigmine. There were no significant differences in age at disease onset, myasthenia gravis activities of daily living score, disease duration, and follow-up duration. Decremental response on the baselinerepetitive nerve stimulation (RNS) test was lower in the R-CMAP group. This group had a higher frequency of side effects and intolerance to, and a lower maximal tolerable dose of pyridostigmine bromide (PB). The appearance of R-CMAPs after a standard dose of injected neostigmine may be a good indicator of the risk of PB side effects. The detection of R-CMAPs after neostigmine injection may help in creating a therapeutic plan for myasthenia gravis patients.