Repetitive nerve stimulation test in myasthenic crisis

Abstract

Introduction Herein we report our experience with the repetitive nerve stimulation (RNS) test in myasthenia gravis (MG) crisis. Methods The various parameters of the RNS tests in 26 patients with MG crisis were analyzed. Results In 18 (69%) patients, MG crisis is the first manifestation of MG. RNS tests were abnormal in 24 (92%) patients by decrement at low‐rate stimulation in any of 4 tested muscles. Three patterns of abnormality were found: MG pattern (decrement at low‐rate stimulation) in 23 patients; Lambert–Eaton myasthenic syndrome pattern in 1 patient; and cholinergic crisis pattern in 1 patient. Discussion During MG crisis, the RNS test can serve as a rapid and sensitive diagnostic tool for MG in a majority of patients. Muscle Nerve 59:544–544, 2019

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“…1. It is generally recommended to perform EMG/NCS 2 weeks after clinical onset of any neurological disease (Rubin, 2012), except for repetitive nerve stimulation and/or single-fiber EMG tests on patients with suspected neuromuscular disease in critical condition (Oh et al, 2019). 2.…”

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“…In order to confirm the diagnosis of seropositive MG, the patient underwent repetitive nerve stimulation (RNS). The RNS testing incorporates the surface stimulation and recording procedures used in routine motor nerve conduction studies and is abnormal in up to 70% of patients with generalized MG. During low-frequency (2-3 Hz) RNS, Ach becomes depleted at the examined neuromuscular junctions, and therefore, less is available for immediate release, resulting in smaller excitatory postsynaptic potentials (22). In patients without MG, all excitatory postsynaptic potentials exceed the threshold to generate an action potential, and no change in the summated compound muscle action potential is noted.…”

“…In patients without MG, all excitatory postsynaptic potentials exceed the threshold to generate an action potential, and no change in the summated compound muscle action potential is noted. In patients with MG, the number of AchRs is reduced, lowering the safety factor, and thus, during RNS, some excitatory postsynaptic potentials may not reach threshold, which means that no action potential is generated (22). This results in a decrement of the compound muscle action potential of 10% or more.…”

Repetitive nerve stimulation testing in myasthenic crisis

“…In the setting of MC, RNS testing is particularly useful—it may provide real‐time information regarding the status of neuromuscular transmission long before antibody testing results become available, thus facilitating timely diagnosis and expeditious treatment. Given the critical distribution of weak muscles underlying MC, one would expect that RNS testing should have good sensitivity to demonstrate deficient neuromuscular transmission in MC, and the retrospective series by Oh and colleagues supports this notion …”

“…Given the critical distribution of weak muscles underlying MC, one would expect that RNS testing should have good sensitivity to demonstrate deficient neuromuscular transmission in MC, and the retrospective series by Oh and colleagues supports this notion. 8 In their series, RNS testing was conducted in a cohort of 26 patients in MC at a single institution between 1997 and 2017. In addition to patients with respiratory failure, the definition of MC utilized included patients with acute inability to swallow.…”