2025 Workshop Handout Bundle-W118 RNS with Clinical Applications in Diagnosis of NMJ Disorders Kesner

W118 RNS with Clinical Applications in Diagnosis of NMJ Disorders Kesner

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This document, authored by Dr. Vita G. Kesner for the AANEM Workshop on Repetitive Nerve Stimulation (RNS), provides a comprehensive overview of neuromuscular junction (NMJ) physiology and the clinical utility of RNS in diagnosing NMJ disorders such as Myasthenia Gravis (MG), Lambert-Eaton Myasthenic Syndrome (LEMS), botulism, and congenital myasthenic syndromes.<br /><br />Key concepts include the structure and function of the NMJ, where motor axons release acetylcholine (ACh) to generate an endplate potential (EPP) that triggers muscle fiber action potentials (MFAPs). Normal transmission features a “safety factor,” where excess ACh release ensures reliable muscle activation.<br /><br />In pathological states, autoantibodies reduce this safety factor. For example, MG involves antibodies targeting post-synaptic ACh receptors, leading to decremental responses on slow RNS due to reduced EPPs and fewer muscle fiber activations. LEMS and botulism involve presynaptic defects lowering calcium influx or vesicle release, causing low baseline compound muscle action potentials (CMAPs) and characteristic post-exercise facilitation evident on rapid RNS.<br /><br />RNS testing involves low-frequency stimulation (2-3 Hz) to detect decrements in CMAP amplitude indicating NMJ transmission failure and high-frequency stimulation (>10 Hz) or brief exercise to evaluate CMAP facilitation, particularly useful in distinguishing presynaptic disorders like LEMS.<br /><br />The document stresses proper technique: supramaximal stimulation, limb immobilization, temperature control, and medication withholding to avoid false results. Interpretation guidelines include a 10% decrement cut-off for diagnosis, with reproducibility checks and post-exercise testing to assess facilitation or exhaustion.<br /><br />Electrophysiologic patterns differ among disorders: MG shows normal baseline CMAP with decrement on slow RNS and partial repair after exercise; LEMS features low CMAPs with marked post-exercise increment; botulism shows similar but sometimes subtler findings with prolonged facilitation.<br /><br />Finally, the paper discusses technical considerations, differential diagnosis, and reporting standards, emphasizing quality control to prevent false positives/negatives.<br /><br />In summary, RNS is a valuable, minimally invasive electrophysiologic tool essential for diagnosing and differentiating NMJ disorders through detailed understanding of underlying physiology and rigorous testing protocols.

Keywords

Neuromuscular Junction

nmj

Repetitive Nerve Stimulation

Myasthenia Gravis

Lambert-Eaton Myasthenic Syndrome

Botulism

Congenital Myasthenic Syndromes

Acetylcholine

Compound Muscle Action Potentials

Electrophysiologic Testing

Safety Factor